Nitrogen Cycling in Coastal Marine Environments
Edited by T. H. Blackburn and 1. S0rensen
cg 1988 SCOPE. Published by John Wiley & Sons LId
CHAPTER 10
Nitrification in Estuarine and Coastal
Marine Sediments
KAJ HENRIKSEN and W. MICHAEL KEMP
10.1
INTRODUCTION
Primary production in coastal waters appears to be regulated largely by the
availability of nitrogen for phytoplankton assimilation (Ryther and Dunstan,
1971;Eppley et al., 1979).In most coastal systems uptake ofthe regenerated form
of inorganic nitrogen, ammonium (NH:) accounts for the majority of phytoplankton production (Nixon, 1981). Nitrate (NOi) and ammonia (NHn are
considered to be the principal form of 'new' (allochthonous) and regenerated
(autochthonous) nitrogen, respectively, in the ocean (Dugdale and Georing,
1967).However, in estuaries and shallow marine systems, NO; may be produced
in situ via rapid oxidation of regenerated NH: (Kemp et al., 1982).In general,
NH: is the preferred form of nitrogen (compared to NO;) for phytoplankton
assimilation at concentrations> 2 J.1M(McCarthy et al., 1977).
Processes of nitrogen regeneration and transformation thus strongly influence
organic production in coastal marine environments. Nitrogen regeneration in
sediments becomes increasingly important in progressively shallower systems
(Harrison, 1980), so that benthic recycling accounts for about 30-80% of the
nitrogen requirement for phytoplankton growth in water columns of 5-50 m (e.g.
Nixon, 1981; Blackburn and Henriksen, 1983; Boynton and Kemp, 1985). As
indicated above, a portion of the NH: produced from decomposition and
deamination of organic matter in sediments may be reoxidized to NO; before
diffusing to the overlying water.
This process of ammonium oxidation, or nitrification, occurs essentially in two
steps (Kaplan, 1983):
NH: + 1.50z ~N02
L1G=
-
+ HzO + 2H+
65 kcal mol N-1
NO; + 0.50z
~
NO;
AG= -18kcalmolN-l
208
Nitrogen Cycling in Coastal Marine Environments
Several intermediate compounds (e.g. hydroxylamine)and byproducts (e.g.
nitrous oxide) of nitrification have been identified and may accumulate under
certain conditions (Kaplan, 1983).N itrosomonasspp. and N itrobacter spp. are the
principal organisms responsible for the two steps, respectively, of nitrification.
Their metabolism is chemoautotrophic, such that growth of nitrifiers is very
inefficientdue to energy requirements for carbon dioxide reduction. Ecologically,
this implies that populations of relatively low abundance can effect high rates of
NHi oxidation (Fenchel and Blackburn, 1979).Other heterotrophic bacteria are
also capable of oxidizing NH:; however, they are less important in most marine
sediment systems (Focht and Verstraete, 1977).
Nitrate produced in nitrification may also be used as an electron-acceptor for
N03 reduction to gaseous forms (N2 or N2O). Since these gases are essentially
unavailableforalgalassimilationand sincere-fixationofN 2 appearsto belimited
in most estuarine systems (Nixon and Pilson, 1983),this coupling of nitrification
and denitrification represents a sink for regenerated nitrogen. To a large extent
rates of denitrification in coastal sediments are limited by N03 supply and hence
by nitrification (Jenkins and Kemp, 1984; Andersen et aZ.,1984). Nitrification
may also cause a significant consumption of oxygen in sediments (Seitzinger
et aZ.,1984;Christensen and Rowe, 1984),and this chemoautotrophic O2 demand
might influence the balance between aerobic and anaerobic heterotrophy in such
systems.
Nitrification processes can therefore, exert considerable influence on marine
production: by modifying the form of recycled nitrogen, by shunting regenerated
NH: into the denitrification sink and by competing with heterotrophic processes for limited supplies of dissolved oxygen. It becomes important then to
understand spatial and temporal patterns of nitrification and factors regulating
its rate.
Nitrification in the water column of shallow marine and estuarine systems
appears to be relatively limited, although in regions of high NH: concentration
(e.g.the oligohaline parts of estuaries or waters adjacent to sewage effluents) high
rates of pelagic ammonium oxidation have been reported (Billen, 1975; Deck,
1980; Elkins et aZ.,1978).During transient periods of estuarine destratification,
elevated rates of NH: oxidation and nitrite accumulation have also been
observed (McCarthy et aZ.,1984). However, temporal patterns of N03 in most
coastal waters appear to be largely a function ofland runoff at low salinities and
flux.esfrom sediments at higher salinities (Nixon and Pilson, 1983). Supplies of
N03 for denitrification in coastal marine sediments appear to be derived almost
exclusivelyfromsedimentnitrification(e.g.Sl6rensenet aZ.,1979;Seitzinger et aZ.,
1983) or direct inputs from rivers (Smith et aZ.,1985).
The present review considers nitrification in sediments of estuaries and shallow
coastal marine systems. Several previous papers have provided thorough reviews
Nitrification in Estuarine and Coastal Marine Sediments
209
of the physiology, biochemistry and population ecology of nitrifying bacteria
(Aleem, 1970; Painter, 1970; Suzuki et al., 1974; Focht and Verstraete, 1977;
Belser, 1979). The paper of Kaplan (1983) provides an excellent summary and
analysis of nitrification in the oceans. Here, we focus on sediment processes and
relatively shallow waters (generally < 50m), and our discussion considers this
process in the ecological context of nitrogen, oxygen and organic carbon
fluxes.
10.2 METHODS
The various techniques available for measurement of nitrification and nitrifying
bacterial abundance have been described in detail elsewhere (Schell, 1978;
Schmidt, 1978).We review, here, only those methods which have been used in the
study of sediment nitrification.
10.2.1 15N Tracers
One of the first direct measurements of nitrification in marine sediments
employed 15N isotope dilution technique with sediment slurries (Koike and
Hattori, 1978). This approach involves addition of lsN-NO; to a mixture of
50ml seawater (amended with NH:) and 5 g (wet) sediment incubated for
48 h open to the atmosphere but without shaking. The observed changes in
concentration
and atom
% enrichment
of NO;
over time are used then to
calculate nitrification and NO; reduction. The method is relatively straightforward in its application, but it suffers from the assumption that atom %lsN-NH:
remains constant throughout the incubation and from the inherent disturbance
of sediment structure and redox conditions.
Other reports on lsN isotopic estimates of nitrification rates have attempted to
use intact sediment systems. Chatarpaul et al. (1980) used a similar lsN-NO;
dilution technique for stream sediments maintained intact after mixing, prior to
laboratory incubation. They interpreted changes in concentration and atom %
lsN for NO; as attributable to a form of net nitrification where NO; reduction
to NH: was not considered (although it was, apparently, unimportant in this
case). Jenkins and Kemp (1984) successfully measured lsN-Nz production for
intact cores of estuarine sediment amended with lsN- NH: to estimate the rate of
nitrification in relation to environmental conditions (e.g. macrofaunal activity,
redox potential). However, problems with variable diffusion and depletion of
substrate during incubations render results difficult to interpret as ambient
(in situ) rates.
A significant improvement in the slurry and batch-core systems was developed
by Nishio et al. (1982). In their system filtered seawater containing lsN-labeled
210
Nitrogen Cycling in Coastal Marine Environments
N03 or NH: was used to continuously renew water overlying an intact
sediment core. Simple isotope kinetic relations allowed an estimate of
nitrification (and that fraction coupled to denitrification), net ammonification,
denitrification and nitrate reduction to NH:. This system is essentially a
chemostat method which forces the coupled process of nitrogen transformationdiffusion to steady state, so that problems of transient rates due to diffusion
limitation or substrate depletion would be avoided. The main drawbacks of this
technique are its laborious nature requiring separate experiments with both 15NNO; and 15N-NH: amendments, and an assumption as to the actual sediment
zone where nitrification is occurring. Whereas 15NO; amendments can be done
at ambient NO; concentration (e.g. 50-100flM), the 15N-NH: amendments
often require use of increased NH: concentration to obtain measurable
percentage 15Nenrichment. To our knowledge it is the only method, however, by
which direct, quantitative measurements of nitrification coupled to denitrification can be made for intact sediments. This approach has been modified (Kemp
and Twilley, unpublished) by adding for each treatment a third experiment with
continuous exchange of filtered seawater only to estimate ambient NO; and
NH: fluxes across the sediment-water interface, and to test for possible effects of
NH: amendments on NO; fluxes across the sediment-water interface.
10.2.2 Specific inhibitors
The chemoautotrophic nitrification process is inhibited by a large number of
(more or less) specific chemical compounds. A good review of these compounds
and their mode of action is given by Hauck (1980). Three of these compounds
(nitrapyrin, allylthiourea and chlorate) have been used in aquatic sediments for
measurement of actual rates in mixed sediment (Billen, 1976), intact cores
(Henriksen, 1980;Hall, 1984),and for measurement of potential nitrification rates
in sediment slurries (Belser and Mays, 1980).
Nitrapyrin or N-serve (2-chloro-6-trichloromethyl-pyridine) is the most widely
used selective inhibitor of the nitrification process. It inhibits the oxidation of
ammonia to hydroxylamine, the first step in the nitrification process (Campbell
and Aleem, 1965).In pure cultures, effectiveblocking is obtained at concentrations
of 1-10 ppm, depending on strain of ammonium-oxidizers (Belser and Schmidt,
1981). In sediments and soils with high organic content, higher concentrations are needed, because nitrapyrin is partially inactivated by absorption and
decomposition (Briggs, 1975; Henriksen, 1980).
In marine sediments nitrite is usually oxidized to nitrate as fast as it is being
produced, and rarely accumulates. If ammonium oxidation is selectively blocked
(e.g. N-serve) without disturbing the rate of NH: production, the difference in
NH: concentration with or without the blockage provides an estimate of
nitrification. Nitrapyrin in concentrations sufficient to block NH: oxidation
Nitrification in Estuarine and Coastal Marine Sediments
211
(20 Jig/cm3 wet sediment) do not appear to affect NH: production or incorporation in aerobic or anaerobic sediment (Henriksen, 1980).Nitrapyrin is rapidly
degraded however, under anaerobic conditions, and it should be added at I-day
intervals by changing the water column. This also serves to maintain near in situ
NH: gradients. In addition, the carrier solvent used for nitrapyrin (acetone) does
not affect the NH: production or incorporation at the concentrations used
(2-6 JiI/cm3 wet sediment), and acetone also inhibits nitrification (Henriksen,
1980). However, at higher concentrations acetone is a non-specific inhibitor of
general bacterial activity (Hauck, 1980). After 3-4 days incubation (22-12 °c)
acetone causes severe immobilization of NH:, probably due to growth of
bacterial populations, capable of using acetone as a carbon source (Henriksen,
unpublished). Nitrapyrin, dissolved in acetone and acetone in equal concentrations both caused a 10-15% immobilization of NH: relative to controls
after 72 hours at room temperature in lake sediments (Hall, 1984).
Hall (1984) introduced another nitrification inhibitor, allylthiourea (ATV),
which is water-soluble and blocks effectively at 2~5 ppm concentrations in
sediment slurries. This inhibitor was tested in parallel with nitrapyrin on lake
sediments (using the intact core method) and consistently higher rates for
nitrification were obtained (Hall, 1984).He attributed this differenceto ineffective
blocking by nitrapyrin. Tests for the effect of ATV on bacterial NH:
production/incorporation in the sediments were not given. In slurries of a marine
sandy sediment (aerobic and anaerobic), ATV (5Jig/cm3 wet sediment) had no
apparent effect on net NH: production and effectivelyblocked the oxidation of
NH: for 5 days. However, incubations of intact marine sediment cores with ATV
in parallel with nitrapyrin resulted in lower and inconsistent rates with ATV
compared to nitrapyrin (Henriksen, unpublished). ATV also interferes with the
commonly used indophenol-hypochlorite assay for NH:. Further testing is
needed for this compound in marine sediments.
An advantage of these nitrification inhibitor/NH: accumulation methods is
that concentration gradients of oxygen and ammonium can be maintained near
in situ conditions during the incubation. In addition they are also technically
simple. The limitations are low sensitivity in sediments with high natural
variation ofNH: gradients associated with benthic infauna or macrophyte roots.
It should be noted that the method does not include nitrification associated with
infauna burrows, since cores are selected to avoid larger zoobenthos. Only where
the benthos consists offew, evenly distributed species,can burrow nitrification be
included, using appropriate core sizes (Henriksen, 1980;Henriksen et ai., 1980).
The methods may also underestimate nitrification rates in sediments, where
dissimilative nitrate reduction back to NH: is of quantitative importance, as for
estuaries with high concentrations of NO; in the water column (Koike and
Hattori, 1978; Nishio et al., 1982).
Dark [14C] bicarbonate incorporation into nitrifying bacteria has been
212
Nitrogen Cycling in Coastal Marine Environments
measured by Billen (1976) in mixed sediment samples with and without
nitrapyrin in short-term aerated incubations (2-5 hours). The CO2 incorporation
rate was related to the rate of ammonium oxidation using a N:C ratio of 8.3.
Later investigations have shown that the N:C ratio can vary from 40 during
exponential growth of marine nitrifiers to 4 in the stationary phase, and the N:C
ratio decreases with decreasing oxygen tension (Kaplan, 1983). Ambient
nitrification rates are also difficult to interpret from the method of Billen, because
aerobic incubations tend to measure potential rather than actual nitrification
(Henriksen et ai., 1981).
10.2.3 Indirectmeasurements
Nitrification rates in sediments can be estimated indirectly from the combined
measurements of denitrification and exchange of nitrate across the sedimentwater interface (nitrification
= denitrification
+ nitrate
flux out of sediment).
The estimate cannot be applied to shallow waters with benthic primary
production (where N03 assimilation is likely to occur), and it will also
underestimate the nitrification rate in sediments, where nitrate reduction to
ammonia is not measured directly but is of quantitative importance (see above
paragraph). First-order estimates of nitrification can also be obtained by nitrogen
mass-balance calculations given direct measurements of nitrogen fluxes across
the sediment-water interface and profiles of nitrogen species in sediments (e.g.
Boynton et ai., 1980;Christensen and Rowe, 1984).
10.2.4 Potential activities
An estimate of the potential activity of nitrifying bacteria present in the
sediments can be obtained by measuring the NO; plus N03 production in
shaken sediment slurries, incubated aerobically at room temperature with
amended substrate concentrations (Hansen, 1980; Belser and Mays, 1980;
Henriksen et ai., 1981).The NO; plus N03 accumulation tends to be linear with
time for the first 24 hours, indicating that little or no growth of nitrifying bacteria
occurs. It should be noted that the amount of sediment relative to incubation
medium (filtered seawater, enriched with NH:) should be small (e.g. 2-3 g wet
weight per 100ml). This will insure full aerobic conditions while still giving
significant accumulation of NO; + N03 over the incubation period.
Sodium chlorate can be used as a nitrification inhibitor in these slurries (Belser
and Mays, 1980). Chlorate (CI03) blocks the oxidation of NO; to N03 at a
concentration of 10mM without affecting the NH: oxidation, and time-course
accumulation of NO; can be assayed. The high sensitivity and ease of NO;
measurements (Strickland and Parsons, 1972)favors this procedure under some
circumstances. The chlorate blocking is, however, not fully efficient. Ammonium
oxidation rates estimated by CI03 inhibition in slurries tend to be 10-15% lower
Nitrification in Estuarine and Coastal Marine Sediments
213
than controls, where NO; + NO; is measured (Belser and Mays, 1980;
Henriksen, unpublished). Hynes and Knowles (1983)found, for mixed cultures of
NH: and NO; oxidizers, that NH: oxidation was inhibited by chlorite (CIO;),
produced from reduction of CIO; by the NO; oxidizers. They caution that this
might occur to a varying degree also in slurries.
The quick and simple measurement of potential nitrification activity has been
used for mapping the distribution of nitrification potential with depth in a variety
of sediments (Hansen, 1980; Hansen, et al., 1981), in microzonations around
infauna burrows (Henriksen et al., 1983;Kristensen et al., 1985)and macrophyte
roots (Andersen and Hansen, 1982).A fairly good correlation (within a factor 2)
between actual nitrification rates (measured in intact cores by the
nitrapyrinjNH: accumulation method) and potential rates in the surface layer
was found for a range of sediments, if the latter was corrected to in situ
temperature and oxygen penetration (Henriksen et aI., 1981).It may therefore be
a useful tool for surveys of nitrification activity, if the range of oxygen penetration
into the sediments is known.
10.2.5 Countingmethods
The most probable number (MPN) technique is a widely used counting
method for nitrifying bacteria. The efficiencyof the method may vary considerably, and it has an inherently low statistical precision. Belser and Schmidt (1978)
did a comparative study with pure cultures of three NH: oxidizers and obtained
counting efficiencies of the MPN method with different media and incubation
times of 3.4-94%. The efficiencyin sediment and soil samples can be much lower
(Belser, 1979).When MPN numbers are being used in a study, it is desirable to
have an idea of their relative counting efficiency. Belser and Mays (1982)
introduced a method for estimating the minimum number of NH: oxidizers
(X min)in a soil or sediment sample from potential nitrification rate measurements. They argued, that the specific activity (K) per cell in the potential rate
measurements must be lower or equal to the maximum specific cell activity (Ko)
obtained in pure culture studies. This X min represents the theoretical minimum
number of NH: oxidizer cells required to produce the amount of activity
observed in the sample. They found no correlation between potential activity
(X min),and MPN counts for seven estuarine stations, where MPN counts ranged
from 0.2 % to 5.1% of the estimated minimum population. MPN counts and
estimated Xminpopulations for six stations (three depths) in Danish waters are
given in Table 10.1. MPN counts ranged from 0.5% to 26% of the estimated
minimum population, but were consistently higher in the surface layer (0-1 cm
depth, mean value 13.5%)compared to 1-2 and 4-6 cm depths (mean value 3.3~;';
and 3.8%, respectively). This may suggest a lower specific cell activity in the
surfacepopulation of NH: oxidizers compared to the deeper anoxic layers.
MPN estimates of nitrifying bacteria in various estuarine and coastal marine
214
Nitrogen Cycling in Coastal Marine Environments
Table 10.1. Numerical abundance ( x 104per ml sediment) of NHZ oxidizing bacteria
estimated by the MPN method and calculated as theoretical minimum population (X min)
at six stations in coastal danish waters
Station number
Depth
(em)
0-1
Variable
Xmin
a
MPN (No.)
MPN (%)b
1-2
Xmin
a
MPN (No.)
MPN (%)b
4-6
Xmin
a
MPN (No.)
MPN (%)b
2
5
6
7
8
Overall
mean
69
18
26
196
10
5.1
23.6
0.3
1.3
23.6
1.5
6.4
21.7
4.7
21.7
158
32
20.3
82
11
13.5
47
212
3.2
1.5
33
0.2
0.6
33
0.9
2.7
36
1.7
4.7
150
0.7
0.5
85
1.9
3.3
225
4.6
2.0
29
0.1
0.3
29
0.9
3.1
29
0.8
2.8
10.9
0.2
1.8
56
1.3
3.8
1
4.5
9.6
10.9
1.4
12.8
a Xminvalues were calculated from potential nitrification rates and a specific cell activity of 0.023 pM
NH4 cell- 1h - 1 (Belser and Mays, 1982).
b The MPN numbers as percentages of estimated theoretical minimum value (Xmin)'
Data from Hansen (1980).
sediments are summarized in Table 10.2. In the surface layer numbers of
nitrifying bacteria range from 0 to 107per cm3 sediment. Highest numbers were
counted in the estuaries Kysing Fjord (Denmark) and Chesapeake Bay (USA),
and large seasonal changes were sometimes observed (Chesapeake Bay, King
Goodie Bay). In general, numbers of nitrifying bacteria decreased with depth.
An immunofluorescent antibody assay (FA) for direct counts of natural
populations of nitrifiers in soils were developed by Schmidt and co-workers
(Schmidt, 1974).This method has been further developed for counting of marine
NH: oxidizing bacteria (Nitrosococcus oceanus and Nitrosomonas marina) by
Ward and Perry (1980). Webb and Wiebe (1975) used the FA technique to
determine populations ofN02 oxidizing bacteria (Nitrobacter agilis) at a coral
reef and found good agreement between nitrification rates (measured by the
nitrapyrin-C4C]bicarbonate method) and nitrifier activities, estimated from FA
counts and published cell-specificactivities. FA counts of marine NH: oxidizers
in combination with 15N-nitrogen measurements of ammonium oxidation were
used to obtain rates of NH: oxidation per cell at different depth of the water
column (Ward et al., 1982). The method could be useful for evaluation of cell
activities in natural sediment populations; however, the highly organic matrix of
coastal marine sediments may pose serious problems in obtaining efficient
staining (see frontpiece). In addition, the existence of multiple serotypes which
requires many FAs for each genus further complicates the technique, although
the total number of serotypes may be more limited in the marine environment.
215
Nitrification in Estuarine and Coastal Marine Sediments
Table 10.2. Numerical abundance of nitrifying bacteria ( x 103per ml sediment) in
various estuarine coastal marine sediments estimated by MPN counts
System
References
NH40xidizers NOzoxidizers
Estuaries
0.15-3
Delware inlet,
New Zealand
King Goodie Bay,
Scotland
Barataria Bay,
Florida
Chesapeake Bay,
Maryland
Kysing Fjord,
Denmark
Ems-DolIard,
HolIand
MaizurujKumihama
Bays, Japan
Coastal waters
Danish waters
(14-200 m)
North Sea
off Belgian coast
a
b
C
d
Balser and Mays, 1982
6-94a
1-88a
0.33
7-90
0.55-120
-
100-1500b
0-10b
11000'
860-2450c
1
25-60
0.1-1
2.8-684d
1.4-4.6d
0.02-0.25
3300
23-11000
0.02
0.1-1
19-444
29-666
McFarlane and Herbert, 1984a
Jones and Hood, 1980
Jenkins and Kemp, 1984
Hansen, 1980
Helder and De Vries, 1983
Sugahara et al., 1974
Hansen, 1980
BilIen, 1978
Seasonal ranges: 0-1 em depth (above); 1-5em depth (below).
Range for 0-6em depth in: spring (above); summer (below).
Data for June: 0-1 em depth (above); 1-8 em depth (below).
Range of six stations for Nov at: 0-1 em depth (above); 1-6em
(below).
10.3 TEMPORAL AND SPATIAL PATTERNS
10.3.1 Temporal patterns
Seasonal trends of nitrification rates have been reported for a few temperate
coastal sediments, and a variety of patterns are evident (Figure 10.1). Hansen
et al. (1981) have suggested that the decrease in sediment nitrification during
summer in Danish coastal waters is attributable to a combination of reduced
depth penetration of °2' increased concentrations of H2S and greater competition for NH: (Figure 10.1a).This seasonality was especially pronounced for the
very shallow (0.5m) Kysing Fjord station. The brief increase of nitrification rates
in June at the deeper (17m) Aarhus Bay site was attributed to warming waters, an
effectoffsetby the abovefactorsin July-October (Hansenet al., 1981).Seitzinger
216
Nitrogen Cycling in Coastal Marine Environments
Hansen et of. (1981)
120
'+r
80
\-. ~/\
40
/
Ky';"'
Fjoe'
I-.
t
.
\
.-2~
~~~hUS---V
?
0
I ).
Seitzinger et of. (1984)
1W~
~
'E
z
Bay
80 ~
-0
E
:!..
C 40fo
.2
c
u
....
'-
0
:;
l
+
Providence/'
River
Macfarlane and Herbert
\
'\
/
j
i
-~
7---1
\
f
///
j
'
/I
Narragansett
I
.r:
N
);
j
\j-~t
(1984)
c
Q)
E
:.0 120
Q)
(j)
80
40
0
Toy Estuary
(Kingoodie Bay)
J
F
M
A
M
J
J
A
S
0
N
D
Figure 10.1. Seasonal patterns of sediment nitrification
measured as: (a) NH: production with N-serve for two
stations in Danish coastal waters (Hansen et al., 1981);
(b) difference between N 2and NO)" fluxes across sedimentwater interface for two stations in Narragansett Bay
(Seitzinger et al., 1984);(c) [14C]bicarbonate incorporation
with N-serve for a Scottish estuary (McFarlane and
Herbert, 1984a)
Nitrification in Estuarine and Coastal Marine Sediments
217
et al. (1983) have explained a similar midsummer depression of nitrification at
their providence River station (salinity 290/00)in Narragansett Bay by O2
limitation in these organically rich sediments (Figure 10.lb). A marked decrease
in nitrification from April to August was also reported from Chesapeake Bay
sediments by Jenkins and Kemp (1984),and this trend corresponded to decline in
bacterial abundance and surficial (0-2 cm) redox potential. Similar trends have
been observed more recently for these and other Chesapeake Bay stations (Kemp
and Twilley, unpublished).
Quite the opposite seasonal sequence has been observed for stations in middle
Narragansett Bay (USA) and in Tay Estuary (Scotland). In both of these cases
temporal patterns of nitrification follow more closely the annual temperature
cycle,with peak rates in June-July (Figure lO.lb, c).Nitrification in these systems
does not appear to have been limited by O2 diffusion into the sediments.
Nitrification and denitrification rates at the station in mid-Narragansett Bay fell
to zero under experimentally induced anoxic conditions in July (Seitzinger et ai.,
1983). The NH: availability may be the key factor regulating nitrification in
Narragansett Bay here, where reported seasonal trends of NH: flux from
sediments to water are congruous to those for nitrification (Nixon et ai., 1976;
Seitzinger et ai., 1983).MacFarlane and Herbert (1984a) suggested that enzymatic effects of temperature, per se, were responsible for the observed seasonality.
Mean nitrification rates summarized from nine different studies (seven in
shallow coastal sediments) are remarkably similar, especially given the range of
sediment types and measurement methods (Figure 10.2).Average values between
50 and 70 lLmolm - 2 h -1 predominate for all studies, which include at least spring
and summer observations. Relatively high rates were estimated from values
reported for Japanese coastal waters in September-November, and low rates
were observed in English lake and Atlantic slope sediments. Not shown here are
the remarkably high values for experimentally fertilized mesocosm studies in
Narragansett Bay (300-6001Lmol m - 2 h - 1) reported by Seitzinger and Nixon
(1985).
Few data are available for temporal sequences of nitrification at time scales
shorter than months. In one recent study daily (1-2-day interval) observations on
net fluxes of O2 and NH: across the sediment-water interface for experimental
intact sediments (darkness with aerated overlying water) were used to examine
the time course for decomposition of sediment organic matter (Boynton,
unpublished). If one assumes constant proportions of both O2 consumption
attributable to other than chemoautotrophic nitrification and NH: regeneration
associated with anaerobic respiration, the ratio of O2:NH: fluxes will be
proportional to nitrification. In this study of a sandy sediment system, net fluxes
ofNH: and O2 diminished to near zero within 30 and 40 days respectively. The
flux ratio (02:NH:) exhibited a periodicity of about 5-lO days and a marked
increase after 30 days, suggesting an increase in nitrification relative to
ammonification rates as regeneration of NH: declined. On shorter time scales,
218
Nitrogen Cycling in Coastal Marine Environments
Method
-----------
Nitrification,
0
50
fmol
100
Author
N m-2 h-1
150
200
Japanese
Coast
15N-Slurries
I
Koikeand(1978)
Hattori
15N-Cores
Japanese
Coast
Nishio et 01
(1983)
11\i- Cores
Chesapeake
Boy
.--------
unpublished
N-Serve
(Cores)
Danish
Coast
Hansen et 01
(1981)
ATU-Cores
Kemp and Twilley,
--------
English
Lakes
.-------
Hall and
Jeffries (1984)
14C-Slurries
Belgian
Coast
Billen (1978)
14C-Slurries
Scottish
-----------------------Estuary
--------
Direct N2
(Cores)
Modeling
I/.1 V/:::::;:.(\>;--;.:.~.~I
(6)
---------
Narragansett
Boy
Atlantic
Slope
MacFarlane and
Herbert (1984)
I
Seitzinger
et 01. (1984)
Christensen and
Rowe (1984)
Figure 10.2. Mean values and ranges (histograms and bars) for nitrification
in various estuarine and coastal marine (stippled) and other (open) sediments,
using different methodologies (seetext for methods description). Numbers in
parentheses indicate number of observations (minimum of four)
Henriksen has observed in shallow tidal flats evidence of diel cycles of
nitrification which may result from interactions with diatom production (see
discussion below). Further studies are needed to examine daily (or shorter)
temporal patterns of nitrification, especially in response to penurbations such as
organic deposition events or macrofaunallife cycles (e.g. larval set).
10.3.2 Spatial patterns
Some broad spatial patterns of sediment nitrification can be discerned from
information reported in the literature. There is some indication that calculated
rates of nitrification tend to decrease with depth across the range of 15-65 m in
Danish waters (Henriksen et ai., 1981).A strong correlation between station
depth (45- 500m) and nitrification potential has been reported for sediments from
the Algerian Atlantic coast (Vargues and Brisou, 1963). Similarly, sediment
nitrification rates for deeper stations (35m) were estimated to be only 60% of
those measured for relatively shallow ( 15m) stations off the Belgian coast (Billen,
Nitrification in Estuarine and Coastal Marine Sediments
219
1978). It might be postulated that higher organic matter deposition rates at
shallower stations (e.g. Hargrave, 1984;Lee this volume) would support greater
rates ofNH: regeneration which would, in turn, enhance nitrification. A positive
relationship between net NH: regeneration and denitrification reported elsewhere for estuarine sediments (Kemp et aI., 1982)further supports this idea. The
relatively low C:N ratio of surficial particulate organic matter at shallower
stations on the Danish coast might indicate that organic matter reaching the
sediments in shallow systems tends to be richer in nitrogen; however, no simple
correspondence betwen depth and NH: regeneration rates could be seen
(Blackburn and Henriksen, 1983).
Although organic carbon and total nitrogen content and NH: regeneration in
Narragansett Bay sediments exhibited a continual decline seaward from the Providence River station. No correlation between these variables and nitrification
was evident for three sites along this inshore-offshore gradient (Seitzinger et at.,
1983). Mean nitrification rates at three stations in a Chesapeake Bay tributary
were generally proportional to rates of both organic deposition and NH:
regeneration fluxes from sediments to overlying water (Kemp and Twilley,
unpublished). Here, highest rates for nitrification as well as deposition and NH:
flux were observed in the 'transient zone' of the Patuxent River Estuary (mean
salinity range 4-12%0),and nitrogen cycling in this region appears to influence
primary production of the entire estuary (Kemp and Boynton, 1984).
The depth distribution of nitrification activity in marine sediments is
ultimately constrained by the limits of downward O2 diffusion. Until recently the
determination of O2 in sediments was hampered by lack of an appropriate
analytical technique. The development of micro electrodes for polarographic
measurements of O2 with high spatial resolution (0.1mm) has allowed direct
observation of O2 penetration in sediment porewater (Revsbech et aI., 1980a),
where typical depths observed for O2 disappearence were 1-6.5 mm (Revsbech
et at., 1980b; S~rensen et at., 1979).
Sharp and opposing gradients of O2 and NH: occur in this surficial redox
interface of marine sediments (Figure 10.3), where highest in situ rates of
nitrification would be expected to occur (Kemp et at., 1982). In typical depth
distribution of potential nitrification and NO; concentration, maximum values
occur in the zone of O2 penetration (Figure 10.3). High values for potential
nitrification deeper in the sediment column may be associated with the walls of
infauna burrows and tubes (e.g.Henriksen et at., 1983)or downward mixing due
to physical resuspension and bioturbation. Other relative measurements of
nitrifying bacterial distributions (MPN) have also indicated highest abundence in
the uppermost sediment strata (Hansen et at., 1981;Vincent and Downes, 1981),
and vertical distributions of atom% 15N-NO; in experiments with 15NH:
amendments have indicated similarly the predominance of surficial nitrification
(Jenkins and Kemp, 1984).By correcting potential nitrification values to ambient
temperature and assuming that this 'potential' is realized only in the region of O2
~
220
Nitrogen Cycling in Coastal Marine Environments
Potential nitrification
(nmol cm-3 d-1)
0
0
400
800
12000
<O2,fLM
N03' I-'M
NH4, I-'M
100 200 300 0 2 4 6 0 100 200 300
2
E
u
.c4
0.
Q)
-0
<=6
Q)
E
is
\
Q)
(f)8
6
Figure 10.3. Profiles with depth of nitrification potential, depth of oxygen
penetration (= actual zone of nitrification) and porewater concentrations of NO;
and NH: in a typical coastal sediment (17 m depth, Aarhus Bay). Insert shows
depth of oxygen penetration observed in variety of sediments from Danish waters
(14-200m depth). (After Henriksen et al., 1981; Revsbech et al., 1980.)
penetration, Henriksen et al. (1981) found a strong and significant correlation
between nitrification rates so calculated and those measured with intact
sediments under in situ conditions. These results further emphasize the correspondence between vertical distribution of O2 and actual nitrification.
Small scale (mm-cm) spatial distributions of nitrification, both vertical and
horizontal directions, are also influenced by discontinuities in the physical
structure of sediments. In particular, macrofaunal burrows and tubes, diatom
mats and macrophyte roots may affect nitrification by altering spatial gradients
of °2' NH:, CO2, pH and so on. These interactions are discussed in a later
section of this paper.
10.4 PHYSICAL AND CHEMICAL REGULATION
10.4.1 Temperature
The optimum temperature for pure cultures of nitrifying bacteria appears to be
in the range of 25-35 DC,and the temperature range in which growth occurs is
usually within 3-45°C (Focht and Verstraete, 1977). Temperature optima and
growth ranges for natural populations seem to depend largely on ambient
temperature regimes. For example Jones and Hood (1980b) reported optimum
temperature of 40°C for a N itrosomonas sp. isolated from a subtropical estuarine
Bay in Florida, whereas Helder and De Vries (1983) observed optimum
temperatures of 25-35 °C for a Nitrosomonas sp. isolated from the temperate
221
Nitrification in Estuarine and Coastal Marine Sediments
Temperature
I
3
~~22 °c
/
2
~1021-26
~/°(Slurries)
~~
12-22°C
[]
2
0
°102.44-2.58
4
8
12
16
20
24
28
°c
4
0
"
D
0
I
+ ".
I
Z
a
I
2
;.
8
v---
--Q
(Slurry)
8
I
I
I
X
I
(])
(])
>
/
x- !f--x>
I
(f)
2
Oxygen ---x
--- -- n-/
x~
(Pure culture)
31-
8
11-.
I
I
0
(])
0::
2
41
3
4
Ammonium
6
8
10
20
--_8
30
40 kPa
-
8
(Slurry)
2
Km
100
200
300
400
500
= 85 p.mal NH; t;1
600
700
800
p.mal NH; t-1
Figure lOA. (a) Relative rates of NH: oxidation at different
temperatures in three coastal/estuarine sediments, measured at 2 DC
and 22°C (one at 3°C and 14°C) in spring (ambient bottom
temperature 1.5-3°C, 0 x 0) and at 12DCand 22°C in autumn
(ambient bottom temperature 12-15 DC,D . 0 ).Allratesareplotted
assuming equal rates at 22°C. (Data from Hansen, 1980 and
Henriksen, unpublished.) (b) Relative rates of NH: oxidation at
different oxygen concentrations in (1) pure culture of Nitrosomonas
sp. (
), slurries of estuarine sediment at oxygen concentrations
below air saturation (-)
and oxygen concentrations above air
saturation (---).
(After Knowles and Hynes, 1984; J~rgensen
et al., 1984; Henriksen, unpublished.) (c) Relative rates of NH:
oxidation in aerated slurries of estuarine sediment at different NH:
concentrations. (After Hansen, 1980.)
222
Nitrogen Cycling in Coastal Marine Environments
Ems-Dollard estuary in Holland. Nitrifiers seem capable of extreme adaptation
to low temperatures; significant nitrification rates have been observed in the
water column at
-
2°C under Antarctic ice (Horrigan, 1981)and at 0.5°C in
sediments of the northern Bering Sea (Henriksen, unpublished).
The effect of temperature on nitrification rates in pure cultures tends to be
described well by the Arrhenius equation within the range 15-35 DC,whereas
rates at lower temperatures tend to decrease rather steeply (Focht and Verstraete,
1977).Values of QlO reported in the literature for nitrification cover the range:
2-3 (18-38 DC)for cultures of marine NH: oxidizers (Carlucci and Strickland,
1968);2.7~3.3(10-20 °C)for isolates of Nitrosomonas sp. and Nitrobacter sp. from
the Ems-Dollard estuary (Helder and De Vries, 1983); 2.1-2.6 (2-22°C) for
potential nitrification rates in three different coastal and estuarine sediments
sampled at low temperature in spring (1.5-3 °C); and 2.4-2.6 (12-22 DC)for the
same sediments sampled at 12-15°C in autumn (Figure lO.4a) (Hansen, 1980;
Henriksen, unpublished). It appears as if nitrifying bacteria may be able to adapt
to cold temperatures over the season, but no information on this important topic
is yet available.
10.4.2 Oxygen
Oxygen is required for each of the two major oxidation steps in nitrification. In
pure cultures the Km value of N itrosomonasis 1611MO2 and for N itrobacter 62 11M
O2. The last step is the most sensitive, which results in NO; accumulation at low
oxygen tensions (Focht and Verstraete, 1977). Heterotrophic bacteria have a
much higher affinity for oxygen (Km < 111MO2) and are likely to outcompete
nitrifying bacteria at low oxygen concentrations in the sediments. The reported
O2 concentrations at which NH: oxidation stops ranges from 1.1 to 6.211MO2
(Gundersen, 1966;Carlucci and McNally, 1969;Sugahara et al., 1974;J~rgensen
et aI., 1984).
Nitrous oxide is produced within the first step in the nitrification process at low
oxygen concentrations (Goreau et al., 1980;Hynes and Knowles, 1984;J~rgensen
et aI., 1984). In simultaneous measurements of nitrification, denitrification and
N 20 production in slurries, N20 production increased sharply below 0.18 11MO2
and showed maximum at complete anoxia. Denitrification was the main source
of N20 production, except for a narrow range between 1.1 and 2.211MO2
(J~rgensen et al., 1984).
In pure cultures of a marine N itrosomonas sp. the ratio of CO2 incorporated to
nitrogen substrate oxidized increased with decreasing oxygen concentration. Cell
production per mole N02 produced was increased by a factor 5 at the lowest
oxygen level (5.611MO2), whereas the production of NO; per cell declined by a
factor 7 (Goreau et al., 1980).As mentioned earlier, this change in C: N ratio with
oxygen tension makes it difficult to use [14C]bicarbonate incorporation as an
indirect measurement for the rate of NH: oxidation.
Nitrification in Estuarine and Coastal Marine Sediments
223
Very little is known about the effect of high oxygen concentrations on the
nitrification process. Nearly pure oxygen was toxic to Nitrosococcus oceanus
growing on agar medium, but less inhibitory in liquid medium (Gundersen, 1966).
Increasing inhibition of potential nitrification activity at increasing O2 levels has
been measured in slurries of estuarine sediment (15% and 25% inhibition at
2 and 2.6 times air saturation respectively; Henriksen, unpublished). Further
investigations of this aspect are needed in relation to diurnal fluctuations in
nitrification rates in shallow water sediments with benthic primary production,
where oxygen concentrations in the top 2-3 mm can reach levels of 2-5 times air
saturation during the daytime (Revsbech et ai., 1981).
10.4.3 Ammonium
The substrate oxidation by nitrifying bacteria follows in general the MichaelisMententype kinetics, and a large number of Km values are reported in the
literature, mostly from pure cultures and sewage treatment systems. Here the Km
values for NH: oxidation are in the range 70-700 f.1MNH: and for NO;
oxidation 350-600 f.1M
NO; (Focht and Verstraete, 1977).Nitrifying bacteria are,
however, capable of adaptation to extremely low substrate concentrations and
Km values as low as 0.1f.1M
for both NH: and NO; oxidation has been reported
for natural populations of open ocean waters (Olson, 1981; Hashimoto et ai.,
1983). The substrate affinity of nitrifiers may therefore depend largely on the
ambient substrate availability in their environment. An isolate (Nitrosomonas sp.)
from the Ems-Dollard estuary had a Kmvalue of 55 f.1M
NH: at 20°C (Helder and
De Vries, 1983).The NH: concentration of the water in this estuary ranged from
20-80 f.1MNH: (Helder et ai., 1983). Hansen (1980) reported Km values of 60100f.1M
NH: for NH: oxidation in slurries of a estuarine sediment (Figure 10.4c),
where porewater concentrations in the surface sediment ranged from 20 to 200 f.1M
NH:. In addition, Km values estimated for natural populations of nitrifiers in the
Thames were related to ambient temperature, decreasing from 76 f.1MNH: at
25°C to 7 f.1MNH: at 5°C (Knowles et ai., 1965). This could indicate that a
considerable variation in Km may occur over the season, especially for shallow
water sediment.
10.4.4 Carbon dioxide and pH
The activity of nitrifying bacteria are optimal in a narrow pH range from
neutral to slightly alkaline (pH 7-8.5), with the limiting range for activity being
slightly wider (pH 6-9.5) (Focht and Verstraete, 1977). The upper limit is
dependent on the concentration of free (undissociated) ammonia (NH3), which is
toxic to the bacteria. In pure cultures nitrification was unaffected up to pH 11.2,
as long as free NH3 was kept below 1.4f.1M(Prakasam and Loehr, 1972).
Nitrobacter is relatively more sensitive, and NO; oxidation is selectively
224
Nitrogen Cycling in Coastal Marine Environments
inhibited in the range 7-70 J1Mfree NH3 (Belser, 1979).The pH of the solution
determines the equilibrium between NH: and NH3 (pK. = 9.24) and at pH
9,215 J1Mof total NH: is required to inhibit NO; oxidation, whereas at pH 9.5
only half of this concentration is needed. On the other hand, it has been suggested
that undissociated NH3 rather than NH: is the substrate for ammonium
oxidizers (Suzuki et al., 1974), such that slightly alkaline conditions would
increase substrate availability, which would be beneficial at low NH: concentrations in surface sediment.
Marine sediments of deeper waters generally encounter only small pH
fluctuations, whereas shallow-water sediments with benthic microalgae can show
dramatic diurnal variations in pH (e.g. pH 7-10) in the surface layer (Revsbech
et aI., 1981; Andersen and Jensen, 1983). Optimum rates at pH 7.5-8.5 were
measured for an estuarine Nitrosomonas sp. with a sharp decrease in activity at
pH 7, whereas the activity at pH 9 was still 50% of maximum (Jones and Hood,
1980b). The pH of these sediments and overlying water varied between pH 6.3
and 8.5, and could be an important factor regulating the nitrification rate in the
sediments.
Carbon dioxide concentrations in marine sediments are usually high and not
considered a limiting factor for growth of nitrifying bacteria. If, however, nitrifiers
use free CO2 instead of bicarbonate as their primary carbon source, as is the case
for most micro algae (e.g.TaIling, 1976),inhibition of growth may occur at high
pH. In an intertidal sand flat with benthic diatoms, only 2% (0.4J1M)of normal
free CO2 in sea water was present in surficial pore water (pH 9.3) during the
daytime (Rasmussen et al., 1983).This was considered a possible limiting factor
for diatom photosynthesis, and nitrifying bacteria would have to compete with
the diatoms for this carbon source.
10.4.5 Salinity
Nitrifying bacteria in marine environments with fluctuating salinities seem
adapted to the prevailing salinity of their growth habitat and are able to acclimate
to a broad range of salinities after a lag phase (Finstein and Bitsky, 1972). For
example, a marine Nitrosomonas sp., isolated from the Ems-Dollard estuary at
15%0,was able to adapt to the whole salinity range (0-35%0)and grew at the same
rate after a lag phase of up to 12 days (Helder and De Vries, 1983). Short-term
fluctuations in salinity may, however, have strong regulating effects on nitrification activity. An estuarine Nitrosomonas sp. (Florida) showed optimum activity
in the range 5-10%0with rapid decrease in activity (within 24 hours) on both sides
(50%ofthe max activity at 1.3%0and 15%0;Jones and Hood, 1980b).MacFarlane
and Herbert (1984b) found a broader salinity tolerance for an estuarine
Nitrosomonas sp. isolated from a Scottish estuary, but no time scale for
measurements after transfer to different salinity media was given.
Natural populations of nitrifiers in ocean waters are inhibited by light
intensities as low as 1% sunlight (Olson, 1981).The NO; oxidizers are the most
Nitrification in Estuarine and Coastal Marine Sediments
225
sensitive and the inhibition is due to oxidation of cytochrome c in the presence of
oxygen and light (Bock, 1965).Light may also influence the nitrification activity
in shallow water sediments. Light penetration into sandy sediments is 2-3 mm,
depending on grain size and organic content (Fenchel and Staarup, 1971), and
nitrification could be inhibited in the top millimeter of the sediments during
daytime, but this effect has not been demonstrated experimentally in marine
sediments.
10.4.6 Toxic and stimulating compounds
Nitrifying bacteria are inhibited by a range of volatile sulfur compounds, of
which CSz is particularly toxic (Powlson and Jenkinson, 1971; Bremner and
Bundy, 1974). Sulfide, the product of anaerobic sulfate reduction, is the
quantitatively most important toxic sulfur compound in marine sediments. Srna
and Baggeley (1975) found complete inhibition at 0.9flM SH- and suppressed
nitrification at 0.4flM SH - in aquaria experiments, whereas Yoshida (1967)
measured full inhibition of NH: oxidizers at 40 flM SH -.
It is difficultto interpret the possible effect of SH - in natural sediments.
Concentrations offree SH- in the surface layer can be high (up to 500 flMSH-) in
sheltered areas with high organic deposition, often with strong diurnal variations
(Hansen et al., 1978) and no nitrification activity is present in such habitats
(Hansen, 1980). In sediments with less dramatic sulfate reduction rates, SHconcentrations are usually low or zero in the upper layers and not well defined in
the narrow zone of active nitrification. A range of other volatile sulfur compounds
are produced during anaerobic decomposition in marine sediments, but very
little is known about their concentrations and distribution in the sediments.
Dissolved organic matter may either stimulate or inhibit nitrification. High
concentrations (> 100mg/l) of yeast extract, peptone and other related compounds inhibited nitrification in batch cultures, whereas low concentrations
(10 mg/I) stimulated growth significantly (MacFarlane and Herbert, 1984b).
Jones and Hood (1980a) found enhanced activity (up to 150%)for an estuarine
Nitrosomonas sp. when grown in the presence of two heterotrophes, isolated from
the same estuarine environment. Several vitamins, amino acids and other
organics have been shown to stimulate NH: oxidation (Clark and Schmidt,
1967).Kaplan (1983)notes that although nitrifying bacteria in general lack Krebs
cycle enzymes, a complete Krebs cycle has been found in Nitrosomonas oceanus
(Williams and Watson, 1968).
10.4.7 Surfaces
Available surface area could be of importance for the maximum nitrification
activity possible in sandy and more fine-grained sediments. If nothing else limits
growth of nitrifying bacteria, growth will continue to a certain point, where
226
Nitrogen Cycling in Coastal Marine Environments
growth ceases, despite surplus substrate availability. This is termed the maximum
carrying capacity (X max)of a soil or sediment (Belser, 1979).Belser points out that
X max may not actually be a result of physical surface area limitation, but a result of
self-inhibition (due to low pH, low oxygen tension, etc.), resulting from diffusionlimited processes. However, this might be represented mathematically as surface
area limitation.
In the sediments, nitrifying bacteria must compete with the much larger
heterotrophic bacterial population for 'apparent' surface area. Limited carrying
capacity of the sediment in the actual nitrification zone could be an important
factor determining the population size of nitrifying bacteria in sediments with
surplus NH: availability, but it is difficult to separate surface area limitation
from other factors affecting the population of nitrifiers.
10.5 BIOLOGICAL AND ECOLOGICAL REGULATION
10.5.1 Diatom interactions
The prc::;enceof benthic diatoms appears to inhibit nitrification activity in
the surface layer of shallow-water sediments. Depth distribution of potential
nitrification activity is shown (Figure 10.5) for two aquaria with sieved sandy
sediment, one kept in the dark and the other with a 12 hour photoperiod for
Potential
0
200
400
nitrification
600
800
L
E
u
~
.<::
+a.
~
rates
(nmol cm-3 d-1)
1000
0
L
J
0
0
2
2
4
4
6
6
8
8
10
10
12
12
Dark aquarium
200
,
400
,
600
,
%:
/;
Light /Dark aquarium
Figure 10.5. Potential nitrification rates in sediment of sandy
aquaria, kept in the dark and in a 12hour light/dark cycle,
respectively, with continuous flow of seawater (14 QC).Measurements were made after 6 weeks. (Henriksen, unpublished.)
Nitrification in Estuarine and Coastal Marine Sediments
227
6 weeks. A dense mat of benthic diatoms was present in the 'light' aquarium and
the nitrification potential in the top 5mm was less than 10%ofthe activity in the
'dark' aquarium. The observed inhibition can be attributed to a combination of
several factors including; NH; limitation, high pH, high O2 concentrations, light
inhibition, CO2 limitation and organic excretion products. Competition for
NH; represents a potentially important factor regulating growth of nitrifiers in
the top layer. In natural populations of diatoms on a tidal sand flat, NH;
concentrations in the top 4 mm were 5-15 pM in the porewater during daylight,
increasing only slightly (15-30 pMNH4) in the dark. Higher NH; concentrations
are encountered at night in older decomposing algal mats overlain with quiescent
water, a situation analogous to that in aquaria after 6 weeks
(Henriksen,unpublished). Diel ranges in pH were 7.6-9.1 in the 'light' aquarium
and 7.8-8.1 in the 'dark' aquarium. In these algal mats diurnal fluc.tuations in O2
depth penetration tend to be 2-5 mm and maximum O2 concentrations may
exceed 2-5 times air saturation (Revsbech et al., 1981).
It is evident that nitrification was inversely related to activity of benthic
diatoms, but it is difficult to evaluate the quantitative importance of any factor
individually, especially since all effectsare not linear and may be interrelated. For
example, the depth of oxygen penetration (zone of actual nitrification) would be
increased by a factor 2-3 during algal photosynthesis, but associated high pH
and O2 concentrations,reducedNH; (andpossiblyCO2)availability,as wellas
possible light inhibition and inhibition by bacteriostatic excretion products from
the diatoms, will depress nitrification activity in the upper 2-4 mm of the
sediment; the zone of peak nitrification will be found deeper in the sediment
during daytime. In the dark period the nitrification zone shrinks due to lower
oxygen penetration, but conditions for nitrification will be otherwise improved.
The effect of benthic diatoms on exchange ofNH; and NO;- is dramatic, both
in aquaria experiments and in nature. In sandy sediments with high benthic
primary production the flux ofNH; and NO;- was directed from overlying water
into the sediments (Henriksen et al., 1984) and a tight coupling between
regeneration and assimilation of NH; seems to exist in the surface layer of such
sediments. The depression of nitrification (and thereby denitrification) makes the
coupling more efficient by reducing the loss of plant nutrients associated with
coupled nitrification-denitrification.
to.5.2
Macrofaunal interactions
High potential nitrification activity is found in the lining of permanent infauna
burrows, and these rates are consistently higher than corresponding nitrification
activity at the sediment surface (Henriksen et al., 1983;Blackburn and Henriksen,
1983; Kristensen et al., 1985).This pattern may be explained in terms of higher
NH: availability in the burrow environment. Ammonium concentrations
around the deeper parts of burrows are often significantly higher than at the
tV
tV
00
~--Head shaft
Tail shaft
Potential nitrification
(nmol cm-3 d-')
0
2
3
4
5
0
0 fLJ
2
4
6
8
10
12
14
16
18
, Gallery
20
il
Net NH; production
2
3
4
5
~
c
...
""
~
;:;
(j
""
Q.
S.
""
1
Gallery
Figure 10.6. Potential nitrification rates and net NH: production rates (measured at 20°C) around the tailshaft of
Arenico/a marina burrows: (-)
inner oxic lining, 0-2 mm; (
) adjacent sediment, 2-8 mm. From a tidal sand flat,
Danish Wadden Sea. (Henriksen, unpublished.)
s.
(j
C
i::>
'"
[
~
i::>
...
S.
~
t>1
;:;
'"
::;.
c
;:;
;:!
~
;;
'"
229
Nitrification in ,Estuarineand Coastal Marine Sediments
sediment surface, and this, in combination with NH: excretion by the
macrofaunal inhabitant, gives a higher substrate availability. Oxygen availability, on the other hand, is in general lower in the burrow lining and may
fluctuate considerably, depending on infaunal irrigation rhythms (Kristensen,
this volume). Simultaneous measurements of net NH: production and nitrification potential were made for sediments surrounding burrows of the lugworm,
Arenicola marina (Figure 10.6).There was a significant increase with depth in the
burrow (tail shaft) of both net NH: production and nitrification potential in the
inner oxic lining (0-2 mm). Rates in the sediment adjacent to the oxic burrow wall
(2-8 mm) were lower, exhibiting no trend with depth. There was a generally linear
relationship between net NH: productions and nitrification potential to
moderately high rates of the former process, after which nitrification activity
showed little response to increasing NH: production (Figure 10.7). Concentrations ofNH: porewater of both the inner and outer layer was low (2-20 J1.M
NH;) in 0-10 cm depth, increasing to 50 J1.MNH: in the deeper layers.
The lugworm lives in V-formed burrows of 15-20 cm depth (Figure 10.6).The
irrigation current is directed in through the tail shaft and out through the
sediment-filled head shaft (funnel).Irrigation activity is almost continuous during
water cover (Kruger, 1971),and NH:, excreted by the polychaete, will mostly be
transported from the gallery up through the head shaft. Thus the high ammonium
production in the inner burrow wall seems to be the main source for nitrification
5000
,
-0
'"
'E 4000
()
0
E
c 3000
c
0
0
()
- 2000
...
.c
~c
a
1000
Q)
-0
CL
1000
2000
3000
Net NH: production
4000
5000
6000
(nmol cm-' d-')
Figure 10.7. Potential nitrification rates versus net NH; production rates in the inner oxic layer of tail shaft (0-2 mm) and gallery of
ArenicQlamarinaburrows(data fromFigure 10.6).(. = Tail shaft;
0 = gallery.)
230
Nitrogen Cycling in Coastal Marine Environments
in this burrow environment, but additional NH: is supplied through diffusion
from the surrounding sediment. Potential nitrification rates in the burrow wall of
another burrowing polychaete, N ereis virens, in organic-poor and organic-rich
sediment were highest in the rich sediment, perhaps reflecting increased NH:
availabilityin the rich sediment(Kristensenet al., 1985).
Factors other than availability of NH: may be responsible for the high
potential nitrification activity measured in burrow environments; these include
absence of light, more stable pH regime and stimulation by organic excretion
products from the animals. Although O2 concentrations in the burrow environment are often low and fluctuating, these extended surfaces seem to offer ideal
conditions for nitrifying bacteria. MPN counts of NH: oxidizers and corresponding rates of potential nitrification are represented for surface and deeper
layers at six stations in Danish waters and from a polychaete (Nereis virens)
burrow (Figure 10.8).Two distinctly different relations are evident for surface and
deeper anoxic layers in the vertical section of the sediments, with higher specific
activity per cell for the deeper layers, but no trend between surface and deeper
layers in the horizontal section of the worm burrow. The source of this difference
is uncertain. The nitrifier population may adapt to temporary and continuous
anoxic conditions in a way that enhances cell activity during the potential
nitrification measurement at full oxygen tension (e.g.internal pools of stimulating
organic compounds), or there may be a selection for different strains/ species of
nitrifiers with higher specific cell activity.
The nitrification potential at the sediment surface is affected by grazing and
fecal pellet formation of the macrofauna. Facal pellets produced by surface
deposit feeders and filter feeders represent a potentially ideal environment for
nitrifying bacteria; however, an aging period of some duration may be required.
The potential nitrification rate of fecal pellets from the surface feeding bivalve,
Macoma baithica (5.2~M NO; cm-3 d-1) was four times higher than that in
surface sediment on which M. baithica was feeding, and seven times higher than
the rate in corresponsing surface sediment of control aquaria (Henriksen et ai.,
1983). Fresh fecal pellets, collected on a tidal sandflat from the same species,
showed equally high activity. Fresh fecal pellets of the filter-feeding bivalve,
Mytilus edulis (sampled in August), showed very low potial nitrification activity
(0.05~mol NO; cm - 3 d - 1),but extremely high net NH: production rates. Aged
fecal pellets in the surface sediment of the dense Mytilus beds had high potential
nitrification rates (2.5-3~mol NO; cm-3d-1) and lower net NH: production
(Henriksen, unpublished). In late autumn, fresh fecal pellets from Mytilus had
high potential nitrification rates equal to those of the sediment surface in the
extensive Mytilus beds along the tidal channel. At this time of the year active
resuspension of the fecal pellet sediment from the mussel beds apparently
enhanced nitrification, resulting in high net NO; production in the tidal water
(Henriksenet ai., 1984).The importance of sediment resuspensionleading to
increased rates of water column nitrification has been demonstrated for an
231
Nitrification in Estuarine and Coastal Marine Sediments
60
..
a
50
x
z 40
a..
~
'"
~
30
-0
)(
a
+.. 20
I
Z
0
z
10
0
400
800
1200
Potential nitrification
1600
2000
2400
rate (nmol cm-3 d-')
Figure 10.8. Numbers (x 104) of NH: oxidizers (MPN counts)
versus potential nitrification rates in surface layer (0-1 em, open
circles) and deeper anoxic layers (1-2 and 4-6 em, closed circles) of
the sediment from seven stations in Danish coastal waters and from
the oxic burrow waIl (0-1 mm, x) and adjacent anoxic sediment
(l-lOmm, 0) of Nereis virens burrows (polychaete).(Data from
Hansen, 1980.)
estuarine system, where highest rates were consistently observed in the physically
active'turbidity maximum'zone (Owens,1985).
Grazing by zoobenthos can also inhibit nitrification activity. Nitrifier populations are particularly susceptible to reductions via grazing pressure due to their
relatively inefficient metabolism and slow growth rate (Fenchel and Blackburn,
1979),and grazing by zoobenthos could be a regulating factor for the population
size of nitrifying bacteria. Meier-Reil (1983)has suggested that seasonal shifts in
zoobenthos grazing pressure might be responsible for observed winter increases
in benthic bacterial abundance at a station in Kiel Bight. If this were the case,
than it is reasonable to postulate that bacterivorous grazing might be at least
partially responsible for similar seasonal trends in nitrifier abundance (e.g.
Hansen et al., 1981).However, direct evidence for grazing control of nitrification
in sediments is lacking.
232
Nitrogen Cycling in Coastal Marine Environments
10.5.3 Macrophyteeffects
Both emergent and submersed macrophytes may influence sediment nitrification through various mechanisms including: oxygenation of deep sediments via
O2 release from roots, increased production ofNH: associated with N2 fixation
and/or decomposition of particulate organics which tend to be 'trapped' from
surrounding waters and deposited in macrophyte beds, and increase in both O2
diffusion and NH: regeneration associated with activities of macrofauna which
tend to be more abundant in vegetated sediments.
The effect of O2 release from roots of macrophytes in oxidizing sediment
porewaters of the rhizosphere has been well documented for both emergent
(Howes et al., 1981) and submersed vascular plants (Wium-Andersen and
Andersen, 1972). Recent experiments with macrophyte leaf and root parts
separated in watertight, two-compartment chambers have demonstrated that
1-100% of photosynthetically produced O2 may be released by macrophyte
roots to the external rhizosphere (e.g. Sand-Jensen et al., 1982). Recently, such
measurements have been related to in situ conditions for an estuarine population
of Potamogeton perfoliatus L. to estimate sufficient O2 flux to support 250350 /lmol m - 2h -1 ammonium oxidation during the macrophyte growing season
(Kemp and Murray, 1986).While intense competition with other redox reactions
would limit the availability of nitrifiers to exploit this O2 source, 10-20% of this
O2 flux would serve to double the ambient nitrification rates in surficial
sediments (Jenkins and Kemp, 1984).
Bacterial N 2 fixation rates associated with leaves and sediments of seagrass
populations are among the highest values reported, and for many tropical
seagrasses measured rates of N 2 fixation are sufficient to support all of the
macrophyte production (Capone, 1983).Abundances of both nitrogen-fixing and
denitrifying bacteria in the rhizosphere of the submersed macrophyte, Myriophyllum spicatum, were shown to be significantly higher than in surrounding
sediment (Blotnick et al., 1980).In addition, increased rates of particulate organic
nitrogen deposition associated with seagrass damping of waves and tidal currents
(Kenworthy et aI., 1982) represent another source of new nitrogen to these
sediments (Kemp et al., 1984).In either case, enhanced rates of ammonification
could be expected in these vegetated sediments, and high rates have been
reported for Zostera marina sediments, sufficient to support plant demands for
gmwth (Iizumi et al., 1982). The increased availability of regenerated NH:,
coupled with active burrowing of relatively abundant benthic macrofauna in
these systems, create an ideal condition for enhanced surficial nitrification.
There are fewdirect measurements of nitrification rates or nitrifier populations
associated with macrophyte sediments. Iizumiu et al. (1980) reported high
concentrations of nitrate (9.8/lM)in pore waters of an Alaskan Z. marina bed, and
directmeasurementsof denitrificationindicatedrates of
~
10-80 /lmol m - 2 h - 1
with highest values in the sediments containing greatest seagrass biomass. Here
233
Nitrification in Estuarine and Coastal Marine Sediments
only 6% of the measured O2 released from roots to rhizosphere would be needed
to support nitrification coupled to the highest observed rates of denitrification.
Measurements of nitrification potential (aerated slurry incubations, 400 pM
NH:) appeared sufficient to support such rates. Limited estimates of nitrification
(using the N-servejNH: production method) for intact sediment cores with and
without P. perfoliatus indicated 20 times greater rates in vegetated sediments
(Kemp et aI., 1982). Recent data for sediments of the submersed macrophyte,
Littorella uniflora, in oligotrophic Danish lakes demonstrated elevated rates of
potential nitrification in surface and deep (root zone) sediments during the
macrophyte growing season (Christensen, 1984). However, direct effects of the
plant on nitrification were not examined. Kaplan et al. (1979) also reported very
high rates of potential nitrification for salt marsh sediments, but they were unable
to estimate actual nitrification rates.
10.6 INTERACTION WITH AMMONIFICATION
DENITRIFICATION
AND
An important process by which nitrogen is made essentially unavailable for
marine biological production is denitrification, the reduction of NO; to N2 gas.
To a large extent rates of denitrification appear to be limited by NO; availability
in estuarine and coastal marine sediments, and therefore sediment nitrification, in
effect,regulates denitrification in these systems. Deamination of organic nitrogen
(ammonification) produces NH: which may support assimilation by photosynthetic organisms. The sequence ofNH: oxidation to NO; by nitrifying bacteria,
and subsequent reduction of NO; to N2 by denitrifiers, provides a shunt of
regenerated ammonia away from assimilative pathways which support photosynthesis. Hence the coupling of nitrification to ammonification and denitrification may directly influence primary production.
Parallel observations on net fluxes of NO; and NH: across the sedimentwater interface reveal some patterns which bear on the coupling of nitrification
both with ammonification and denitrification (Boynton and Kemp, 1985). For
two benthic communities dominated by burrowing macrofauna in Narragansett
Bay, inverse relationships between NO; and NH: fluxes were observed in the
data of Nixon et al. (1976) (Figure 10.10a,b). This relation, however, is not oneto-one. For example, a large decrease ( 250 pmol Nm - 2 h -1) in NH: recycling
flux between September and June from sediments dominated by the amphipod,
Ampelisca sp., corresponded to a much smaller increase in NO; flux ( 90 pmol
N m -2 h -1) from these sediments (Figure lO.lOa).Presumably some of the NO;
formed from NH: oxidation was reduced to N2. No relationship between the
two fluxes is evident in the winter data for any of the three stations. The more
negative slope of the NH: versus NO; flux relation for the Nephtys-Nucula
system may indicate closer coupling of nitrification and denitrification. Low
NO; fluxes from the sediments dominated by the hard shell clam Mercenaria
~
~
234
Nitrogen Cycling in Coastal Marine Environments
4 Ha)
Coastal
waters
3
2
0
0
c
0
0
0
....
8
10
30
20
Water depth (m)
40
(b) Danish
coastal
waters
-c
...
OJ
"C
c
0
4
0
0
....
";::
-c
0
0
0
20
e::
40
Water
depth
10
60
(m)
(c)
Aarhus
Bay
5
00
4
Temperature
8
(Oe)
12
Figure 10.9. Ratios of sediment nitrification to denitrification
(N:D): (a) annual means of N:D for systems with data available
versus depth of overlying water column; (b) N:D for stations in
Danish coastal waters in July (open circles) and November (closed
circles) versus water depth; (c) N:D for Aarhus Bay station on five
dates plotted versus sediment temperature
Nitrification in Estuarine and Coastal Marine Sediments
235
may similarly result from a tighter connection between nitrification and
denitrification process.
For the very limited number of coastal marine sites where annual estimates of
sediment nitrification and denitrification were both available, a positive relationship appears between the ratio of nitrification to denitrification (N:D) and water
column depth (Figure 1O.9a).A similar pattern is evident for sediments in Danish
coastal waters at stations spanning a wider depth regime (14-65 m), and values of
N:D range from
~
1 to > 9 (Figure 1O.9b).The inverserelationshipsobserved
between depth and organic deposition to the sediment surface in marine waters
(Suess, 1980; Hargrave, 1984) suggest that the strength of N:D coupling may
depend, in part, on supply of organic matter. Increased organic deposition would
tend to increase both energy substrate for denitrification and NH: regeneration
rates (Nixon, 1981), which would, in turn, enhance nitrification (Kemp et aI.,
1982).Increased abundance of macrofauna, associated with more organic foods,
may further contribute to greater rates of both nitrification and denitrification
(Henriksen et al., 1983, Kristensen et al., 1985) and possibly to N:D coupling
(Aller, this volume). In addition, lower temperatures at the sediment surface in
most deeper waters may contribute to decoupling of the two processes, if
nitrifying bacteria are more capable (than denitrifiers) to adapt to low temperatures. Such an inverse relationship between temperature and N:D was evident in
the seasonal data from the Aarhus Bay station, for which estimates of nitrification
and denitrification were available (Figure 10.9c).
Establishing a mechanism by which nitrification and denitrification can be
efficiently connected is complicated by the fact that they require extremely
different redox conditions; nitrification and denitrification are obligately oxic and
anoxic processes, respectively (Focht and Verstraete, 1977).The two-layer model
developed by Vanderborght and Billen (1975), Vanderborght et al. (1977) and
Patrick and Reddy (1976)defines surficial oxic and deeper anoxic sediment strata
separated by a redox discontinuity profile across which NO; and NH: diffuse
downward and upward, respectively. Applying simple one-dimensional diagenic
equations to this model, and using measured vertical profiles of NO;,
Vanderborght and Billen (1975) were able to estimate rates of nitrification and
denitrification for sandy coastal sediments. Henriksen (1980) used this model to
demonstrate that the upper nitrification zone must be limited to 0-5 mm to
reproduce observed NO; profiles for Danish coastal sediments. However, NO;
fluxes across the sediment-water interface could not be predicted from observed
vertical distribution of porewater NO; based on molecular diffusivity alone
(Henriksen et aI., 1981).
Conditions of very efficient coupling of sediment nitrification and denitrification (where the two rates are approximately equal with little loss of NO; from
the sediments) are difficult to explain using the two-layer model. With nitrification occurring in the surficial layer we would expect that NO; diffusion upward
across the sediment-water interface would be at least equal to the downward flux
236
Nitrogen Cycling in Coastal Marine Environments
300
(a)
200
100
,
.r:
0
';'E
(b)
0
E
~
100
)(
:J
;;::
I
I
I
I
I
I
I
I
E
:J
'c
0
50
E
E
0
+Q)
z
03~
0
Jon-Mar
(c)
Mercenaria
200
100
Nov -Apr
0
-20
0
Net nitrate
40
flux
60
80
(j.Lmol m-2 h-1)
Figure 10.10. Net fluxes of NH: versus NO;- across the
sediment interface for three stations in Narragansett Bay
dominated by: (a) the amphipod Ampelisca sp.; (b) the polychaete and bivalve, Nephtys sp. and Nucula sp.; (c) the bivalve
M ercenariasp. Data are separated according to warm and cold
seasons, with numbers beside points indicating month of
observation. (Adapted from data of Nixon et aI., 1976.)
Nitrification in Estuarine and Coastal Marine Sediments
237
to the dentrification zone, depending on the depth of maximum nitrification
relative to the depth of O2 penetration. Bioturbation of macrofauna from above,
and ebullition of methane bubbles from below, the zones of nitrification and
denitrification would both contribute to reduced coupling and increa3ed N03
flux from the sediments to overlying water. Jenkins and Kemp (1984) demonstrated tight coupling of these two N-transformation processes using 15N-NH:
enrichment experiments with intact estuarine sediment cores in which> 99%
of the 15N-N03 produced in nitrification was reduced to 15N-N2 via denitrification. They proposed that such close coupling of the two reactions could be
explained by denitrification occurring in reduced microzones within the upper,
oxic zone of nitrification. They further suggested that invertebrate fecal pellets of
100-200 flM diameter might provide the appropriate spatial scales of redox
structure, for such tight coupling of the diffusion-reaction system. The existence
of such reduced micro sites in marine sediments has been demonstrated using
redox-sensitive tetrazolium salts (Pearl, 1984).
Other investigators have shown that denitrification rates or capacities tend to
be greatest in the uppermost sediment strata into which O2 penetrates (S16rensen
et ai., 1979; Kasper, 1982; Andersen et ai., 1984)and that denitrification readily
occurs in aerated soil or sludge slurries (Focht and Verstraete, 1977).J9>rgensen
(1977) suggested that similar anoxic microsites must exist in surface sediments
where high rates of sulfate reduction were observed. Sayama (1983) also inferred
anoxic microsites in fecal pellets of a marine polychaete (Neanthes japonica) to
explain higher (10 x ) denitrification rates in undisturbed versus crushed fecal
pellets overlain by oxic water.
There are several other possible explanations for observed close coupling of
nitrification and denitrification. The simplest of these would be to invoke aerobic
denitrification, such that the two processes could coexist in a narrow redox
gradient. While recent evidence suggests that bacteria capable of aerobic
denitrification may exist (e.g. Robertson and Kuenen, 1984), it is generally
thought that the process is unimportant in nature (Focht and Verstraete, 1977).A
three-layer one-dimensional model was proposed by Grundmanis and Murray
(1977), with a deeper oxic layer, maintained by macrofaunal irrigation, underlying the two-layer system of Vanderborght and Billen (1975). This model is of
limited generality, requiring a seldom-observed bimodal vertical distribution of
N03 concentrations in porewaters. The transient ventilation patterns described
by Kristensen, V (this volume) for N ereis virens result in oscillations of O2
concentration in the polychaete tubes from near-saturation to zero within 20 min.
It is conceivable that temporal rather than spatial separation of nitrification and
denitrification in such a system might lead to complete coupling.
Finally, the elegant diagenic model of Aller (this volume) predicts close
coupling of the two processes via radial diffusion properties with vertically
oriented macrofaunal burrows. Here, N03 produced by nitrification in the
burrow wall will tend to diffuse radially outward into reduced regions of
238
Nitrogen Cycling in Coastal Marine Environments
denitrification more readily than the inward diffusion toward the tube. Clearly,
there is need for direct experimentation to test the applicability of these
alternative models, which describe the important interaction between nitrification and denitrification processes in coastal sediments.
10.7 NITRIFICATION IN NITROGEN AND OXYGEN BUDGETS
One way to measure the relative significance of an ecological process such as
sediment nitrification is to compare its associated elemental fluxes to those of
('.'\
60
'_/
'I
"
'"I
E
0'"
c:
0
~E
::!
U)
c:
0
u
0'"
c
Q)
.§
"Q)
U)
20
c
"0
I-
2
4
6
°2 Consumed in nitrification (mmol °2 m-2d-')
Figure 10.11. Oxygen consumption in sediment nitrification
processes compared to total sediment oxygen demand for:
Cheasapeake Bay (CB, squares-from Kemp and Twilley, unpublished); Narragansett Bay (NB, circles-calculated from data of
Seitzinger et al., 1984);and Danish coastal waters (DK, trianglesfrom Hansen et al., 1981 and Revsbech and Jy)rgensen, unpublished). Open and closed symbols indicate warmer and colder
seasons, respectively
239
Nitrification in Estuarine and Coastal Marine Sediments
related individual processes or to the net fluxes across system boundaries. Here
we consider nitrogen and oxygen fluxes associated with nitrification for coastal
sediments where simultaneous measurements of other fluxes were available.
Oxygen is consumed by sediment microbial and metazoan communities in
aerobic heterotrophic respiration and in certain chemoautotrophic processes
such as nitrification. The stoichiometric molar ratio of O2 consumed per NH:
oxidized in nitrification is 2: 1 (e.g. Christensen and Rowe, 1985). Using this
relation to estimate O2 consumption by nitrification, the calculated rate can be
compared to total sediment O2 demand (SOD) (Figure 10.11).
A reasonably consistent relation between O2 consumption by nitrification and
SOD can be seen for sediments in Danish coastal waters, with the former
constituting 4-14% (mean 7%) of the latter. A similar proportion of total O2
consumption was attributable to nitrification in Chesapeake Bay sediments
(mean 8%); however, the fractional contribution to SOD was more variable
(range 1-14%). In contrast, nitrification in Narragansett Bay sediments comprised a generally higher fraction of total O2 demand, averaging 14% (range 224%). Similar contributions by nitrification to total sediment O2 consumption
have been reported for the Baltic coast (5%, Nedwell et ai., 1983) and English
lakes (8-10%, Hall and Jeffries, 1984). Considerably greater effects (35%) of
nitrification on O2 budgets of continental slope and abyssal plain sediments were
estimated by Christensen and Rowe (1984).
The impact of nitrification on sediment nitrogen budgets can be assessed by
comparison with rates of ammonification and denitrification in five coastal
Table 10.3. Summary ofmajor nitrogen transformation processes in various estuarine and
marine sediments
N-Fluxes (mmol N m-2d-l)
Coastal
ecosystem
Time
period
North Sea
Annual
Sluice Dock
?
Nov.
Ju!.
Ju!.
Kattegat
Narragansett
Bay
Patuxent
Estuary
a
b
Annual
This is 'net ammonification
Mean
depth
(m)
Ammonificationa
Nitrification
Denitrification
Referencesb
15
35
1.5
17
17
7
5.3
2.4
6.7
2.3
2.4
7.6
3.4
1.5
4.5
0.8
0.8
2.8
1.2
0.4
2.1
0.3
0.5
2.4
(I)
3
11.2
1.7
3.2
(5)
above assimiliation
and sorption
(2)
(3)
(4)
in sediments.
References: (1) Billen, 1978; (2) Vanderborght et aI., 1977; (3) Blackburn and Henriksen, 1983;
(4) Seitzinser et al., 1984; (5) Boynton et al., 1980; Jenkins and Kemp, 1984; Kemp and Twilley,
unpublished data.
240
Nitrogen Cycling in Coastal Marine Environments
systems for which such data are available (Table 10.3). For example the
proportion of NH: which is produced in organic decomposition and then
oxidized by nitrification for these selected sediment systems ranges from 15% in
the Patuxent Estuary to 67% in Sluice Dock. A simple explanation for such
variation is not immediately apparent. Higher percentages of NH: being
oxidized might be expected in sediments where much of the NH: is regenerated
anaerobically via sulfate reduction occurring well below a relatively thick
nitrification zone (Vanderborght et aI., 1977). In this case NH: molecules
diffusing upward would have a higher probability of being oxidized before
escaping to the overlying water. Smaller percentages might be more common in
sediments where surficial aerobic ammonification dominates (Kemp et aZ.,1982)
such that the oxidation of regenerated NH4 molecules would be less likely prior to
diffusing from the sediment porewaters.
10.8 CONCLUSIONS
In this review we have tried to stress the importance of understanding sediment
nitrification in the context of the entire nitrogen cycle of estuarine and coastal
marine sediments. The major elements of this system of nitrogen transformation
and diffusion reactions are depicted in Figure 10.12. Here we focus on three
principal routes of nitrogen transformations: (1) ammonification, via both oxic
and anoxic pathways; (2) nitrification, both NH: and NO; oxidations; and (3)
NO; reduction to both N 2 and NH:. Reactions involving other N-species such
as nitric and hitrous oxides and hydroxylamine are not included. Even at this
scale of representation it is immediately obvious that nitrification rates and
growth of nitrifying bacteria depend on other N-transformation pathways, as well
as on diffusion across substantial redox gradients. Furthermore, the activities of
macrofauna and meiofauna in these sediments exert a myriad of influences on
nitrification and its coupling with ammonification and denitrification. Mechanisms of animal influence (identified by numbered circles in the figure) include,
respectively: (1)irrigation and ventilation; (2)tube and burrow building; (3)fecal
pellet production; (4) sediment mixing; (5) bactivorous grazing.
Conceptual models (such as that in Figure 10.12) provide a framework from
which we can organize and sharpen our thoughts and experiments on sediment
nitrification and related processes. However, this exercise can be formalized using
mathematical description of the sediment nitrogen cycle. One approach which
has been widely utilized for geochemical analyses is the 'diagenic modeling'
developed by Berner (1980) and others. Here, the sediment-nitrogen system is
described largely in terms of diffusion-reaction couples, with partial differential
equations integrated over time and space in the vertical dimension only. In the
scheme of Vanderborght and Billen (1975) the sediment system is generally
partitioned into two redox zones, with an oxic layer overlying an anoxic stratum.
This approach has been very effectivein reproducing observed vertical profiles of
~
...
...........
............
s;'"'
~
o.
;:
s.
tl']
'"
i:
!:>
...
/
--;':;:0/
I
I
I
,I
s.
~
!:>
;:
!:>..
()
0
!:>
'"
[
~
!:>
...
s.
~
C;)
~
!:>..
§.
~
--~
Figure 10.12. Conceptual representation of nitrification in relation to the nitrogen cycle of estuarine and
coastal marine sediments. Animal effects depicted include (circles): (1) ventilation; (2) tube and burrow
building and associated effects on radial diffusion of NO; (2a,2b); (3) production of fecal pellets and other
structures with anoxic microsites; (4) burial of particulate N; (5) bactivorus grazing. Hexagons represent
heterotrophic organisms including bacterial groups: ah = aerobic heterotrophs; sr = sulfate reducers;
ni = NH: oxidizers; nb = NO; oxidizers; nr = NO; reducers; dn = denitrifiers. Squares represent various
chemical species. Line types indicate: double thickness = particulate transport; continuous, smooth = Ntransformations; continuous, wavy = diffusion of chemical species; dashed == O2 uptake or S04 reduction;
dotted = animal interactions
~
'"
tV
.j::>.
242
Nitrogen Cycling in Coastal Marine Environments
porewater NO; and NH: concentrations, and for estimating actual rates of
nitrification, denitrification and ammonification from such profiles (Vanderborght et al., 1977; Billen, 1978).
With the traditional diagenic modeling techniques the temporal rate of change
for porewater concentrations of NH: (Cn) and O2 (Co) associated with
nitrification (only) can be described as follows:
ot
oCo
at
02Cn
( )
= D'
( )
oCn = D'
n
-
OX2
Vrn[R(Cn)] [R(Co)]
(1)
Vrn[R(Cn)] [R(Co)]
(2)
a2Co
0 ax2
-
where x is vertical distance, t is time, Dare diffusitivity coefficients, and Vrnis
maximum nitrification rate which is directly proportional to nitrifier bacterial
abundance. R(Cn)and R(Co)represent nitrification dependence on NH: and O2
concentrations, respectively. This approach does not directly consider various
effectsof benthic macrofaunal and meiofaunal activities on nitrogen cycling, even
though the importance of these organisms is well documented experimentally
(previous section). However, it is common to employ operational (rather than
molecular) diffusivity coefficient (D') which account for the increased vertical
dispersion (above molecular scale) associated with bioturbation (Berner, 1980;
Billen, 1982).When portrayed in this fashion other mechanisms of interaction
between animals and nitrification can be included in the simple model. For
example, bactivorous grazing on nitrifiers could be described as decreases in Vrn,
and the effects of animal burrows enhancing diffusion of O2 and NH: might be
approximated also by increasing the 'effective distance' over which rates are
integrated to calculate vertical fluxes. However, as with the definition of
'operational diffusivity' coefficients, the later approach requires arbitrarily
establishing an 'effective distance' for integration.
As previously discussed in this paper, the one-dimensional, two-layer diagenic
model (Vanderborght and Billen, 1975)cannot readily account for the coupling of
sediment nitrification and denitrification often observed in coastal systems
(Jenkins and Kemp, 1984; Seitzinger et al., 1984). The three-layer model of
Grundmanis and Murray (1977), with an irrigated oxic layer underlying an
intermediate anoxic stratum, helps to resolve this problem; however, the rarity of
reports of the bimodal vertical profiles of NO; predicted by this model suggests
that it is not very general. Perhaps the most intuitively appealing approach to
modeling this nitrification-denitrification coupling is that proposed by Aller (this
volume), where spatial relations are defined three-dimensionally around animal
burrows and tubes. The radial diffusion geometry of this model is sufficient to
explain the range in nitrification-denitrification coupling observed (Figure 10.9).
Other factors such as anoxic microsites (Jenkins and Kemp, 1984)probably also
contribute to this coupling, but direct evidence is lacking. In addition, a
Nitrification in Estuarine and Coastal Marine Sediments
243
consistent rational way to incorporate anoxic microsites into the mathematical
structure of a sediment nitrogen model awaits further development.
It is clear at this stage that sediment nitrification is an important process in
nitrogen budgets of most estuarine and coastal marine systems, and that O2
consumption by nitrification can constitute a significant fraction of total benthic
O2 demand. Patterns of ambient sediment nitrification have been described
reasonably well at a few selected sites over relatively large temporal (e.g.seasonal)
and spatial (e.g. 5-50 m depth gradients) scales, and much is known about the
physiology of nitrifying bacteria. Considerably less information is available
concerning changes in ambient nitrification over short and intermediate scales,
and regarding ecological. effects of interactions of nitrifiers with animals and
plants. There are various useful methods available for measuring sediment
nitrification, each with its particular strengths and weaknesses, and these very
different techniques have yielded remarkably consistent estimates of nitrification
rates. However, much of the future progress in this field (as in other disciplines)
will depend on methodological developments, and on the application of clever
new experimental and analytical approaches.
ACKNOWLEDGEMENTS
We wish to thank R. Twilley, W. Boynton, S. Seitzinger, J. Jenkins, T. Fisher,
C. Stevenson, L. Murray, T. H. Blackburn, J. I. Hansen, P. Bondo Christensen
and B. Soeland, who have provided us with stimulating discussions and data
during the preparation of this chapter. K. Henriksen was supported by funds
from National Science Foundation Grant DPP-8405286, and W. M. Kemp
was supported by funds from Maryland Sea Grant College.
REFERENCES
Aleem,M. I. H. (1970).Oxidation of inorganic nitrogen compounds. Ann. Rev. Plant
Physiol.,21, 67-90.
Andersen, F. 0., and Hansen, J. I. (1982).Nitrogen cycling and microbial decomposition
in sedimentswith Phragmitesaustralis(Poaceae).Hydrobiol. Bull., 16, 11-19.
Andersen, T. K., and Jensen, M. H. (1983).Nitrogen transformations in coastal sediments,
investigated by use of the acetylene inhibition technique with special reference to diurnal
and seasonal variations in denitrification rates. M.Sc. thesis, University of Aarhus,
Denmark.
Andersen, T. K., Jensen, M. H., and S9Irensen,J. (1984). Diurnal variation of nitrogen
cycling in coastal marine sediments. I. Denitrification. Mar. Bioi., 83, 171-6.
Belser,L.W. (1979).Population ecologyof nitrifyingbacteria. Ann. Rev.Microbial., 33,
309-33.
Belser, L. W., and Mays, E. L. (1980). Specific inhibition of nitrite oxidation by chlorate
and its usein assessingnitrificationin soilsand sediments.Appl.Environ.Microbial., 39,
505-10.
Belser, L. W., and Mays, E. L. (1982).Use ofnitrifier activity measurements to estimate the
244
Nitrogen Cycling in Coastal Marine Environments
efficiency of viable nitrifier counts in soils and sediments. Appl. Environ. M icrobiol., 43,
945-48.
Belser, L. W., and Schmidt, E. L. (1978). Diversity in the ammonium-oxidizing
nitrifier
population of a soil. Appl. Environ. Microbiol., 36, 584-8.
Belser, L. W., and Schmidt, E. L. (1981). Inhibitory effect of nitrapyrin on three genera of
ammonia-oxidizing nitrifiers. Appl. Environ. Microbio/., 41, 819-21.
Berner, R. A. (1980). Early Diagenesis: a theoretical approach. Princeton University Press,
Princeton.
Billen, G. (1975).Nitrification in the Scheidt estuary (Belgium and The Netherlands). Est.
Coast. Mar. Sci., 3, 79-89.
Billen,G. (1976).A method for evaluatingnitrifyingactivityin sedimentsby dark 14Cbicarbonate incorporation. Water Res., 10, 51-7.
Billen, G. (1978). A budget of nitrogen recycling in North Sea sediments off the Belgian
coast. Est. Coast. Mar. Sci., 7, 127-46.
Billen, G. (1982).An idealized model of nitrogen cycling in marine sediments. Am. J. Sci.,
282, 512-41.
Blackburn, T. H., and Henriksen, K. (1983).Nitrogen cycling in different types of sediment
from Danish waters.Limnol.Oceanogr.,28, 477-93.
Blotnick, J. R., Rho, J., and Gunner, H. B. (1980). Ecological characteristics of the
rhizospheremicroflora of Myriophyllum heterophyllum.J. Environ.Qual.,9, 207-10.
Bock, E. (1965). Untersuchung uber die wirkung sichtbaren lichtes auf Nitrosomonas
europaea und Nitrobacter winogradsky. Arch. Mikrobiol., 51, 18-41.
Boynton, W. R., and Kemp, W. M. (1985). Nutrient regeneration and oxygen consumption by sedimentsalongan estuarinesalinitygradient.Mar. Eco/.Progr. Ser.(In press).
Boynton, W. R., Kemp, W. M., and Osborne, C. G. (1980). Nutrient fluxes across the
sediment-water interface in the turbid zone of a coastal plain estuary. In: Kennedy, V. S.
(ed.),EstuarinePerspectives,pp. 93-109. AcademicPress,New York.
Bremner, J. M., and Bundy, L. G. (1974). Inhibition of nitrification in soils by volatile
sulfurcompounds.Soil BioI. Biochem.,6,161-5.
Briggs, G. G. (1975).The behavior of the nitrification inhibitor N-Serve in broadcast and
incorporated applicationsto soil. J. Sci.Fd. Agric., 26, 1083-92.
Campbell, N. E. R., and Aleem, M. I. H. (1965).The effect of 2-chloro, 6 (trichloromethyl)
pyridine on the cheomoautotrophic metabolism of nitrifying bacteria. Antonie van
Leuwenhoek, 31, 124-44.
Capone, D. G. (1983). N2 fixation in seagrass communities. Mar. Techn. Soc. J., 17,
32-7.
Carlucci, A. F., and McNally, P. M. (1969). Nitrification by marine bacteria at low
concentrations of substrate and oxygen. Limnol. Oceanogr., 14, 735-9.
Carlucci, A. F., and Strickland, J. D. H. (1968). The isolation, purification and some
kinetic studies of marine nitrifyingbacteria. J. Exp. Mar. BioI. Ecol., 2, 156-66.
Chaterpaul, L., Robinson, J. B., and Kaushik, N. K. (1980).Effects oftubificid worms on
den(trificationand nitrificationin streamsediments.Can.J. Fish.Aquat. Sci.,37,65063.
Christensen, 1. P., and Rowe, G. T. (1984). Nitrification and oxygen consumption in
northwest Atlantic deep-sea sediments. J. Mar. Res., 42,1099-1116.
Christensen, P. B.(1984).The influence of aquatic macrophytes on nitrogen cycling. M.Sc.
thesis, University of Aarhus, Denmark.
Clark, c., and Schmidt, E. L. (1967). Uptake and utilization of amino acids by resting
cellsof Nitrosomonaseuropaea.J. Bacteriol.,93, 1309-15.
Deck, B. (1980).Nutrient element distribution in the Hudson estuary. Ph.D. thesis,
Columbia University, New York.
Dugdale, R. c., and Goering, J. 1.(1967).Uptake of new and regenerated forms of nitrogen
in primary production. Limnol. Oceanogr., 12, 196-206.
Nitrification in Estuarine and Coastal Marine Sediments
245
Elkins, J. W., Wofsy, S. C, McElroy, M. B.,Kolb, C E.,and Kaplan, W. A.(1978).Aquatic
sources and sinks for nitrous oxide. Nature (Lond.), 275, 602-6.
Eppley, R. W., Renger, E. H., and Harrison, W. G. (1979). Nitrate and phytoplankton
production in southern Calfornia coastal waters. Limnol. Oceanogr., 24, 483-94.
Fenchel, T., and Blackburn, T. H. (1979). Bacteria and Mineral Cycling. Academic Press,
London.
Fenchel, T., and Straarup, B. J. (1971).Vertical distribution of photosynthetic pigments
and the penetration of light in marine sediments. Oikos 22, 172-182.
Finstein, M. S., and Bitsky, M. R. (1972). Relationship of autotrophic ammoniumoxidizing bacteria to marine salts. Water Res., 6, 31-40.
Focht, D. D., and Verstraete, W. (1977). Biochemical ecology of nitrification and
denitrification. Adv. Microbiol. Ecol., I, 135-214.
Goreau, T. J., Kaplan, W. A., Wofsy, S. C, McElroy, M. B., Valois, F. A., and Watson,
S. W. (1980). Production of N02 and N20 by nitrifying bacteria at reduced
concentrations of oxygen. Appl. Environ. Microbiol., 40,526-32.
Grundmanis, V., and Murray, J. W. (1977). Nitrification and denitrification in marine
sediments from Puget Sound. Limnol. Oceanogr., 22, 804-13.
Gundersen, K. (1966). The growth and respiration of Nitrosocystis oceanus at different
partial pressures of oxygen. J. Gen. Microbiol., 42, 387-96.
Hall, G. (1984).Measurements of nitrification rates in lake sediments: comparison of the
nitrification inhibitors Nitrapyrin and allylthiourea. Microb. Ecol., 10, 25-36.
Hall, G. H., and Jeffries, C (1984).The contribution of nitrification in the water column
and profundal sediments to the total oxygen deficit of the hypolimnion of a mesotrophic
lake. Microb. Ecol., 10, 37-46.
Hansen, J. I. (1980).Potential nitrification in marine sediments. M.Sc. thesis, University of
Aarhus, Denmark.
Hansen, J. I., Henriksen, K., and Blackburn, T. H. (1981). Seasonal distribution of
nitrifying bacteria and rates of nitrification in coastal marine sediments. Microb. Ecol.,7,
297-304.
Hansen, M. H., Ingvorsen, K., and Jorgensen, B. B. (1978). Mechanisms of hydrogen
sulfide release from coastal marine sediments to the atmosphere. Limnol. Oceanogr.,23,
68-76.
Hargrave, B. T. (1984).Sinking a particulate matter from the surface water of the ocean. In
Hobbie, 1. E., and Williams, P. J. Le B.(eds),Heterotrophic Activity in the Sea, pp. 15578. Plenum Press, New York.
Harrison, W. G. (1980). Nutrient regeneration and primary production in the sea. In:
Falkowski, P. G. (ed.),Primary Productivity in the Sea, pp. 433-60. Plenum Press, New
York.
Hashimoto, L. K., Kaplan, W. A., Wofsy, S. C, and McElroy, M. B. (1983). Transformations and fixed nitrogen and N20 in the Cariaco Trench, Deep-Sea Res., 30,575-90.
Hauck, R. D. (1980). Mode of action of nitrification inhibitors. In: Stelly, M. (ed.),
Nitrification I nhibitors- Potentials and Limitations, ASA Spec. Publ. No. 38,pp. 19-33.
American Society of Agronomy, Madison, Wisconsin.
Helder, W., and de Vries, R. T. P. (1983).Estuarine nitrite maxima and nitrifying bacteria
(Ems-Dollard Estuary), Neth. J. Sea Res., 17, 1-18.
Helder, W., de Vries, R. T. P., and Rutgers van der Loeff, M. M. (1983). Behavior of
nitrogen nutrients and silica in the Ems-Dollard estuary. Can. J. Fish. Aq. Sci., 40, 188200.
Henriksen, K. (1980). Measurement of in situ rates of nitrification in sediment. Microb.
Ecol.,6, 329-37.
Henriksen, K., Hansen, 1. I., and Blackburn, T. H. (1980).The influence of benthic infauna
on exchange rates of inorganic nitrogen between sediment and water. Ophelia,Suppl. 1,
249-56.
246
Nitrogen Cycling in Coastal Marine Environments
Henriksen, K., Hansen, 1. I., and Blackburn, T. H. (1981). Rates of nitrification,
distribution of nitrifying bacteria and nitrate fluxes in different types of sediment from
Danish waters. Mar. Bioi., 61, 299-304.
Henriksen, K., Rasmussen, M. B., and Jensen, A. (1983). Effect of bioturbation on
microbial nitrogen transformations in the sediment and fluxes of ammonium and
nitrate to the overlaying water. Eco/. Bull., 35, 193-205.
Henriksen, K., Jensen, A., and Rasmussen, M. B. (1984). Aspects of nitrogen and
phosphorus mineralization and recycling in the northern part of the Danish Wadden
Sea. Neth. Inst. Sea Res. Pub!. Series,1O,51-69.
Horrigan, S. G. (1981).Primary production under the Ross Ice Shelf, Antarctica. Limnol.
Oceanogr.,
26, 378-82.
Howes, B. L., Howarth, R. W., Teal, J. M., and Valiela, I. (1981). Oxidation-reduction
potentials in a saltmarsh: spatial patterns and interactions with primary production.
Limnol. Oceanogr., 26, 350-60.
Hynes, R.K., and Knowles,R (1984).Production of nitrous oxide by Nitrosomonas
europaea: effectsof acetylene,pH and oxygen.Can.J. Microbiol., 30,1397-1404.
lizumi, H., Hattori, A., and McRoy, C P. (1980).Nitrate and nitrite in interstitial waters of
eelgrass beds in relation to the rhizosphere. J. Exp. Mar. Bioi. Eco/.,47, 191-201.
lizumi, H., Hattori, A., and McRoy, C P. (1982). Ammonium regeneration and
assimilationin eelgrass(Zosteramarina) beds, Mar. Bio/., 66, 59-65.
Jenkins, M. C, and Kemp, W. M. (1984).The coupling of nitrification and denitrification
in two estuarinesediments.Limno/.Oceanogr.,29, 609-19.
Jones, R. D., and Hood, M. A. (1980a). Interaction between an ammonium oxidizer,
Nitrosomonas sp. and two heterotrophic bacteria, Nocardia atlantica and Pseudomonas
sp.: a note. Microb. Ecol., 6, 271-6.
Jones, R D., and Hood, M. A. (1980b).Effects of temperature, pH, salinity and inorganic
nitrogen on the rate of ammonium oxidation by nitrifiers isolated from wetland
environments. Microb. Ecol., 6,339-47.
J~rgensen, B. B.(1977).Bacterial sulfate reduction within reduced microniches of oxidized
marine sediments.Mar. Bioi., 41, 7-17.
J~rgensen, S. K., Jensen, H. B., and S~rensen, J. (1984). Nitrous oxide production from
nitrificationand denitrificationin marinesedimentat lowoxygenconcentrations.Can.
J. Microbiol., 30, 1073-8.
Kaplan, W. A.(1983).Nitrification. In: Carpenter, J. E., and Capone, D. G. (eds),Nitrogen
in the Marine Environment,pp. 139-90. AcademicPress,New York.
Kaplan, W. A.,Valiela, I., and Teal, J. M. (1979).Denitrification in a saltmarsh ecosystem.
Limnol. Oceanogr., 24, 726-34.
Kaspar, H. F. (1982).Denitrification in marine sediment: measurements of capacity and
estimate of in situ rate. Appl. Environ.Microbiol., 43, 522-7.
Kemp, W. M., and Boynton, W. R. (1984). Spatial and temporal coupling of nutrient
inputs to estuarine primary production: the role of particulate transport and
decomposition. Bull. Mar. Sci., 35, 522-35.
Kemp, W. M., and Murray, W. (1986). Oxygen release from roots of the submersed
macrophyte, Potamogeton perfoliatusL.: regulating factors and ecological implications.
Aquat.Bot. (In press).
.
Kemp, W. M., Wetzel, R L., Boynton, W. R, D'Elia, C F., and Stevenson, J. C (1982).
Nitrogen cycling and estuarine interfaces, In: Kennedy, V. S. (ed.), Estuarine Comparisons, pp. 209-30. Academic Press, New York.
Kemp, W. M., Boynton, W. R, Twilley, R. R, Stevenson, J. C, and Ward, L. G. (1984).
Influencesof submersedvascularplants on ecologicalprocessesin upper Chesapeake
Bay, In: Kennedy, V. S. (ed.), The Estuary as a Filter, pp. 367-94. Academic Press, New
York.
Nitrification in Estuarine and Coastal Marine Sediments
247
Kenworthy, W. J., Zieman, J. C, and Thayer, G. W. (1982).Evidence for the influence of
seagrasses on benthic nitrogen cycling in a coastal plain estuary near Beaufort, North
Carolina (USA), Oecologia, 54, 152-8.
Knowles, G., Downing, A. L., and Barrett, M. J. (1965).Determination of kinetic constants
for nitrifying bacteria in mixed culture, with the aid of an electronic computer. J. Gen.
M icrobiol., 38, 263-78.
Koike, I., and Hattori, A. (1978).Simultaneous determination of nitrification and nitrate
reduction in coastal sediments by an 15Ndilution technique. Appl. Environ. Microb., 35,
853-7.
Kristensen, E. (1985). Oxygen and inorganic nitrogen exchange in a Nereis virens
(Polychaeta) bioturbated sediment-water system. J. Coast. Res., 1, 109-16.
Kristensen, E., Jensen, M. H., and Andersen, T K. (1985). The impact of polychaete
(Nereis Virens Sars) burrows on nitrification and nitrate reduction in estuarine
sediments. J. Exp. Mar. Bioi. Ecol., 85, 75-91.
Kruger, F. (1971). Bau und leben des wattwurms Arenicola marina. Helgolander Wiss.
Meeres-unters,22, 149-200.
MacFarlane, G. T, and Herbert, R. A. (1984a). Dissimilatory nitrate reduction and
nitrification in estuarine sediments. J. Gen. Microbiol., 130, 2301-8.
MacFarlane, G. T., and Herbert, R. A. (1984b). Effect of oxygen tension, salinity,
temperature and organic matter concentration on the growth and nitrifying activity of
an estuarine strain of Nitrosomonas. FEMS Microbiol. Leu., 23, 107-11.
McCarthy, J. J., Taylor, W. R., and Taft, J. L. (1977). Nitrogenous nutrition of the
plankton in the Chesapeake Bay. 1. Nutrient availability and phytoplankton preferences. Limnol. Oceanogr., 22, 996-10 11.
McCarthy, J. J., Kaplan, W. A., and Nevins, J. L. (1984). Chesapeake Bay nutrient and
plankton dynamics. 2. Sources and sinks of nitrite. Limnol. Oceanogr., 29, 84-98.
Meier-Reil, L. A. (1983). Benthic response to sedimentation events during autumn to
spring at a shallow water station in the Western Kiel Bight. Mar. BioI., 77, 247-56.
Nedwell, D. B., Hall, S. E., Anderson, A., Hagstrom, A. F., and Linstrom, E. B. (1983).
Seasonal changes in the distribution and exchange of inorganic nitrogen between
sediment and water in the Northern Baltic (Gulf of Bothnia). Est. Coast. Shelf Sci., 17,
169-79.
Nishio, T, Koike, I., and Hattori, A.(1982).Denitrification, nitrate reduction, and oxygen
consumption in coastal and estuarine sediments. Appl. Environ. Microbiol., 43, 648-53.
Nishio, T, Koike, I., and Hattori, A.(1983).Estimates of denitrification and nitrification in
coastal and estuarine sediments. Appl. Environ. Microbiol., 45, 444-50.
Nixon, S. W. (1981).Remineralization and nutrient cycling in coastal marine ecosystems.
In: Nielson, B. J., and Cronin, L. E. (eds),Estuaries and Nutrients, s. 111-138. Humana,
Press, Clipton, N1.
Nixon, S. W., and Pilson, M. E. Q. (1983). Nitrogen in estuarine and coastal marine
ecosystems. In: Carpenter, E.1., and Capone, D. G. (eds), Nitrogen in the Marine
Environment, pp. 565-648. Academic Press, New York.
Nixon, S. W., Oviatt, C A., and Hale, S. S. (1976).Nitrogen regeneration and metabolism
of coastal marine bottom communities. In: Anderson, J., and MacFeydan, A. (eds),
The Role of Terrestrial and Aquatic Organisms in Decomposition Processes,
pp. 269-83. Blackwell, Oxford.
Olson, R. J. (1981). Differentiel photoinhibition of marine nitrifying bacteria: a possible
mechanism for the formation of the primary nitrite maximum. J. Mar. Res., 39, 227-38.
Owens, N. (1986). Estuarine nitrification: a naturally occurring fluidized bed reaction?
Estuar. Coast. Shelf Sci., 22, 31-44.
Painter, H. A. (1970). A review of the literature on inorganic nitrogen metabolism in
microorganisms. Water Res., 4, 393-450.
248
Nitrogen Cycling in Coastal Marine Environments
Patrick, W. H., and Reddy, K. R. (1976).Nitrification-denitrification reactions in flooded
soils and water bottoms: dependence on oxygen supply and ammonium diffusion. J.
Environ. Qual., 5, 469-72.
Powlson, D. S., and Jenkinson, D. S. (1971). Inhibition of nitrification in soil by carbon
disulfide from rubber bungs. Soil. Bioi. Biochem., 3, 267-9.
Pearl, H. (1984). Alteration of microbial metabolic activities in association with detritus.
Bul/. Mar. Sci., 35, 398-408.
Prakasam, T. B. S., and Loehr, R. C. (1972).Microbial nitrification and denitrification in
concentrated wastes. Water Res., 6, 859.
Rasmussen, M. B., Henriksen, K., and Jensen, A. (1983). Possible causes of temporal
fluctuations in primary production ofthe micro-phytobenthos in the Danish Wadden. Sea.
Mar. Bioi., 73, 109-14.
Reise, K. (1983).Biotic enrichment of intertidal sediments by experimental aggregates of
the deposit-feeding bivalve. Macoma balthica. Mar. Ecol. Progr. Ser., 12, 229-36.
Revsbech, N. P., ~rensen, J., Blackburn, T. H., and Lomholt, J. P. (1980a).Distribution of
oxygen in marine sediments measured with microelectrodes. Limnol. Oceanogr., 25,
403-11.
Revsbech, N. P., J~rgensen, B. B.,and Blackburn, T. H. (1980b).Oxygen in the sea bottom
measured with a microelectrode. Science, 207, 1355-6.
Revsbech, N. P.,J~rgensen, B. B., and Brix, O. (1981).Primary production of microalgae
in sediments measured by oxygen microprofile, H 14C03 fixation, and oxygen exchange
methods. Limnol. Oceanogr., 26, 717-30.
Robertson, L. A., and Kuenen, 1. G. (1984).Aerobic denitrification: a controversy revived.
Arch. Microbio/., 139, 351-4.
Ryther, J. H., and Dunstan, W. M. (1971). Nitrogen, phosphorus, and eutrophication in
the coastal marine environment. Science, 171, 1008-13.
Sand-Jensen, K., Prahl, c., and Stokholm, H. (1982). Oxygen release from roots of
submerged aquatic macrophytes. Oikos, 38, 349-54.
Sayama, M. (1983).Relationship between burrowing activity of the polychaetous annelid,
N eanthesjaponica (Izuka) and nitrification-denitrification processes in the sediments. J.
Exp. Mar. Ecol., 72, 233-41.
Schell, D. M. (1978). Chemical and isotopic methods in nitrification studies, In:
Schlessinger, D. (ed.),Nitrification and Reduction of Nitrogen Oxides, pp. 242-95. Am.
Soc. Microbiol. Publ., Washington, DC.
Schmidt, E. L. (1974). Quantitative autoecological study of microorganisms in soil by
immuno-fluorescence. Soil Sci., 118, 141-9.
Schmidt, E. L. (1978). Nitrifying organisms and their methodology. In: Schlessinger, D.
(ed.), Nitrification and Reduction of Nitrogen Oxides, pp. 288-91. Am. Soc. Microbiol.,
Washington, DC.
Seitzinger, S. P., and Nixon, S. W. (1985). Eutrophication and the rate of denitrification
and N20 production in coastal marrine sediments. Limnol. Oceanogr., 30, 1332-39.
Seitzinger, S. P., Pilson, M. E., and Nixon, S. W. (1983). N20 production in near-shore
marine sediments. Science, 222, 1244-6.
Seitzinger, S. P., Nixon, S. W., and Pilson, M. E. Q. (1984). Denitrification and nitrous
oxide production in a coastal marine ecosystem. Limno/. Oceanogr., 29, 73-83.
Smith, C. 1.,Delaune, R. D., and Patrick, W. H. (1985).Fate of riverine nitrate entering an
estuary. 1. Denitrification and nitrogen burial. Estuaries, 8, 15-21.
Sma, R. F., and Baggaley, A. (1975).Kinetic responses of perturbed marine nitrification
systems. J. Water Pol/ut. Control Fed., 47, 472-86.
Strickland, J. D. H., and Parsons, T. R. (1972).A Practical Handbook of Seawater Analysis.
Bull. Fish. Res. Bd. Can. 67, 2nd edn.
Nitrification in Estuarine and Coastal Marine Sediments
249
Suess, E. (1980). Productivity, sedimentation rate and sedimentary organic matter in the
oceans. 6. Vertical carbon flux. Nature, 288, 260-3.
Sugahara, I., Sugiyama, M., and Kawai, A. (1974). Distribution and activity of nitrogen
cycle bacteria in water-sediment systems with different concentrations of oxygen. In:
Colwell, R., and Morita, R. Y. (eds), Effect of the Ocean Environment on Microbial
Activities, pp. 327-40. University Park Press, Baltimore, Maryland.
Suzuki, I., Dular, V., and Juok, S. C. (1974). Ammonia or ammonium as substrate for
oxidation by Nitrosomonas europaea cells and extracts. J. Bacteriol., 1230, 556-558.
Sprensen,1. Jprgensen, B. B.,and Revsbech, N. P. (1979).A comparison of oxygen, nitrate
and sulfate respiration in coastal marine sediments. Microb. Ecol., 5, 105-15.
Tailing, J. F. (1976).The depletion of carbon dioxide from lake water by phytoplankton. J.
Ecol., 64, 79-121.
Vanderborght, J. P., and Billen, G. (1975).Vertical distribution of nitrate concentration in
interstitial water of marine sediments with nitrification and denitrification. Limnol.
Oceanogr., 20, 953-61.
Vanderborght, J. P., Wollast, R., and Billen, G. (1977). Kinetic models of diagenesis in
disturbed sediments. 2. Nitrogen diagenesis. Limnol. Oceanogr., 22, 794-803.
Vargues, H., and Brisou, J. (1963). Researches on nitrifying bacteria in ocean depths on
the coast of Algeria. In: Oppenheimer, C. H., Symposium on Marine Microbiology,
pp. 415-26. C. C. Thomas, Springfield, Illinois.
Vincent, W. F., and Downes, M. T. (1981).Nitrate accumulation in aerobic hypolimnia:
relative importance of benthic and planktonic nitrifiers in an oligotrohpic lake. Appl.
Environ. Microbial., 42,565-73.
Ward, B. B., and Perry, M. J. (1980).Immunofluorescent assay for the marine ammoniumoxidizing bacterium Nitrosococcus oceanus. Appl. Environ. Microbial., 39, 913-18.
Ward, B. B., Olson, R.1., and Perry, M. J. (1982). Microbial nitrification rates in the
primary nitrite maximum off Southern California. Deep-Sea Res., 29, 247-55.
Watson, S. W. and Waterbury, J. B. (1971).Characteristics oftwo marine nitrite oxidizing
bacteria, Nitrospina gracilisnovogen. sp. and Nitrococcus mobilisnovogen. novosp. Arch.
Microbial., 77, 203-30.
Webb, K. L., and Wiebe, W. J. (1975).Nitrification on a coral reef. Can. J. Microbiol., 21,
1427-31.
Williams, P. J. Le B., and Watson, S. C. (1968). Autotrophy in Nitrosocystis oceanus. J.
Bacterial., 96, 1640-8.
Wium-Andersen, S., and Andersen, J. M. (1972).The influence of vegetation on the redox
profile of the sediment of Grane Langsoe, a Danish Lobelia lake. Limnol. Oceanogr.,17,
948-52.
Yoshida, Y. (1967). Studies of the marine nitrifying bacteria, with special reference to
characteristics and nitrite formation of marine nitrite formers. Bull. Misaki Kenkyu
Hokoku Maizuru, 11, 2-58.