Journal of General Microbiology (198 I), 123, 297-3 12. Printed in Great Britain
297
Differences in Microbial Decomposition Processes in Profundal and
Littoral Lake Sediments, with Particular Reference to the Nitrogen Cycle
B y J . G W Y N F R Y N J O N E S * A N D B E R N A R D M. S I M O N
The Freshwater Biological Association, Windermere Laboratory, The Ferry House,
Ambleside, Cumbria LA22 OLP
(Received 11 July 1980)
~~
An investigation of sediments from the littoral (shallow water) and profundal (deep water)
zones of Blelham Tarn, a shallow eutrophic lake, showed marked differences in the microbial
decomposition processes. These differences were due largely to differences in the degree of
oxygenation, supply of electron acceptors, and mean summer temperature at the two sites.
The changes in the hypolimnion (the deep water zone formed on thermal stratification, which
may be treated essentially as a closed system) could be used to calculate profundal rates of
aerobic respiration, NO, and SO:- reduction, and methanogenesis, relative to the
accumulation of CO,. Laboratory measurements demonstrated that N H t accumulation,
SO:- reduction and methanogenesis were more intense in the profundal than in the littoral
zone. Anaerobic processes that occurred in the littoral sediments did so at greater depths than
in the profundal sediments. The release of CH, and N, bubbles also provided estimates of the
importance of these processes at the two sites. At both sites aerobic respiration was the most
important component (about 50 %) of carbon mineralization; SO:- reduction was the least
important, accounting for only a small percentage of carbon turnover. Pathways of NO;
reduction and methanogenesis accounted for approximately equal proportions (varying
between 15 and 25%) of the carbon mineralized. When the results were adjusted to account
for the relative areas of the profundal and littoral zones, the former was the more important
site of methanogenesis and SOP reduction, whereas aerobic respiration and NO, reduction
were greater in the littoral zone. The major end-product of NO; reduction was NH,+ in the
profundal and N, in the littoral zone. The higher and continued levels of nitrification, which
recycled the NHt in the littoral sediments, were thought to contribute to this.
INTRODUCTION
Decomposition processes in lakes have been studied largely by comparison of
mineralization with net primary production. Sediment respiration has often been taken as a
measure of decomposition (Hargrave, 1969; Wetzel et al., 1972; Jones, 1976) and has been
shown to be related to lake trophic status (Ohle, 1956; Edmondson, 1966) and sediment
particle size (Hargrave, 1972; Jones, 1980). A few studies have attempted to determine the
role of processes other than aerobic respiration (nitrification, oxidation of sulphur and iron)
in oxygen consumption. Burns & Ross (19 72) attributed 12% of the oxygen loss in Lake Erie
to inorganic processes, whereas Hall et al. (1978) calculated that nitrification alone accounted
for 25% of the oxygen consumption in Grasmere, English Lake District. It is clear that
although oxygen consumption may overestimate aerobic respiration, measurement of this
single process of mineralization will lead to a serious underestimate of carbon turnover.
Recent studies (Rudd & Hamilton, 1978; Barber & Ensign, 1979; Robertson, 1979; Fallon et
al., 1980) have demonstrated the importance of methanogenesis, but very few investigations
have attempted to determine the relative contributions of the various anaerobic decom0022-l287/8l/0000-9430 $02.00
01981 SGM
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J . G . J O N E S A N D B. M . S I M O N
position processes in carbon mineralization in freshwater systems. This may reflect a
recognition that the interactions of anaerobic bacteria in aquatic sediments are extremely
complex (Abram & Nedwell, 1978a, b; Cappenberg & Jongenjan, 1978; Winfrey & Zeikus,
1977). In a preliminary analysis of CO, accumulation in the profundal zone of Blelham Tarn,
Jones & Simon (1980) concluded that aerobic respiration accounted for about 42%,
denitrifkation for 17%, SO$- reduction for 2 % and methanogenesis was equivalent to 25 %.
In contrast, detailed studies of sublittoral marine and estuarine sediments (Fenchel &
Jorgensen, 1977; Fenchel, 1978) have shown SO:- reduction to be the most important
anaerobic process (equivalent, in carbon terms, to 60 % of aerobic respiration) whereas
denitrification accounted for only a small percentage and methanogenesis was considered to
be negligible.
Almost all freshwater sediment microbiology has been confined to studies of the profundal
(deep water) sediments, but recently (Jones, 1980) distinct differences have been observed
between these and littoral (shallow water) sediments. This paper examines how these
differences are reflected in the relative importance of the various anaerobic decomposition
processes, particularly those associated with the nitrogen cycle.
METHODS
Sampling. The methods and sites used for sampling the sediment in Blelham Tarn (English Lake District: 54'
24' N, 2O 59' W) were those described by Jones (1980). The profundal site was in the centre of the northern basin
of the tarn (water depth 13.5 m) and littoral samples were taken about 10 m offshore beyond a bed of Phragmites
and at a depth of about 1.5 m. Water samples were taken at a series of fixed depths (up to 2 m) above the
sediment-water interface with a Friedinger water bottle. Sub-samples for chemical analyses were taken
immediately into acid-washed polythene bottles. Glass bottles were used to contain samples for dissolved gas and
S2- determinations and these were flushed with at least twice their own volume of water before they were
stoppered. The sediment traps and gas traps were those described by Jones & Simon (1980).
Sample preparation. Sediment was either removed from the cores by extrusion using the device described by
Jones (1976), or sub-samples were taken with syringes through holes drilled at 0.5 cm intervals down the tube and
sealed with polythene adhesive tape. For most analyses of interstitial water, sediment samples were centrifuged at
5500 g for 45 min at 2 "C, and the supernatant was then removed for analysis. In all cases internal standards were
added to the samples before treatment, to determine the efficiency of the analytical procedure. Preliminary trials
with the colorimetric procedure for S2- showed that significant quantities of H,S were lost during the handling
process. To overcome this, samples of whole sediment were removed with a syringe and injected immediately into
excess reagent. Thus the analysis was one for labile S2- rather than specifically for HIS.
Physical and chemical analyses. Dissolved 0 , and temperature were measured in the field with a combination
oxygen electrode and thermistor (Mackereth, 1964). Dissolved CO, and CH,, the contents of the gas traps, and
particulate material in the sediment traps were analysed by the methods described by Jones & Simon (1980).
NO,-N was determined as NO, (Benschneider & Robinson, 1952), after reduction by the method of Elliott &
Porter (197 1). NH,-N was determined by the method of Chaney & Marbach (1962), and S2- by that of Rees et al.
(197 1). Full details of these methods and their performance characteristics when applied to freshwater samples are
provided by Mackereth et al. (1978). The method of Tabatabai (1974) was used for the determination of Sotconcentration in interstitial waters. E , profiles of sediment cores were obtained with a bright platinum electrode as
described by Jones (19794.
Dissolved CH,, CO, and H,S in acidified sediment samples were also analysed by a head-space, gas
chrqmatographic technique (Jones, 1979b). The carbon and nitrogen content of sediments dried to constant
weight at 80 OC was determined in a Hewlett-Packard F & M 185 CHN analyser.
Cellulolytic activity. The activity of cellulose decomposers in the sediment was determined as loss of colour from
strips of cellophane dyed with Remazol Brilliant Blue R according to Moore et al. (1979). The strips of cellophane
(2 x 25 cm), attached to glass rods with elastic bands, were inserted into sediment cores and incubated at 15 OC
for 4 d. The strips were then removed and cut into 1 cm segments; the remaining dye was extracted and its
absorbance was measured at 595 nm.
Enumeration of denitrifying bacteria. The most probable number (MPN) of denitrifiers and NOT-reducing
bacteria was estimated by preparing a tenfold dilution series of the samples for six dilutions, with eight replicates at
was used for all samples. The medium of Stanier et al. (1966) was
each dilution. A preliminary dilution of
used and 10 ml samples were incubated at 15 OC for 14 d in test tubes, each containing an inverted Durham tube,
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Decomposition processes in lake sediments
299
under sterile liquid paraffin. Two batches of media were prepared with and without (NH4),S04. The final KNO,
concentration was 2 mM. At the end of the incubation period the gas in the Durham tubes was analysed for
nitrogen as described above. The ability of the organisms to reduce NO; to NO, and beyond was tested according
to Jones (1979 b).
Enumeration of SO:--reducing bacteria. The MPN was obtained using the same dilution series as that for
denitrifiers except that the initial dilution was lo-'. The medium used was that of Postgate (1963) with thioglycollic
acid adjusted to pH 7.6 and sterilized separately by membrane filtration. Sediment samples were taken from the
core with a syringe and injected directly into the above medium made up without agar. The sample was
homogenized and serial dilutions were made in the liquid medium. Portions were then transferred to replicate tubes
of the agar-containing medium held at 47 "C. The tubes were plunged immediately in an ice bath and topped up
with a plug, at least 3 cm deep, of molten medium. The tubes were incubated at 30 "C for 14 d.
Measurement of coenzyme F 4 2 W An attempt was made to determine the concentration of this electron carrier in
the sediment and to use this as a relative estimate of the numbers and distribution of methanogenic bacteria. The
method was based on that of Delafontaine et al. (1979). Sediment samples were mixed with an equal weight of
distilled water and placed in a boiling water bath for 15 min. Three volumes of 2-propanol was added to the cooled
sample which was then mixed and centrifuged at 4500 g for 45 min at 4 "C. The fluorescence spectrum of the
supernatant was read against a distilled water/2-propanol blank in a Perkin Elmer 204 fluorescence
spectrophotometer. The excitation wavelength was 420 nm and the emission spectrum was scanned from 450 to
500 nm. The maximum emission of F42, occurred at 470 nm as a shoulder peak, and the background fluorescence
was subtracted to provide relative estimates of the coenzyme.
Measurement of inorganic nitrogen transformations. Rates of NO; removal and NH: accumulation in
unstoppered sediment cores with 1 litre of overlying water were measured over 5 d. The cores were incubated at
15 "C with gentle stirring and samples for analysis were removed at daily intervals. Apparent rate constants for the
two processes were calculated.
Potential rates of denitrification were measured in vials. Sediment samples were removed from the zone of
maximum NOT-reducing activity (Jones, 1979b) and diluted with an equal volume of membrane-filtered core
water. The diluted samples were dispensed into vials and flushed thoroughly with purified helium before the vials
were sealed. To some vials 0.2 ml of N,O was injected into the head space, equivalent to a final concentration of
0.17 mM in the aqueous phase. The vials were incubated at 15 " C for 24 h, after which the products in the heak
space were analysed by gas chromatography. Autoclaved controls were also incubated. Denitrification was
measured as the appearance of N, gas in the presence and absence of N,O, and also as the rate of uptake of N 2 0 .
Several attempts were made to use C,H, to block denitrification (Balderstone et al., 1976; Sorensen, 1978a) but
inhibition was incomplete, varying between 7 % and 54 %.
Nitrification was measured both in intact cores and in diluted sediment samples. In both cases N-serve
[2-chloro-6-(trichloromethyl)pyridine~,
stock solutions of which were made up in acetone, was used as an inhibitor
(Goring, 1962; Billen, 1976). Intact cores were treated overnight with N-serve to a final concentration of 10 mg 1-'
in the overlying water, The following morning a second portion of N-serve was added (it is rapidly adsorbed and
inactivated in the presence of particulate organic matter) and the cores were sealed and incubated for 24 h at
15 "C. The change in 0, concentration with time was measured with a Yellow Springs Instruments oxygen
electrode (model 57). Nitrification in diluted sediment samples was measured as 0, uptake, NH: uptake and NOT
accumulation in the presence and absence of 5 mg N-serve 1-'. The surface sediment samples were diluted low2in
.
membrane-filtered core water to which (NH,),SO, had been added to a final concentration of 10 y ~ For
respirometric determinations the samples were aerated and then 0, uptake was measured kinetically in 100 ml
Pyrex bottles fitted with an EIL dissolved oxygen probe with stirrer (model 80 12-1). Rates of change in NH: and
NO; concentrations were measured over a 5 d period in samples incubated in conical flasks in an orbital
incubator. Samples for chemical analyses were removed at daily intervals and a second portion of N-serve was
added after 3 d
Measurement of SO:- reduction. The method used was essentially that of Jmgensen (1978). Sediment cores
were taken in the usual manner and extruded until the sediment-water interface was within 1 cm of the top of the
core tube. Smaller Perspex cores (20 cm long, 2.4 cm internal diam.) were inserted into the sediment and then
removed. These cores were stoppered and carrier-free 35SO:- (The Radiochemical Centre, Amersham) was
injected via holes drilled at 1 cm depth intervals and sealed with silicone rubber. The cores were incubated for 4 h
and then treated as described by Jsrgensen (1978) except that the 35S2-formed was trapped in 0.1 M-NaOH rather
than zinc acetate. We ran extensive trials on the efficieiicy of traps for S2- using both non-radioactive S2- (with
colorimetric determinations) and 35S2-.The most efficient trap was NaOH, but even so recoveries of the S2- were,
on average, only 80%, and all SO:- reduction estimates were corrected for this.
Analysis of data. Where possible 95 % confidence limits of variables have been calculated, but when there were
insufficient data to provide information on the parerit distribution of any given variable, the results are presented as
means with standard deviations. The procedures for calculation of exchange between sediment and the water
column on an area basis and the bathymetric data were those used by Jones (1976).
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Fig. 1. Sediment cores from the profundal ( a ) and littoral (b) zones of Blelham Tarn at the onset of
summer stratification. The depth of the oxidized zone (about 1 cm) is seen clearly in the profundal
sample.
Table 1. Chemical and physical features of the sediment and the overlying water at the
littoral and profundal sites in Blelham Tarn during summer strat8cation
The values in parentheses are the seasonal standard deviations, which were based on 8 values for
sediment samples and 24 values for water samples.
Sediment characteristics
Carbon content (mg g-')
Nitrogen content (mg g-l)
Depth of E , = +250 mV (mm)
Overlying water
Temperature ("C)
Dissolved 0, ( p ~ ~ o l l - ~ )
NO5 (pmoll-l)
NHZ (kmol 1-l)
S2-(pmol l-l)
SO:- (pmoll-l)
ND,
Littor a1
Profundal
135 (16)
1 3 - 2 (1.0)
15-3 8
181 (15)
21.8 (3.8)
0-12
14.5
256
48.9
4.1
(3.1)
(37.5)
(33.4)
(4.8)
ND
144.8 (15.6)
6.8 (0.6)
2 1 - 9 (31.3)
1.5 (1.2)
105.5 (31.9)
10.8 (6.3)
131.3 (13.5)
Not detectable.
RESULTS
During the period of summer stratification the conditions surrounding the littoral and
profundal sediments (Fig. 1) of Blelham Tarn differed quite markedly, and it was the purpose
of this paper to examine how such differences were reflected in decomposition processes at the
two sites. The means of the major variables (Table 1) during the summer period provide a
useful summary of prevailing conditions. The profundal sediments were significantly richer in
carbon and nitrogen (P = 0*05),more reducing, and were in contact with water which was
colder, 0, depleted and had higher concentrations of NH; and S2-. The changes in
concentration of these inorganic nutrients which occurred in the hypolimnetic water during
summer stratification were due largely to the activity of micro-organisms in the sediment. The
hypolimnion could be considered, to a large degree, as a closed system where turbulence is
relatively low, and therefore rates of change in nutrient concentration in the water column in
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301
Table 2. (a)Field measurements of sediment metabolic activity in the littoral and profundal
zones of Blelham Tarn during summer stratijkation
Activity* (pmol m-' d-')
Liquid phase
NO? consumption
NH: accumulation
S2- accumulation
CO, accumulation
CH, accumulation
0, consumption
Littoral
Profundal
479-1 129
ND
ND
779-2600
571-1 179
409-553
5950-10900
235-1244
6700-9400
503
248
86-239
296- 156 5
ND
ND
ND
Gaseous phase
N, gas released
CH, gas released
(b)Particulate nutrient input to the profundal sediments
Nutrient inputt (pmol m-* d-l)
Year
I
C
N
>
Reference
1971 36000
ND
Pennington (19 74)
1972 16800
1860
Jones (1976)
1977 37 200 (18 100) 4360 (2500) Jones & Simon (1980)
ND,Not determined.
* The values are ranges obtained over the three years 1978 to 1980.
t The values in parentheses are the seasonal standard deviations, based on 24 values.
the profundal zone could be integrated and expressed per unit area of sediment. The littoral
sediments, by contrast, were in open, turbulent water constantly replenished by inflowing
rivers and wind activity. Consequently only very approximate estimates of nutrient exchange
rates could be made and these only on calm days. The values shown in Table 2 ( a ) represent
ranges obtained over three years. Changes in concentrations of dissolved gases and NH; and
S2- could not be detected in the water in the littoral zone because of exchanges with the
atmosphere and oxidation in the water column. Clearly, rates varied considerably from year
to year, and in the one instance where comparable data were obtained (for NO, uptake), the
ranges for the two sites overlapped, A better comparison of methanogenesis and denitrification
was obtained from the release of gas bubbles from the sediment. (The N, values had to be
corrected for gas stripping by CH,; see Discussion.) Rates of methanogenesis were
consistently higher in the profundal zone but the reverse was true of N, release. Once again,
considerable variability in the rates was observed from year to year; this may have reflected
variation in the annual input of carbon and nitrogen to the sediment (Table 2 b).
Potential rates of microbial decomposition were consistently higher in the profundal than in
the littoral sediments, and the degradation of cellulose, a major carbon polymer generated by
primary producers in the lake ecosystem, was chosen to illustrate this (Fig. 2). These rates of
decomposition, in turn, controlled the extent to which end-products of anaerobic metabolism
accumulated in the sediment interstitial water. The CH, concentrations at the end of summer
were at least an order of magnitude higher in the profundal than in the littoral zone (Fig. 3)
and this difference was still noticeable in early spring before the onset of stratification (Fig. 4).
CH, concentrations also correlated reasonably well with levels of coenzyme F,,, and were
probably a good indication of the distribution of methanogens in the sediment (Fig. 4). Rates
of methanogenesis in samples incubated in vials, under conditions similar to those used for
denitrification studies, were equivalent to 2600 (s.D. = 1190) pmol CH, m-' d-' for
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I
I
10‘
I
Dissolved CH, concn (pmol 1-I)
lo2
103
I
I
n
8
W
5 10
2c
-
e’
10
Fig. 2
’
Fig. 7
Fig. 2. Cellulose decomposition in profundal (e)and littoral (0)sediments of Blelham Tarn before the
onset of summer stratification in 1980. Cellophane strips stained with Remazol Brilliant Blue R were
incubated in sediment cores for 4 d at 15 OC and the loss of dye (AASg5)was measured.
Fig. 3. Dissolved CH, concentrations in profundal (e)and littoral ( 0 )sediments of Blelham Tarn
sampled shortly before the autumn overturn in 1979. Sub-samples were taken with syringes and the
CH, concentration was determined by gas chromatographic analysis of the head space.
Coenzyme F,,,
Dissolved CH, concn
fluorescence
(emission intensity units)
(pmol 1-l)
0
100 200 300 0
5
10
15
20
I
Fig. 4. Dissolved CH, concentrations (a) and relative fluorescence intensity of coenzyme F,,, (b) in
profundal ( 0 )and littoral ( 0 )sediments of Blelham Tarn before the onset of summer stratification in
1980.
profundal and 790 (s.D. = 410) pmol CH, m-2 d-I for littoral sediments. The differences in
S2- concentration between the two sites (Fig. 5 a) were of the same order as those found for
CH,, but whereas S2- concentrations deeper in the sediments decreased after the overturn in
the littoral zone, those in the profundal sediments increased two- to threefold. SO:concentrations decreased rapidly with depth below the sediment surface, but surprisingly this
was more marked at the littoral site (Fig. 5b). The numbers of SOf--reducing bacteria (Fig.
6) and rates of S0,2-,reduction (Fig. 7) were consistently higher in the profundal zone. In the
profundal zone the highest numbers and activity were found at or near the sediment surface.
whereas in the littoral sediment population densities (and, to a much lesser extent, activity)
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Decomposition processes in lake sediments
S2- concn (pmol g-')
10'
lo2
100
-
3
5
.n
10
0
303
SO:- concn (pmol g-I)
5
10
U
5
3
15
Fig. 5. Concentrations of labile S2- (a) and SO:- (b) in profundal ( 0 )and littoral ( 0 )sediments of
Blelham Tarn before (symbols connected) and after (symbols not connected) the autumn overturn in
1979. Determinations of S2- were performed on whole sediment, and those of SO:- on interstitial water
separated by centrifugation.
No. of SO:- reducers g-'
103
104
105
n
E
5
W
5a
6 10
15
Fig. 6
Fig. 7
Fig. 6. Depth distribution of the MPN of SOt--reducing bacteria in profundal (,*) and littoral ( 0 )
sediments of Blelham Tarn in August 1979. The 95 % confidence limits on log,, MPN = k0.52.
Fig. 7. Rates of SO:- reduction in profundal ( 0 )and littoral (0)
sediment cores during summer
stratification (a) and after the autumn overturn (b) in 1979. The horizontal bars in (a) indicate 95%
confidence limits; those for the littoral samples fell within the size of the symboi.
were greater at a depth of 4 to 5 cm. The greater intensity of decomposition in the profundal
zone that was observed in Blelham Tarn could also be seen in other lakes of a similar degree
of enrichment. The amounts of CO,, CH, and H,S, major end-products of decomposition,
were all much higher in the profundal than in the littoral sediments of nearby Esthwaite Water
(Fig. 8), a slightly larger water body, the hypolimnion of which also became anoxic in the
summer.
Although concentration gradients of metabolic end-products provided relative information,
other measurements of microbial activity in the sediments were available. The release of gases
from the sediments provided estimates of methanogenesis and denitrification. Methanogenesis
started earlier in the summer in the littoral zone (Fig. 9) but by August the rate of release of
CH, was consistently higher from the profundal sediments and this persisted well into the
winter. A similar seasonal pattern was observed for N, release (Fig. 10) except that much
greater quantities were evolved from the littoral than from the profundal zone (note the
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304
lop3x Peak integrals
CH4
0
100
200
0
co2
0
200
100
H2S
100
200
2
5
h
W
s 4
8
CI
6
Fig. 8. Gas chromatographic analysis of CO,, CH, and H,S in the profundal (0)and littoral (0)
sediments of Esthwaite Water during summer stratification in 1979. Sub-samples were taken with
syringes and acidified, and the head space was then analysed.
L W
J
J
A
S
O
Month
Fig. 9
N
D
J
J
J
A
S O
Month
N
D
Fig. 10
Fig. 9. CH, release from profundal (0)and littoral (0)sediments in Blelham Tarn during 1979.
Bubbles were trapped in inverted polythene funnels (0.14m2 area) and analysed by gas
chromatography.
Fig. 10. N, release from profundal (0)and littoral (0)sediments in Blelham Tarn during 1979.
Bubbles were trapped in inverted polythene funnels (0.14 m2 area) and analysed by gas
chromatography.
logarithmic scale on the ordinate) and that this persisted until mid-September. The average
contents of the gas traps were 5 1 % (vlv) N, and 46 % CH, in the profundal and 76 % N, and
15% CH, in the littoral zone. Although there was consistency in these trends, that more N,
was released from the littoral and more CH, from the profundal zone, the actual volumes at
each site varied considerably from year to year. For example, much more gas was released
between July and January in 1978 than in 1979: the values for the profundal zone during
1978 and 1979 were, respectively, 338 and 265 ymol N, m-2 d-’ and 1012 and 536 pmol
CH, m-2 d-l.
Of the differences in microbial activity observed between the littoral and profundal
sediments, one of the more interesting was the coupling of the nitrogen cycle to carbon
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305
10
-
n
I
4
E,
W
g 10
C
0
0"
z
10'
lo2
./.'
z
2
n
I
d
10'
23
+*
X
100
M
J
J A
Month
S
z
O
Fig. 11. Seasonal changes during 1977 in the concentrations of NO; (a) and N H t (b) in water overlying profundal).( and littoral (0)
sediments in Blelham Tarn.
n
n
E
NH: concn (pmol I-')
500
0
NO, concn (ymol I-')
2
4
5
v
Fig. 12. Depth distribution of NHt (a) and NO; (b) concentration in the interstitial water of profundal
(0)and littoral (0)
sediments of Blelham Tarn immediately before the onset of summer stratification in
1980.
turnover, raising the question why more N, should be released at the littoral site, where all
other decomposition processes were slower. The seasonal changes in NO, and N H t in the
water overlying the sites are illustrated in Fig. 11. The rate of NO; removal was more rapid
and extensive in the profundal zone and corresponded almost exactly, in time, with the
increase in N H t concentration. NO, removal and N H t accumulation in the littoral zone
could not be attributed entirely to sediment activity and probably also reflected assimilation
by the plankton and reduced input from inflowing streams in the drier summer months. There
was little doubt, however, that more N H t was generated in the profundal than in the littoral
sediments (Fig. 12a). The NO, analyses, on the other hand, were extremely variable (Fig.
12b) and were not easily explained. In the absence of satisfactory field data (except for the
gas trap results) the nitrogen transformations were examined under experimental conditions.
NO; removal and N H t accumulation were measured in the water overlying sediment cores
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J . G. J O N E S A N D B . M. S I M O N
Table 3. Experimental determination of NO, reduction in littoral and profundal sediments
from Blelham Tarn at the onset of summer stratiJication in 1980
(a)NO5 uptake and N H t accumulation in undisturbed sediment cores
Lit tor a1
NO;
Rate constants
Zero order (pmol l-' d-l)
First order (d-l)
Rates (pmol mL2d-l)
Zero order
First order
NHI
Profundal
,
NO;
NW
1.9
0.046
2.1
0.106
3.2
0.109
7.9
0.152
493
593
543
279
821
1280
2030
964
(b)N,O uptake and N , evolution in diluted sediment samples*
Littoral
Profundal
N,O uptake (pmol m-', d-l)
4650 (1800)
4630 (2600)
N, accumulation (pmol m-' d-l)
Unamended samples
Samples + N,O (0.17 mM)
3650 (1880)
5500 (1040)
3000 (23 10)
4700 (850)
(c) Numbers of denitrifying and NOT-reducing bacteria in the
sediments?
No. producing N, (g-l)
Medium with NH:
Medium without NH;
No. reducing NO; to NO; or beyond (g-')
Medium with NH:
Medium without NH:
Littoral
Profundal
1-11 x lo5
23-250 x lo5
21-230 x lo5
6.6-73 x lo5
4-44 x 109 1.2-13.2 x 109
0-6-6.9 x lo9 24-23.2 x lo9
*The values in parentheses are standard deviations based on 8 determinations for N,O uptake and 10
determinations for N, accumulation.
The ranges represent 95 % confidence limits for 5 samples.
from the profundal and littoral sites (Table 3 a). We chose to measure exchange between the
sediment and the overlying water for two reasons. Firstly, this was more relevant to changes
in the lake as a whole, and therefore more directly comparable with field observations.
Secondly, rates of change within the sediment were so rapid, and the analytical methods,
particularly for NO, in the interstitial water, too inaccurate to obtain a satisfactory rate on an
area basis. Cores were taken during the first half of the summer before NO, concentrations in
the water became too low. Rate constants were calculated for the kinetics, which were
generally zero order in early spring, and first order later in the summer. Flux rates were
calculated as a product of the constant and the concentration of nutrient in the overlying
water at the time of the experiment. These experimental rates were in good agreement with the
results obtained from the analysis of field data (Table 2 a).
There was little difference in the capacity of the two sediments to denitrify, whether this
was measured by the uptake of N,O or the accumulation of N, gas (Table 3 b). The MPN of
denitrifying bacteria depended to a considerable extent on the inorganic nitrogen sources in
the medium (Table 3c). In the presence of N H t larger numbers of denitrifiers were found in
the profundal than in the littoral samples whereas the reverse was true in the absence of NHt.
The appearance of gas in the MPN tubes could not be taken as a sign of denitrification since
its N, content could be extremely variable. The percentage of N, in the gas produced in the
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Decomposition processes in lake sediments
307
Table 4. Measurement of nitrBcation in littoral and profundal sediments from Blelham
Tarn by inhibition with N-serve
-
Samples were taken in May 1980, before deoxygenation of the profundal zone. N-serve [2-chloro-6(trichloromethyl)pyridinel was applied as a solution in acetone to a final concentration of 10 mg 1-'
for the sediment cores and of 5 mg 1-' in the diluted sediment samples. The 0, uptake results have
been converted to the equivalent NH: oxidation value. The values in parentheses are standard
deviations based on 5 determinations.
Activity (pmol m-z d-l)
Littoral
Inhibition of 0, uptake in intact
sediment cores
Kinetic measurements of 0, uptake
with diluted sediments
NH: oxidation by
diluted sediment
NO; accumulation due to
nitrification by diluted sediment
Profundal
2240
5380
3750
3450
3680 (590)
2760 (3 10)
1660 (1 140)
980 (1 180)
tubes scored as positive for denitrification ranged from 8 to 14% in littoral samples and from
4 to 9 % in the profundal samples. Gas chromatographic analysis was essential to confirm
denitrification. The MPN of bacteria capable of NO; reduction, as opposed to
denitrification, was four orders of magnitude higher. The results were not always significantly
different between the sites and again depended on whether or not the medium contained NHt.
Larger numbers of littoral bacteria were capable of NO; reduction in the presence of N H t
than in its absence. We were able to obtain mixed cultures of bacteria which produced N H t
from NO; in liquid media. Technical problems, such as the generation of N H t in anaerobic
jars and illogical distribution of positive scores in a dilution series (which depended on the
carbon source supplied) prevented the calculation of a valid MPN. The few results that were
acceptable suggested that there was no difference in the numbers of bacteria capable of
reducing NO; to N H t in the two zones.
Bearing in mind the differences in E , characteristics and the N, evolution rates at the two
sites, the sediments were examined for sources of NO, other than input from the general
water body. Estimates of nitrification were made in three ways (Table 4) using the
nitrification inhibitor N-serve. 0, uptake in intact cores was inhibited to a greater degree in
the profundal samples, but kinetic measurements of 0, uptake in diluted, well-aerated
sediment samples indicated little difference in the degree of inhibition. Rates of N H t uptake
and NO, accumulation were, however, consistently higher in the littoral samples. Since
N-serve also inhibits CH, oxidizers, and the concentrations of CH, were higher in the
profundal samples, the changes in inorganic nitrogen species were considered to give a more
reliable estimate of nitrification than the 0, uptake measurements. The depth to which
nitrification occurred in the sediments also differed between the two sites. In the experiments
listed in Table 4, nitrification inhibited by N-serve occurred to a depth of 40 mm in the littoral
but only 2 mm in the profundal samples. Bearing in mind the seasonal changes in E , values at
the two sites even while 0, persisted in the hypolimnion, the potential for nitrification on an
area basis was approximately 30 times greater in the littoral zone.
DISCUSSION
To put the results obtained from Blelham Tarn in perspective, the rate of each microbial
decomposition process, expressed on an area basis, was compared with those available in the
literature. The rate of aerobic respiration in Blelham Tarn has already been shown to be
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308
J . G. JONES A N D B. M. SIMON
comparable with the rates in eutrophic lakes in other temperate zones (Jones, 1976). The rates
of methanogenesis, on the other hand, were much lower (sometimes by an order of magnitude)
than those reported for several North American lakes (Hayward, 1968; Howard et al., 1971;
Rudd 8z Hamilton, 1978, 1979; Robertson, 1979; Strayer 8z Tiedje, 1978) and were
equivalent to 10 to 20%of the particulate carbon input. This proportion was also much lower
than the values of 36 to 60%, 5 5 % and 54% reported by Robettson (1979), Rudd 8z
Hamilton (1979) and Fallon et al. (1980), respectively. The rates of methanogenesis ir
Blelham Tarn were closer to the 0.7 mmol m-2 d-l reported by Barber 8z Ensign (1979) for
Lake Wingra, Wisconsin. The inorganic nitrogen transformations were, however, quantitatively more important. NO, reduction and denitrification, in the range 0.8 to 2.6 mmol rn-’
d-l, were comparable with the rates reviewed by Kamp-Nielsen 8z Anderson (1977) for
sediment-water exchange and within the range 1.5 to 6-0 mmol rn-, d-I obtained by Graetz
et al. (1973) for Wisconsin lakes. Similar measurements (Jones et at., 1980) to those described
here showed that in Grasmere, a lake near Blelham Tarn and considered to be less eutrophic,
the total NO, reduction was less (0.7 mmol rn-, d-l) than in Blelham Tarn, but at a basin
receiving an NHr-rich sewage effluent the quantity of N, gas released from the sediment was
greater. The N H t concentration in the profundal sediment interstitial water of Blelham Tarn
was almost identical to that reported for marine fjord sediments (Fenchel 8z Blackburn,
1979), yet rates of denitrification (Sorensen, 1978 a) and nitrification (Fenchel 8z Blackburn,
1979) in marine samples appear to be slightly lower. The major differences between microbial
decomposition processes in freshwater and marine sediment-water systems is the relative role
of SO:- reduction. Carbon mineralization by SO:- reducers in marine sediments has been
reported to be equivalent to 60% of that respired by aerobic bacteria (Fenchel 8z Jsrgensen,
1977) and rates of SO:- reduction in marine sediments (Jsrgensen, 1978) and salt marsh
sediments (Howarth 8z Teal, 1979) were one to three orders of magnitude higher than those
observed in Blelham Tarn. We have not attempted to provide an exhaustive comparison of
decomposition processes and those requiring further information on rates observed in the
natural environment are referred to Fenchel8z Blackburn (1979).
The complexity of interactions between anaerobes in the sedimentary environment has
been adequately demonstrated (Winfrey 8z Zeikus, 1977; Cappenberg 8z Jongenjan, 1978;
Abram 8z Nedwell, 1978a, b) as has the relative effect of various controlling factors on
individual groups, such as SO:--reducing bacteria (Abdollahi 8z Nedwell, 1979; Nedwell 8z
Abram, 1979). These interactions are still imperfectly understood, but in this paper an
attempt has been made to quantify the contribution of each microbial group to carbon
turnover in two distinct zones of a lake.
The data obtained with the gas traps have provided useful information on carbon and
nitrogen losses from sediments. They may not, however, provide accurate estimates of the
volumes of gases released as bubbles. To the best of our knowledge, values published to date
have not been corrected for the following errors: (a) solubility of CH, bubbles on standing;
(b) oxidation of CH, in the traps by methylotrophs; and (c) stripping of N, from solution into
the CH, bubbles as they are formed in the sediment, as they travel through the water column,
and in the trap itself. Corrections were made in this paper from experimental measurements of
(a), (b)and the last two components of (c). Microbial oxidation of the CH, could occur at the
littoral site but not in the profundal zone where the trap was suspended in anoxic water.
Therefore the results presented may have slightly overestimated denitrification in the
profundal zone. The considerable variability in annual gas release rates suggested that
controlling factors may vary from year to year, and therefore only results from the same year
should be compared. Analysis of coenzyme F,,, may be a useful method for detecting
methanogens in sediments, but requires further development to obtain quantitative data.
The rates of SO:- reduction and numbers of SO:- reducers were significantly higher in the
profundal zone, with activity and population densities being greatest at or near the sediment
surface in the profundal samples and at a depth of 4 to 5 cm in the littoral samples. The depth
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Decomposition processes in lake sediments
Table 5 . Comparison of decomposition processes in littoral and profundal sediments during
summer stratijication in Blelham Tarn
The activity values represent the total activity of each process for the whole period of thermal
stratification; those for O2 uptake, NO: reduction, NH: accumulation, N, evolution and SO:reduction have been converted to CO, equivalents according to the stoicheiometric equations of
Richards (1965). With the exception of N, and CH, evolution, all the values for the littoral zone were
derived from experimental systems. The values for the profundal zone indicate the ranges over three
seasons (1978-80); the individual values marked * are those that are directly comparable with the
values in the littoral zone. The relative contributions of the two zones to the processes in the lake as a
whole are based on the areas of anoxic and oxygenated sediment at the end of the period of
stratification.
r
CO, evolution
0, uptake
NO; reduction
NH: accumulation
N, evolution
SO:- reduction
CH, evolution
Activity (mmol m-,)
Littor a1
Profundal
Relative contribution
to the whole lake
Littoral :Profundal
1600
1280
3 20
460
115
15
50
940-1 800*
750-980*
220*-230
3 10*-3 20
30*-3 10
30-55*
80*-420
1.2: 1
1.2: 1
1.2: 1
1.2: 1
3.1: 1
0.3 : 1
0.5 : 1
distribution of Sop reducers and their activity reflected prevailing E h conditions in the cores
(Fig. 1). All rates of SO:- reduction were corrected for inefficiencies in trapping Sz-; the
reasons for this are discussed in the description of the method. The concentrations of Sz- in
the sediments were consistent with the observed rates of reduction, but the SO:concentrations could not always be considered reliable since considerable variability in the
recoveries of internal standards was encountered. Similarly, the NO; values for the interstitial
water were treated with suspicion because of evidence of significant interference in the
cadmium reduction step by unidentified solutes. No such problems have been encountered
with anoxic water in the hypolimnion.
The relative importance of microbial decomposition processes over the whole lake during
the period of thermal stratification (Table 5 ) could be assessed only if certain extrapolations
were made. Calculations of rates of change in the profundal zone were known to provide
reasonable estimates of activity (Jones, 1976), and these provided the basic data. The littoral
sediments were not, however, part of a closed system and therefore such calculations could
not be made. Relative rates of NO, reduction, NHt accumulation, denitrification, SO:reduction and methanogenesis were, however, obtained from the laboratory experiments.
Although these rates were always higher than those observed in the field (probably due to
disturbance of E h and nutrient gradients during manipulation of the sediments) their ratio was
accepted as a reasonable estimate of relative rates in the littoral and profundal zones. When
considered over the season the differences between the two zones (Table 5 ) were not as
marked as might be expected from the results presented earlier. Two factors contributed to
this. In the first place, NO, reduction in the profundal zone became electron acceptor limited
before the end of the season. Similarly, the rate of methanogenesis (Fig. 9, and Jones &
Simon, 1980) tended to decline rapidly before the onset of overturn; the reasons for this are
not clear. In contrast, there was a constant replenishment of electron acceptors in the littoral
zone, and therefore all processes could be assumed to continue until the end of the season.
Secondly, the temperature rose steadily in the littoral zone during the season (mean values
were twice those of the profundal) thus contributing to higher microbial metabolic activity.
The values given in Table 5 re-emphasize the importance of aerobic respiration and the
small contribution of SO:- reduction to carbon turnover in Blelham Tarn. NO; reduction and
methanogenesis were each responsible for 15 to 25 % of the carbon mineralization. When
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J . G . JONES A N D B. M. S I M O N
Table 6. Relative importance of components of the nitrogen cycle in Eittoral and profundal
sediments in Blelham Tarn
Approximate rates (mmol rn-, d-l)
f
Littoral
Particulate nitrogen input 43.0*
NO: reduction 0.5
NO;-
0.5
N,
\
Profundal
Particulate nitrogen input 3.0
NO: reduction 1 -0
NO;-
0.2
N,
* Assumed to be less than that in the profundal zone because of resuspension.
these contributions were calculated for the whole lake (the rate multiplied by the area of each
zone) the profundal zone was the major site for SO; reduction and methanogenesis, whereas
the littoral was the more important site of aerobic respiration and NO; reduction. This was
particularly true of N, release; the reasons for this require further brief discussion.
The major end-products of inorganic nitrogen metabolism appeared to be NHf in the
profundal zone and N, in the littoral, in spite of the absence of a consistent difference between
the two sites in numbers of denitrifiers and NOT-reducing bacteria. However, the media used
for enumeration of NOT reducers require more careful consideration before they could be
considered to provide ecologically relevant information. This was particularly true of
enumeration of NHf producers. The relative importance of the various components of the
nitrogen cycle at the two sites is summarized in Table 6. The potential for conversion of NO;
to NHf appeared to be greater in the profundal zone. The actual quantity of N H t generated
was approximately 28% greater than that of NO; reduced, a value close to that predicted
(23 %) when the stoicheiometric formula of Richards (1965) was adapted for NO, reduction
to NHt. Further evidence for the existence of this pathway was the fact that accumulation of
NHQ stopped when the hypolimnion became depleted in NO;, although further experimentation with 15N03would be required to provide absolute proof of its existence in the field. The
importance of NH; as a major product of NO: reduction has been demonstrated in marine
sediments (Sorensen, 1978b) and has been discussed recently by Cole & Brown (1980)
particularly in relation to conditions of 0, limitation (Dunn et al., 1979). There is some
evidence of greater accumulation of N H I when the input of organic particles is rich in
nitrogen, whereas N, is the major end-product in more nutrient-poor sites (Jones & Simon,
1980). The main reason for the accumulation of N, as the major end-product of NO;
reduction in the more oxidized littoral zone appeared to be the higher and continued levels of
nitrification which recycled the NH;. Even if relatively constant proportions of NO; were
converted to N, and NHt, the recycling of the latter would automatically result in a net
enrichment in N,. The role of nitrification in nitrogen cycling in lakes should not, therefore, be
underestimated. Oxidation of N H t in the profundal zone was quantitatively less important at
the beginning of the season, and stopped as soon as the hypolimnion became deoxygenated.
The coupling of nitrification and denitrification in sediment systems has been reported
(Knowles, 1979) and our results are consistent with other reports of higher rates ot
denitrification in epilimnetic sediments (Chan & Campbell, 1980). The stimulation of
denitrification and nitrification by the activity of sediment macrofauna (bioturbation) was
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Decomposition processes in lake sediments
311
recently reported (Chatarpaul et al., 1980). The numbers and variety of such animals are
greater in the littoral than the profundal zone of Blelham Tarn (Jones, 1980) and may have
contributed to the results obtained.
We wish to thank E. Rigg and N. Hetherington who performed some of the chemical analyses, T. I. Furnass
who prepared the figures and Miss E. M. Evans who typed the script. This research was financed by a grant-in-aid
from the Natural Environment Research Council.
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