DISTRIBUTION
LAKE
AND FORMS OF NITROGEN
ONTARIO
SEDIMENT
CORE
IN A
A. L. W. Kemp and A. Muckochova
Canada
Ccntrc
for Inland
Waters,
Bllrlington,
Ontario
ABSTRACT
Fixed ammonium nitrogen is the dominant inorganic form of nitrogen in the scdimcnts
of Lake Ontario.
The fixed ammonium
nitrogen concentration
is around 300 ,ug/g of
sediment at the top of a 10-m core and increases gradually
to 525 pg at 150 cm, below
which it remains constant.
As nitrification
is precluded
in the rcduccd sediments below
3 cm, the ammonium ion is tither fixed within the scdimcnt clay lattices or migrates upward
in the sediment
interstitial
waters.
The uniform
fixed and exchangcablc
ammonium
concentrations
below 150 cm in the core indicate that the sediment is saturated with
rcspcct to ammonium
fixation,
and the dccrcasc in thcsc concentrations
above 150 cm
in the core suggest that equilibrium
is not attained with the ammonium ion. The dccpcr
scdimcnts probably
do not regenerate nitrogen to Lake Ontario, but most of the nitrogen
relcascd to the hypolimnion
by the sediments is from nitrification,
denitrification,
and
ammonification
reactions
at the sediment-water
intcrfacc.
A minimum
of 20% of the
organic nitrogen input to the scdimcnts is rcgcneratcd
to the lake from the top G cm
of sediments.
About 90% of the nitrogen
in the surface muds is organic:
28-46%
as amino acid-N, 4-7s
as hcxosamine-N,
and 21-3170 as hydrolyzable
unidentified-N,
From 29-57s
of the total nitrogen could not be accounted for as amino acids, hcxosamincs,
fixed and cxchangeablc
amnonia, nitrate, and nitrite in the surface sediments.
INTRODUCTION
Until rcccntly there have been few studies of the distribution of nitrogen in scdimcnts. The sediment organic-N distribution is uniform for a number of Wisconsin
lakes and also similar to that rcportcd for
soils by Kccncy et al. ( 1970). The dccrcasc in hcxosaminc-N, accompanied by
an increase in amino acid-N with incrcasing lake fertility,
suggested to them a
greater microbial
turnover of organic-N
in oligotrophic
lakes. Organic-N,
fixed
ammonium-N,
and cxchangcablc
ammonium-N were the predominant
folms of
nitrogen in a number of Wisconsin lake
cores (Konrad et al. 1970). Fixed ammonium-N, amino acid-N, and hcxosaminc-N
accounted for about 50’% of the total nitrogcn in the Moholc sediments (Stcvcnson
and Tile 1970); about 20% of the total
nitrogen was prcscnt as ammonium-N,
fixed within the mineral clay lattices. The
nitrogenous part of the organic mnttcr dcposited in marinc basins was found to bc
partly rcgcncratcd as ammonia under rcducing conditions and became oxidized to
nitrate in sediment of positive Eh (Rittenberg ct al. 1955).
LIMNOLOGY
AND
OCEANOGRAPHY
Large amomits of individual amino acids
arc found in the 6 N HCl hydrolysatcs of
sediments (c.g. Jones and Vallcntync 1960;
Swain 1961; Brchm 1967; Dcgcns 1970); it
is not clear whcthcr they exist as intact
pcptidcs or proteins or whcthcr they arc
associated with the sediment humic acids.
Free amino acids, glucosamine and galactosamine, and purinc and pyrimidinc bases
have also been dctcctcd in scdimcnts in
lcsscr amounts ( Dcgens 1965), About 2050% of the scdimcnt organic-N probably
exists as complex hctcrocyclic
nitrogen
compounds.
Comprehensive
rcvicws reccntly published on the nitrogen cycle in
soils (Campbell and Lees 1967) and on
the nature of soil nitrogen compounds
(Bremner 1967) arc applicable to scdimcnt
studies.
Thcrc is little information
on scdimcnt
nitrogen and its role in the Great Lakes.
It has been estimated that 35% of the nitrogcn input is rctaincd in Lake Ontario,
with the scdimcnts probably acting as an
important sink (Int. Jt. Con-m. Rep. 1969).
The quantity of nitrogen in the surface
scdimcnts of Lakes Ontario, Erie, and
IIuron was found to bc directly propor-
855
NOVEMl3ER
1972,
V. 17(6)
856
A.
L.
W.
KEMP
AND
tional to the organic carbon content with
an average C : N ratio of 8.2 ( Kemp 1971).
About 45% of the organic nitrogen deposited in central Lake Eric was cstimatcd
to be returned to the lake waters from the
sediment-water
interface under oxic conditions ( Burns and Ross 1972); a deficit
of inorganic nitrogen under anoxic conditions was attributed to the conversion of
nitrate to nitrogen gas. Nitrifying
b’actcrial densities and nitrification
decrcascd
sharply at the central Lake Eric sedimcnt-water interface at the onset of anoxic
conditions ( Mcnon ct al. 1971).
This study reports the forms and distribution of nitrogen in a 10-m sediment core
from Lake Ontario and rclatcs these to the
rcdox potential and the organic-C content,
The forms of nitrogen found are compared
with those obscrvcd in a number of surfact sediment samples from the lake, and
the potential for the regcncration of nitrogen from the sediments is discussed.
WC wish to thank C. B. J, Gray and
Dr. C. I?. M. Lewis for assistance in collecting and measuring the samples. WC
would also like to thank N. Harper for
carrying out organic and carbonate carbon analysts and for preparation of the
diagrams, L. Johnston for some of the
nitrogen analysts, and C. 13. J. Gray for
critically reviewing the manuscript.
METIIODS
Sampling
methods
A 10-m core was collected from close to
the deepest point in Lake Ontario (77”
54.3’ W, 43” 30.5’ N; depth, 225 m) with a
540-kg Alpinc piston corer. Two 1.5-m
Benthos gravity cores were also collected
The
scdimcnt at
at the same location.
this location is a soft, fine, silty clay mud
overlying firm glaciolacustrine
clay with
the glacial-postglacial
contact at 740 cm
(about 11,000 B.P.), Surface Shipek bucket
samples, reprcscntativc of the major sedimcnt types in Lake Ontario, wcrc also c01lccted. Samples of silty clay mud wcrc
collected from the Rochester, Mississauga,
and Kingston basins of Lake Ontario and
A.
MUDROCIIOVA
Hamilton Harbor. Sand and glaciolacustrine clay samples wcrc collected from
north of the Mississauga basin (Thomas et
al. 1972). A short bluff at 50 Milt Point,
Ontario, consisting of the Grimsby Series
clay loam, overlying a glacial till, was also
sampled.
Analytical
methods
The cores and bucket samples were
subsamplcd and pH and Eh measured on
retrieval of the sediment (Kemp 1969;
Kemp ct al. 1971). Half of the wet scdimcnt was immediately frozen and stored
for analysis. The remaining sediment was
frcezc-dried on shipboard and the water
content dctcrmincd at the same time. The
top 150 cm of the sedimentary column is
dcscribcd by analyses on the two 1.5-m
cores. One core was used for the organicN fraction analyses and the other core for
the remaining analysts. Piston core subsamples were used for the analyses below
150 cm.
The dried sediment was ground to 280
mesh, after removal of shells and bottom
a 35-mesh
fauna, by sieving through
screen. Organic and carbonate carbon
were determined‘ by dry combustion in
an induction furnace (Kemp and Lewis
1968). Total nitrogen was determined by
the Dumas method (Keeney and Bremncr
1967). Nitrate-N, nitrite-N, and exchangeable NHh-N were determined
by the
method of Bremner (1965a), which involves removal of the ions with 2 N KC1
followed by steam distillation
of the extract. It should be noted that part of the
exchangeable
ammonium
is in solublc
form in the scdimcnt. These analyses wcrc
carried out on the freeze-dried sediment;
exchangcablc NHd-N, NOS-N, and NOB-N
concentrations were found to change when
stored wet, presumably due to microbial
alteration of the sediment and changes in
the equilibrium
between fret ( soluble),
exchangeable, and fixed ammonium ions.
Methods of measuring nonexchangcablc
(fixed)
ammonium in soils were evaluated by Mogilevkina
( 1969)) who found
the method of Silva and Bremner ( 1966))
NITROGEN
IN
LAKE
ONTARIO
SEDIMENTS
857
which is used here, to bc the most Ambrosia pollen concentration was deterIt involves the removal of mined at 2-cm intervals in the core and
satisfactory.
exchangcablc NH*-N and organic-N by al- rose from a few hundreds per gram of dry
scdimcnt below 19 cm to more than 20,000
kaline hypobromitc followed by treatment
above 19 cm, The Ambrosia horizon at 19
with 5 N I-IF-1 N HC1 to libcratc fixed
N&-N.
Howcvcr, it is possible that am- cm is intcrprcted as being about 120-140
monia or organic-N is still trapped in 13.P. ( McAndrcws 1972).
silicate rnincrals, not decomposed by HF
RESULTS
AND DISCUSSION
Complex ammonium
(Brcmncr
1965n).
General core charucteristics
phosphate minerals arc not dctcrmined by
this method and these may be present in
The general characteristics of the core
the Lake Ontario scdimcnts. The fixed
arc shown in Fig. 1. The water content
NIXi-N values dctcrmined in this study arc decreases from 93% at the surface to 40%
considered the best possible at this time.
at the postglacial-glacial
clay contact, rcConsistently
higher fixed NI-I,i-N values
fleeting the increase in compaction of the
wcrc obtained when frcezc-dried sediment
scdimcnt. The pH decrcascs from 7.7 at
was used (Kccncy ct al. 1970); here, all the surface to 7.0 at 150 cm. Eh is posifixed NI-14-N determinations were made on
tivc in the top 3 cm, decreases to -0.240 V
fresh wet scdimcnt.
at 12 cm and remains around zero to 1 m
The organic-N fraction was determined
and then bccomcs slightly positive for the
on the extract obtained by 6 N IICl hy- rcmaindcr of the core; thcsc changes arc
drolysis of wet sediment for 17 hr. Total
rcflectcd in the color of the modern scdihydrolyzable, ammonium, amino acid, hcx- ments which change from reddish brown
osamine, and hydroxyamino
acid nitrogen
at the surface to black at 12 cm and gray
1965b). No for the remainder of the core. Black lcnscs
( Brcmner
wcrc mcasurcd
difference was found in the organic-N
or laminations occur frequently in the top
fractions bctwccn wet ‘and frcezc-dried
5 m of scdimcnt and are believed to bc
scdimcnt. The differcncc between the to- local zones of more intcnsc reduction of
tal hydrolyzable-N
and the sum of the the scdimcnt.
hydrolyzable
NIId-N, amino acid-N, and
Carbonate carbon dccrcases from 1.3%
hcxosaminc-N is termed unidcntificd
hy- at the surface to about zero at the Amdrolyzablc-N.
Thcrc is no r&able method
brosia horizon and is again high above the
of measuring total organic-N in sediments;
postglacial-glacial
contact. The high surit is estimated as the differcncc bctwccn
fact carbonate values arc attributed
to
the total-N and the sum of the exchangeprecipitation
of calcium carbonate from
able NH‘t-N, NOS-N, NOZ-N, and the fixed
CO2 libcratcd by the decomposition of orNILt-N.
ganic matter (Kemp ct al. 1972). The high
Stevenson and Chcng (1970) pointed
concentration of carbonates in the lower
out that a large part of the scdimcnt nitroportion of the core is probably a result of
gcn is liable to be fixed in argillaceous
the larger input of crodcd carbonate tills
sediments and rccommcnded an I-IF predeposited in the lake on the retreat of the
trcatmcnt of the sediment. The organic-N
last glaciers (Kemp 1969).
fraction in the core was dctcrmincd using
Organic carbon and total nitrogen contheir method and that of 13remncr ( 1965h);
tents decrease sharply to the Ambrosia
both gave similar results for organic-N,
horizon and then gradually to 3 m. The
suggesting that it is not fixed to any de- threefold incrcasc in carbon and nitrogen
grce in the Ontario scdimcnts.
above the Ambrosia horizon is due to inCarbon dates of 3,440 B.P., 4,020 BP.,
crcascd organic inputs to the sediments
and 5,040 B.P. were obtained at the 101-, and to diagenetic loss (Kemp et al. 1972).
131-, and 161-cm levels of the core by The rise in organic carbon at 350 cm has
the Geological Survey of Canada. The been obscrvcd in a number of cores from
858
A.
sediment
type
L.
W.
water
content
KEMP
PH
AND
A.
MUDROCIIOVA
Eh (volts)
%org-C
%CO,-C
%- N
4020
5040
!i
500 -
postglacial
-------e----m
m-1-1
glacial
sediment
----m
-----
11000
sediment
1000
40
FIG.
1.
Distribution
60
I
80
I
of carbon
100
-400-200
I
I
I
7.0 7.5 8.0
ancl nitrogen
0
I
200
I
0
2
and the sediment
r4
-?A,
6
characteristics
0
.25 .50
.75
of the Lake
Ontario
core.
the main basin of Lake Ontario and is
attributed to events during the Hypsitherma1 period (Kemp 1969).
A present-day sedimentation rate of 350
g m-2 yr-l (2-3 mm yr-l) is estimated
from the water content and bulk density
of the scdimcnt, assuming a constant rate
of scdimcntation since the Ambrosia horizon. This yields a sediment rcsidcnce time
of lo-15 years in the top 3 cm before
NITROGEN
sediment
type
100
IN
LAKE
ONTARIO
exchangeable
NH,-N
exchangeable
(NOs+NO&N
859
SEDIMENTS
org - C
fixed
NH,,- N
organic
org - N
-N
-.-------.
r
dark gray
clay mud
many
black
laminations
glacial
I
1000 I0,-+.
15
30
0
FIG. 2.
micrograms
I
,- .-. r- L
100
50
Distribution
of inorganic
and organic
per gram dry weight of sediment.
further burial. Sedimentation rates wcrc
high immcdiatcly
after the rctrcat of the
glaciers and began to dccrcasc by 11,000
B.P. (McAndrews
1972). The three carbon dates attest to the low scdimcntation
rates in the last 5,000 years. Until WC have
further carbon dates, WC cannot dcterminc
changes in the organic input to the scdimcnts throughout postglacial times.
The quartz and clay mineral content of
the sediment is uniform in the top 1.5 m
of scdimcnt
(R. L. Thomas, personal
200
_-~M-_
400
I
I
600
0
I ---WI
6wo
0
nitrogen
in the Lake
,
2000
30
core, concentrations
in
I
.4ooo
Ontario
15
communication).
Quartz constitutes 25%
of the sediment weight and clay minerals
55%. Illitc is the predominant clay mincral (60-70%) with kaolinite (lo-20%) and
chlori tc (lo-20%) p rcscnt to lcsscr dcgrces.
The characteristics
of this core arc
typical of Lake Ontario silty clay muds.
Similar nitrogen distributions
have been
obtained from a number of Benthos and
piston cores collected from different rcgions in the lake (Kemp 1969; Kemp and
Mudrochova, unpublished data).
860
A.
L.
W.
KEMP
AND
Fixed NH,,-N content of shore bluff
from 50 Mile Point, Ontario.
(Concentrations in g/g dry wt of soil)
TABLE
1.
material
Horizon
Fixed NHq-N
A
111
B
254
C
255
Inorganic nitrogen fraction
The distribution of ( NO3 + NOz)-N, cxchangeable NHd-N, and organic-N in the
Lake Ontario core is shown in Fig. 2.
Fixed NHd-N is the predominant form of
inorganic-N in the Lake Ontario core. The
concentration of fixed NIL-N
rises from
248 pg/g of sediment at the water interfact to around 520 ,ug at 190 cm and fluctuates bctwecn 400 ancl 500 pg for the
remainder of the core. A large part of the
fixed NHd-N is presumably derived from
terrcs trial sources. Fixed NH4-N was dctermincd on a shore bluff profile from 50
Milt Point, Ontario (T,able 1). The bluff
consisted of a surface clay loam (A horizon, 0.3 m), overlying a firm brown clay
(B horizon, 2.0 m). The base of the bluff,
above the waterline, consistccl of a stiff
gray glacial till ( C horizon).
Thcsc horizons can each bc considered as potential
source material for the fine-graincd scdimcnts in Lake Ontario. The fixed NHd-N
concentrations of the B and C horizons are
TABLE
surface
A.
MUDROCHOVA
similar to those found at the surface of
the core and in the silty clay and glaciolacustrine clay surface samples (see belozo).
The concentration of ( NOs + NOz) -N is
low
throughout
the core except in the
oxidized
sediment-water
interface
zone
whcrc a maximum concentration of 28 pg
/g of sediment is found. The concentration
of NOZ-N was found to bc <l pg/g
throughout the core and the results are cxprcsscd as the sum of the NO:, and NO2
concentrations.
In contrast, cxchangcablc
NH‘k-N is low at the surface and incrcnscs
to a maximum of 99 pg/g at 130 cm. The
exchangeable NH‘I-N decrcascs below 300
cm to a low of 53 pg/g at 740 cm. Organic-N is the predominating
form of nitrogcn at the scdimcnt-water
intcrfacc,
accounting for 94% of the total-N.
The organic-C : organic-N
ratio (C : N
ratio) increases irregularly from 8.2 at the
surface to around 10.0 at 1 m, remains
fairly constant to 580 cm, and then incrcascs to 21.2 at the bottom of the core.
The surface ratio is similar to the average of 8.2 rcportcd for a number of Great
Lakes surface samples ( Kemp 1971). The
incrcasc in ratio in the top meter of
sediment indicates that the organic-N compounds arc being mineralized
prcfercntially over the bulk of the organic matter.
The dcclinc in amino acid-N, discussed
later , supports this. Similarly the incrcasc
in ratio at the bottom of the core is followed by a sharp decline in amino acid-N.
2. Organic carbon (OC), carbonate carbon (CC), ancl nitrogen distribution
sediments.
(Concentrations
in g/g
dry wt of secliment; figures in brackets
age of each fraction
relative to total nitrogen)
oc
cc
Total-N
Org-N
Fixed
NH4-N
in Lake Ontario
show the percent-
(No3+No2)-N
NH/,-N
Silty
clay
25,100
420
2,736
2,403(88)
307(U)
6(x1)
20(1)
Silty
clay
47,600
1,460
5,260
4,832(92)
324(6)
9(<1>
95 (2)
Silty
clay
51,000
5,770
5,542(96)
168(3)
5(<1)
65(1)
270
1 (<I)
260(97)
b(2)
3(l)
b(2)
4(l)
790
Glacial
clay
210
10
Glacial
clay
410
210
310
15 (5)
285 (92)
330
210
410
313(76)
48(12)
Sand
4(l)
45(U)
NITROGEN
IN
LAKE
ONTARIO
861
SEDIMENTS
(NN), total hydroTAULE 3. Organic nitrogen forms in the Lake Ontario core. Nonhydrolyxable-N
lyzable-N
(THN),
hydrolyzable
ammonium-N
(AN), hyclrolyxable
hexosamine-N
(HN),
hydrolyzable
amino acid-N (AAN),
hydrolyzable
hydroxyamino
acid-N (IIAAN),
and unidentified
hyclrolyxable-N
(UN).
(Percentage
of fixed NH,,-N in brackets; hyclroxyamino
acicl-N cletermined as part of amino
acid-N)
Depth (cm)
NN
THN
AN
FIN
MN
HMN
UN
% of Total-N
o-3
16
84
19 (6)
7
37
7
21
9-12
19
81
27(18)
10
30
6
14
20-30
8
92
30(21)
10
29
6
23
97-103
41
59
30(17)
5
20
4
4
148-154
35
65
28(16)
6
21
4
10
351-357
36
64
30(17)
10
21
3
3
527-533
3
97
40(25)
10
27
6
20
671-677
3
97
69 (50)
12
13
2
3
749-755
2
98
70(64)
25
5
0
0
The transformations of scdimcnt inorganic
forms of nitrogen arc discussed below.
The nitrogen distribution in the surface
scdimcnt samples shows trends similar to
the core (Table 2). The silty clays were
collcctcd from the Kingston Basin, Mississauga Basin, and Hamilton Harbor, rcspcctivcly. The nitrogen in the surface silty
clay muds is predominantly
organic and
the fixed NH4-N content is around 300 p,g
/I;.
The surface glaciolncustrinc
clays,
which are low in organic carbon, have a
high fixed NIIJ-N content like the glaciolacustrine part of the core. In contrast, the
sand sample, also low in organic carbon,
has high organic-N and low fixed NILI-N,
consistent with the high quartz content of
the sand ( 92%) and the low clay mineral
con tent ( 6% ) .
Organic nitrogen fraction
The distribution of organic forms of nitrogen was dctcrmincd on 9 Ontario core
subsamples (Table 3). The amino acid
content of the top 3 cm is similar to that
of 17 Wisconsin lakes whose surface scdimcnts had remarkably uniform organic-N
compositions,
irrespective
of lake type
(Kccney ct al. 1970). Surface soils also
show little variation in organic-N distribution with soil types and cultivation (Brcmncr 1967).
The dctcrmination of the organic-N fractions in the scdimcnts is based largely on
the estimation of N-compounds rcleascd
by hot acid trcatmcnt, which quantitativcly rclcascs individual
amino acids and
hcxosamincs from proteins, pcptidcs, and
chitin ( Brcmncr 1965n, 73). To detcrminc
whcthcr the amino acids and hexosamines
exist in a fret form in the Ontario scdimcnts, the top 3 cm of the core was ultrasonicatcd in qucous solution for 30 min;
free amino acid-N and hcxosaminc-N wcrc
not cletcctablc in the extract after this
trcatmcnt.
WC assume that the amino
acids exist in the form of peptidcs, protcins, or humoprotcins and that the hcxosamincs exist in the .form of chitin or cell
wall materials and not as free compounds.
The organic-N fractionation
scheme is
basal on the 6 N HCl hydrolysis of a separatc scdimcnt subsamplc. It is assumed
that the ( NO:, + NO:!) -N and cxchangeable NH4-N is solubilizcd by this treatment
togcthcr with all or part of the fixed NH,,-
862
A.
L.
W.
KEMP
AND
N. Hexosamine-N, amino acid-N, hydroxyamino acid-N, and NHJ-N arc measured
in the hydrolysatcs. Total hydrolyzable-N
is also mcasurcd and this togcthcr with
total sediment nitrogen yields values for
the nonhydrolyzable-N
fraction and the
unidentified
hydrolyzable-N
fraction,
IIexosaminc-N
dccrcascs from 420 pg/g
at the surface to 140 at the base of the
core, with the hcxosaminc-N content of the
total-N increasing from 7-25% (Table 3).
Hexosamines form a large pcrcentagc of
bacterial cell walls. High hcxosaminc content in soils has been attributed
to the
large-scale turnover of soil organic matter
by microorganisms (Brcmncr 1967). The
hcxosaminc-N content of the Ontario surface sediment is higher than that rcportcd
for the Wisconsin lakes and that normally
found in soils (Table 3). Although Great
Lake bacterial concentrations arc grca tcs t
at the scdimcnt-water
intcrfacc (Vandcrpost and Dutka 1971; Menon et al. 1971))
it seems unlikely to us that the high hcxosaminc-N content is due to an unusually
large microbial
turnover of the surface
sediment organic matter. It seems more
likely to be due to a large hcxosaminc
input to the scdimcnt and the nature of
the hcxosaminc material. Chitin is known
to bc more resistant to decomposition than
the hcxosaminc linkages in bacterial cell
walls. Zooplankton,
benthic fauna, and
fungi contain chitin and these organisms
may play a larger role in Lake Ontario
than in the Wisconsin lakes.
Amino acid-N dccrcascs from 2,400 pg
/g at the surface (37% of the total-N) to
50 at the base of the core (5% of the totalN ) . The amino acid-N content of the
Ontario surface scdimcnt hydrolysatcs is
similar to that in the Wisconsin scdimcnts
and higher than normally found in soils.
The high protein content of algae and
plankton, togcthcr with a low microbial
turnover of the sediment organic matter,
is most likely rcsponsiblc for the high
amino acid-N content of the lake scdimcnts. The dccrcasc in amino acids content down the core probably reflects their
1964).
slow mincraliza tion (Vallcntyne
A.
MUDROCHOVA
The uniform amino acid-N content bctwccn 1 and 4 m is accompanied by an
incrcasc of nonhydrolyzablc-N
(Table 3))
with which some amino acids may bc
associated.
The hydroxyamino acid-N content of the
core dccreascs from 400 pg/g at the surface to zero in the glacial sediment, indicating that serine and threonine are less
stable than some of the other amino acids
and in accord with the thermal stability
series of amino acids (Vallcntyne 1964).
The hydrolyzable
NIL-N
content increases from 19% at the surface to 70% at
the bottom of the core (Table 3). This incrcasc parallels the fixed and cxchangcablc
NH4-N contents of the sediment (Fig. 2).
It would appear that the fixed NHJ-N in
the Ontario scdimcnts is released by the
6 N I-ICI hydrolysis.
The remaining hydrolyzablc NHd-N can be accounted for
primarily as a hydrolysis product of the
decomposition of hcxosamincs and amino
acids. The surface sediment from the core
was hydrolyzed for 48 and 90 hr while
the amino acid-N and hydroxyamino acidN contents showed a 10% dccrcasc. The
hcxosaminc,
amino aci‘d and hydroxyamino acid dccrcascs were matched by an
cqunl incrcasc in hydrolyzable NH4-N.
The unidentified-N
content of the Ontario sediments is lower than that rcportcd
for the Wisconsin lakes and in soils (Table
3). A small part of this nitrogen is probably in the form of purine and pyrimidine
bases, but dcspitc intensive rcscarch in
soils there is as yet no information on the
nature of thcsc compounds (Bremncr 1967).
The nonhydrolyzablc-N
fraction has also
been the subject of much speculation and
is bclicvcd to be a part of the soil huminN (Brcmncr 1967). Nonhydrolyzablc-N
is
generally low except between 1 and 4 m
in the core (Table 3). At first, WC bclicvcd that there was an error in our
duplicate detcrminaanalyses. Howcvcr,
tions and USC of the altcrnativc HF pretreatment method yielded the same results.
The region of the core between 1 and 4
m reprcscnts the zone of lowest scdimcntation rates. The scdimcnt organic matter
NITROGEN
4.
TABLE
Orgajaic
nitrogen
NN
IN
LAKE
fornas in Lake
THN
ONTARIO
Ontario
AN
% of Total
Kingston
Harbour
HN
MN
Synabols as in Table
HAAN
- N
21
95
20 (4)
40
31
0
100
20 (5)
46
8
29
35
65
18(6)
28
5
22
transformations in
the sediment
The scdimcntary organic matter is dcrived from the lake plankton and from
tcrrcstrial litter and humus. Microscopic
examination of the Ontario surface sedi-
3
UN
37
is linblc to bc humified to a much greater
cxtcnt during periods of slow scclimcntation, since the bacterial population and the
benthic fauna have a much longer time
to dcgradc the scdimcnt organic matter
bcforc it is buried. The incrcasc in nonhyclrolyzablc-N
and the mliformity
of the
amino acid-N, hcxosaminc-N, and C : N ratios in this zone could bc due to the formation of a relatively large humic fraction.
The organic-N fraction was also detcrmined on Kingston Basin, Rochester Basin,
Mississauga I3asin, and Hamilton IIarbor
surface silty clay samples (Table 4). The
first three silty clay samples have varying amino acid, hcxosaminc, and unidcntificd-N concentrations, due to differcnccs
in the nonhydrolyzablc-N
conccn tra tions.
The IIamilton IIarbor sample differs radically from the other silty clay muds
bccausc of its high nonhydrolyzable-N
content. A postglacial mud thickness of 1 m
at our Hamilton Harbor sample location
indicates a very slow scdimcnta tion rate.
The high nonhydrolyzablc-N
con tent may
again bc due to incrcascd humification
at
the sediment-water
interface.
The surfact sediment samples arc generally similar to the core with the nonhydrolyzable-N
fraction again contributing
to the major
diffcrcnccs.
Nitrogen
sediments.
19 (6)
Rochester
Hamilton
surface
84
16
Mississauga
863
SEDIMENTS
4
mcnts rcvcals plankton remains, pollen,
plankton fecal pcllcts, bacterial colonies,
and brown amorphous humus particles (A.
Nauwerck, personal communication).
Estimatcs indicate that more than 90% of the
organic input to Lake Eric is autochthonous ( Kemp 1971). WC bclicve that a
similar situation exists in Lake Ontario
and that the larger part of the sedimcntary organic matter is autochthonous. The
results of this study show that most of the
nitrogen is in the organic form in the surfact silty clay muds. The transformations
of nitrogen in lake sediments arc mainly
the result of microbial proccsscs. The role
of bottom fauna in the transformation of
nitrogen is unknown and is only likely to
bc significant in the top 3 cm of scdimcnt.
The more important transformations
are
shown in Fig. 3 (see Campbell and Lees
1967).
In the scdimcnt environment, there are
continuous transformations of nitrogen opcrating at various rates, but some of the
proccsscs only operate under specific conditions. Ammonification
(Fig. 3) can procccd aerobically and anaerobically over a
wide PI-I and tcmpcraturc range; most hctcrotrophic bacteria produce ammonia in
the clccomposition of organic matter, Howcvcr, nitrification
can only bc carried out
by two groups of obligate acrobcs, Nitrosomonas and Nitrobacter, which arc sensitivc to “pH and have an absolute rcquircmcnt for oxygen. Denitrification
(Fig. 3)
is accomplished by a limited number of
bacteria; since nitrate rcplaccs molecular
oxygen, it only occurs in an anoxic envi-
864
A.
L.
W.
KEMP
A.
MUDROCEIOVR
transformations
of nitrogen
A) ammonification
D) denitrification
B)
immobilization
E) N2 fixation
C)
nitrification
FIG.
3.
Schematic
of major
AND
ronnicnt.
Bat tcrial nitrogen fixation has
been demonstrated in L&c Eric surface
sediments ( IIoward et al. 1970). A recent
study of a sediment core taken from the
same Ontario location as this study showed
that heterotrophic bacterial dcnsitics wcrc
greatest at the sediment-water
interface.
The heterotrophic
bacterial dcnsitics dccrcascd from 107/g of sediment at the
surface to 10” at 20 cm (Bell and Dutka
1972 ) *
An examination of E:h in the core (Fig.
1) shows that nitrification
can only procccd in the top 3 cm and denitrification
below this depth. The changeover of Eh
from positive to negative potentials occurs
at 3 cm and it is prcsumcd that no oxygen
is present below this depth. The top 3 cm
is bclicvcd to bc the zone of mixing in
the Ontario silty clay muds allowing for
rcplcnishmcnt
of oxygen (Kemp et al.
1972). The (NO3 -k N02)-N concentration
decreases from 28 pg/g at the surface to
in lake sediments.
2 at 3 cm, indicating that nitrification
is
maximum at the water interface where
oxygen supply is maximum.
The transformations of nitrogen can then
bc clcscribcd in a general way. Ammonia,
the first product of the microbial dccomposition of freshly dcpositcd organic matter,
can be nitrificd or immobilized by microbial proccsscs (Fig. 3) or can migrate into
the overlying waters or bc fixed by the
scdimcnt clay minerals ( Fig. 3). Although
denitrification
is favored in the reduced
sediment below 3 cm, it is unlikely to take
place owing to the abscncc of nitrate ion;
it is only liable to occur above the 3-cm
lcvcl whcrc oxygen concentrations arc low
within microcnvirorm~cnts and nitrate conccn trations become significant.
The high
organic carbon content and bacterial dcnsitics in the top 3 cm of sediment indicate
that much of the nitrogen rcgencrated by
decomposition of organic matter can bc
returned to the lake water as nitrogen gas,
NITROGEN
IN
LAKE
nitrate, nitrite, and ammonia. Ammonification, migration of ammonia in the pore
waters, and ammonia fixation are the main
processes taking place in the anoxic scdimcnts below 3 cm in the core.
The anaerobic zone is characterized by
increasing concentrations of exchangeable
and fixed NHd-N with depth of burial to
about 1 m (Fig. 2). The large fixed
NH4-N values observed arc due to the
nature of the Lake Ontario clay minerals.
Illitc was found to fix more NHd-N than
montmorillonite
and kaolinitc in a number
of soil profiles (Stevenson and Dhariwal
1959). The high illite content of the Ontario sediment, mentioned previously, acts
of the
as a site for the immobilization
ammonia produced by the ammonification
of sediment organic matter. The uniformity of the fixed and cxchangcablc NHb-N
below 150 cm suggests that the limit of
NH4-N fixation has been achieved in the
dccpcr scdimcnts, and the lower fixed and
exchangeable NIL-N concentrations in the
upper sediments suggest that equilibrium
is not cstablishcd in this region of the
core. This would suggest that present-day
ammonia, migrating upward in the pure
waters, would bc fixed by the clay minerals between 3 and 150 cm. Oxidation of
the ammonia at the surface by nitrifying
bacteria products the gradient in fixed and
exchangcablc NHJ-N between the surface
and 150 cm.
It is not clear why the ( NOs + NOa) -N
values incrcasc to 10 lug/g of scdimcnt at
60 cm and then fluctuate bctwecn 10 and
18 below 60 cm (Fig. 2). The values
could bc artifacts of the sample prcparation, by nitrification
of the scdimcnt NI-ItN during frozen storage or oxidation during frcczc-drying.
IIowcvcr,
this seems
unlikely
as lower values arc observed
above 60 cm. The nitrate might result from
groundwater seepage, as in Lake Mendota
(Zee 1970; Kecney et al. 1971)) but this
seems unlikely as the Ontario glacial clays
h avc a low water content and should
form an effcctivc barrier to sccpagc. The
( NO3 + NOz) -N values below 60 cm arc
also surprising as denitrification
would bc
ONTARIO
SEDIMENTS
865
cxpectcd to proceed in this zone. HOWcvcr, in gcncral the results obtained in the
Ontario core fit the nitrogen transformations cxpccted. The sharp decrease in organic-C and organic-N below 530 cm is
due to tither lower organic productivity
at that time or higher sedimentation rates
after the melting of the last glaciers.
The variable rates of scdimcntation, lack
of knowlcdgc on variations in organic input, and the abscncc of data on nitrogen
gas concentrations preclude any detailed
calculations of the quantity of nitrogen regcneratcd to the lake waters from these
core data. The regeneration of nitrogen
from California basin sediments was calculated by subtracting the weight of inorganic-N present in the sediment from the
weight loss of organic-N (Rittenbcrg et al.
1955). A similar calculation has been carried out for the Ontario core between 19
cm ( 140 B.P. ) and 100 cm (3,440 B.P. ). It
is assumed for this calculation that the annual organic input and sedimentation rate
did not fluctuate too much in this period;
the jaggedness of the total-N curve indicates that this is only a fair assumption
( Fig. 1). The weight loss of organic-N for
a square-centimeter column of sediment is
calculated to be 10.3 mg and the weight
of total inorganic-N to bc 14.5 mg in this
zone. The crude calculation indicates an
imbalance; however, it must be borne in
mind that part of the inorganic-N is fixed
to the clays before their deposition as sediment. The inorganic-N
distribution
and
the above calculation suggest that prcscntday regeneration of nitrogen from the historic zone is unlikely.
As mcntioncd previously, the increase in
organic-N above the Ambrosia horizon is
attributed to diagenesis and to the accelerating input of nitrogen to the Ontario sediments in rcccnt years; therefore, it is not
possible to make the above calculations for
About 45% of the nitrogen dcthis zinc.
posited in Lake Ontario biota and scdiment probably will bc regenerated to the
lake waters under oxygenated conditions,
as has been cstimatcd for Lake Eric (Burns
and Ross 1972). A surface-water plankton
866
A. L. W. KEMP
AND
sample collected at the same location and
time as our core had a C : N ratio of 4.5.
The surface scdimcnt has a C : N ratio of
8.2 and the 6-cm lcvcl of the core a C : N
ratio of 10.7 (Fig. 2). The clccrcasc in
C : N ratio in the top 6 cm togcthcr with
the decrcasc in organic nitrogen content
from 94 to 81% indicates that a minimum
of 20% of the nitrogen loading to the Ontario scdimcnts is lost to the overlying waters via ammonification,
nitrification,
and
denitrification
reactions.
The complcxitics of the organic-N fractionation scheme have already been discussed. The hcxosamine-N, amino acid-N
together with the inorganic-N fraction rcpresent the known nitrogen components in
the Ontario scdimcnts. The hydrolyzable
N&-N consists mainly of the inorganic nitrogen fraction ancl hydrolysis products of
amino acicls and hcxosamincs. The nonhydrolyzablc-N
and the unidcntificd
hydrolyzablc-N make up the forms of nitrogen
that have not been identified in this study;
from 29-57% of the nitrogen has not been
iclcntified as known compounds in the surfact Ontario muds (Table 4) and from 245% in the core (Table 3). The nature
of the nonhydrolyzablc-N
and of the unidentified hydrolyzable-N,
which is prcsumably in an organic form, is unknown
and prcscnts as complex a problem as in
soils.
1972.
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ONTARIO
SEDIMENTS
867
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