55
Hydrobiologia 364: 55–63, 1998.
H. L. Golterman (ed.), Sediment-Water Interaction 8.
c 1998 Kluwer Academic Publishers. Printed in Belgium.
Bacterial and phosphorus dynamics in profundal Lake Erken sediments
following the deposition of diatoms: a laboratory study
Erik Törnblom & Emil Rydin
Institute of Limnology, Uppsala University, Norbyvägen 20, 752 36 Uppsala, Sweden
Received 5 November 1996; accepted in revised form 4 December 1997
Key words: bacterial activity, bacterial biomass, phosphorus fractionations
Abstract
The benthic microbial response to the deposition of natural seston and the microbial impact on nutrient dynamics
was studied in an experimental system using whole sediment cores equipped with flow-through systems for the
overlying water. For 20 days, changes in sediment bacterial activity, total metabolic activity (heat production),
bacterial biomass, phosphorus fractions and basic chemistry were followed, as well as the exchange of nutrients
between sediment and water. Microbial activity and biomass increased immediately in response to the deposition
of seston, peaked after seven days and then decreased linearly over the remaining time of the experiment. Cosettled bacteria were suggested to play an important role in the microbial response. Changes in bacterial biomass
production, bacterial biomass and the NaOH-nrP extractable phosphorus fraction were concurrent in response to
seston additions. The sediment acted as a trap for SRP from the overlying water when bacterial activity was high
and as a source when the bacterial activity decreased. Altogether, the results suggest an important role of bacteria
in the regeneration of seston P. Mineralization rates estimated from sediment heat production showed that ca. 11%
of the added seston carbon was oxidized in the sediments during the experiment.
Introduction
Bacterial degradation and transformation of particulate and dissolved organic matter are key processes
determining turnover rates of organic and inorganic
substances in aquatic systems (Fenchel & Blackburn,
1979. In sediments from both marine and freshwater
environments, bacterial productivity has been shown
to be most closely linked to substrate supply and temperature (Cole et al., 1988; Sander & Kalff, 1993). In
regions with a marked seasonality, pulses of organic
matter input to the sediments occurring during spring
and autumn as settling diatoms blooms, can therefore
be expected to be of major importance for microbial
sediment communities. Studies of pelagic-benthic coupling in marine environments have shown that benthic
sediment communities quickly can respond to inputs of
fresh organic matter in terms of: increasing microbial
activity (Meyer-Reil, 1983; Graf et al., 1982), increasing O2 consumption causing a shift towards anaerobic
conditions (Kelly & Nixon, 1984; Graf, 1987; van
Duyl et al., 1992) and a release of nutrients from the
sediments to the overlying water (Garber, 1984; Kelly
& Nixon, 1984; Enoksson, 1993). Few studies performed in freshwater have focused on the utilization of
the organic matter by benthic organisms (Fitzgerald &
Gardner, 1993; Goedkoop & Johnson, 1996) and little
is known how the normally observed increase in bacterial activity and biomass in response to the deposition of
organic matter, is related to fluxes of nutrients and especially the P-turnover. The mobilization/immobilization
of phosphate in lake sediments has traditionally been
considered to be governed mainly by redox-controlled
interactions with Fe and Mn. The activity of microorganisms has, since a long time been known to indirectly
affect P-cycling through the impact of mineralization
processes on redox conditions (Einsele, 1936; Mortimer, 1941, 1942). More recent studies have suggested
that uptake, storage and release of P by microorganisms in surface sediments can be of major importance
in P-cycling in lakes (cf. Boström et al., 1988; Gächter
et al., 1988; Gächter & Meyer, 1993). The amount
Pipsnr. 159558; Ordernr.:7011636-avg; spcode: GO BIO2KAP
hydr3811.tex; 7/06/1998; 17:00; v.5; p.1
56
of P in bacteria has been shown to be large enough
to explain most of the P-release observed from some
lake sediments (Boström, 1988). P may be bound or
released by sediment bacteria through metabolic reactions, extracellular release and lysis of bacterial cells.
Bacterial P-dynamics in sediments are dependent on
factors like: type, amount and quality of organic matter, P-availability in the surrounding water, NO3 - concentration, P and redox (Boström et al., 1988). In oligotrophic and mesotrophic lakes, the P-content of settling seston alone is too low to fulfil the high P- requirements of the mineralizing bacteria (Gächter & Meyer,
1993). Hence, decomposing bacteria in these systems
may use o-P from the surrounding water (Gächter &
Mares, 1985). In oxidized surface sediments, possible P-sources for bacteria include o-P from the overlying water, P bound to sediment organic matter or
inorganic compounds in the sediment (Boström, 1988;
Istvánovics, 1993).
This paper reports the results of a laboratory experiment using intact sediment cores collected in the profundal of Lake Erken. The study was performed in
order to study the short-term effects of the deposition
of natural seston on the microbial sediment community
and the role of sediment bacteria in P-regeneration.
Materials and methods
Sediment and seston collection, experimental setup
During the autumn diatom bloom 1994, sediment cores
were collected from a boat at 13 1 m depth in the
profundal zone of the eastern basin Lake Erken, Sweden. Lake Erken, situated approximately 65 km north
of Stockholm at 59˚ 250 N, 18˚ 150 E, has a surface
area of 23.7 km2 , a mean depth of 9 m, a maximum
depth of 21 m and can be characterized as mesoeutrophic (Blomqvist et al., 1994). 52 sediment cores
(inner diameter 64 mm, length 500 mm) were collected using a plexiglass core sampler. The surface
sediments (0–0.5 cm) were well oxidized upon collection with redox values well above + 300 mV. The
water temperature at 13 m depth was 12.5 ˚C. The top
rubber stoppers were removed and cores were placed
in a dark water bath regulates at in situ lake temperature using lake water continuously pumped from 4 m
depth. Magnetic stirrers (speed 50 rpm) were placed in
the water 8 cm above the sediments. The cores were
also equipped with a flow-through system operated by
peristatic pumps connected to a 50 l container with
aerated lake water (daily collected at 4 m depth and filtered through phytoplankton nets (mesh size 20 m)).
The renewal time of the water overlying the sediments
(450–600 ml) was approximately 20 h. Cores were left
to equilibrate to laboratory conditions for 48 h.
Seston was collected at 4 m depth using photoplankton nets (mesh size 20 m), rinsed with lake water
and again concentrated with phytoplankton nets. Subsamples were taken for the determination of bacterial
abundance and production, C-, N-, P- and chlorophyll a
concentrations. Live subsamples were also investigated microscopically in order to quantify the proportion
of living diatoms. Before seston additions, peristaltic
pumps and magnetic stirrers were stopped. 20 ml of
the concentrated seston suspension was carefully pipetted into the water close to the sediment surface in 24
randomly chosen cores. The added seston immediately formed a flocculent green layer on the sediment
surface. After 5 h this layer had settled into a thin
3–4 mm) and smooth-surfaced carpet. Water circulation was restarted and samples of in-and all outflowing water were collected for chemical analyses and to
enable the determination of the exact flux in each core.
The stirring was not restarted until day 1 in order to
prevent resuspension of the added seston. When the
stirring was restarted and during the experiment no
visible resuspension of seston or sediment occurred.
On day 0, four replicate cores (controls) and on days
1, 2, 4, 7, 12 and 20, four replicate seston and control
cores were randomly chosen for analyses of microbial activity and biomass, sediment chemistry and Pfractionations (day 1 no P-fractionations). Before sectioning, the O2 -concentration in the overlying water
and sediment redox potentials were determined in all
cores. The overlying water in each core was then carefully removed using a syringe and the sediment was
sectioned into 0–0.5, 0.5–2 and 2–4 cm layers. When
sectioning cores with seston additions care was taken
to include both the seston carpet and the underlying
0.5 cm of sediment. The experimental temperature followed changes in in situ lake temperature at 14 m depth
but was on average 2 ˚C lower due to cooling in the
ground-based part of the water pipe between the lake
and the laboratory. The first 7 days, the experimental
temperature varied around 9 ˚C ( 0.2 ˚C) and then
decreased over time to 6.5 ˚C on day 20.
Analyses
Sediment redox potentials were measured using a platinum Eh-electrode (P 10401, Radiometer A/S) in con-
hydr3811.tex; 7/06/1998; 17:00; v.5; p.2
57
junction with a calomel reference electrode (K 401,
Radiometer A/S). Measurements were performed at
the sediment surface and at 0.5, 1, 1.5, 2, 3, 4 and 5 cm
depth. Water samples were collected on days 0, 1, 2, 4,
7, 12, 15 and 20 from the experimental systems chosen
for sediment analyses (2 replicates of inflowing and 4
for outflowing water).
Heat production measurements were performed on
subsamples from the 0–0.5 cm layer using a fourchannel heat conduction multichannel microcalorimeter (Thermal Activity Monitor 2277, Thermometric).
Sample treatment and measurements were performed
according to Boström and Törnblom (1990). All heat
production measurements were performed at 12 ˚C.
Heat production at in situ temperature was estimated assuming a Q10 -value of 3 according to Törnblom
(1996). Heat production was converted to carbon units
assuming RQ = 1 and using the aerobic catabolism
of glucose as a model process assuming k H˚O2 = 469 kJ mol 1 O2 (Gnaiger & Kemp, 1990).
Incubations of sediment samples with [3H]thymidine
were performed at the ambient water temperature, in
duplicates plus additional formaldehyde killed controls, according to Bell and Ahlgren (1987). 0.2 g
of sediment and 0.2 ml seston suspension (only on
day 0) was incubated in darkness was 20 Ci of
[3 H]thymidine (45–55 Ci mmol 1 , Amersham) for
30 min. Additional [3 H]thymidine-incubations were
performed at 9 ˚C on days 12 and 20 due to the temperature decrease during the last week of the experiment. The degree of participation of [3 H]thymidine
was determined from isotope dilution experiments performed on day 4 according to Pollard and Moriarty
1984. An average degree of participation of 46 5%
was found in both the seston treatment and the control.
Bacterial production was calculated using a theoretical
conversion factor of 0.44 1018 cells mol 1 assuming
a DNA content of 2.5 fg per cell (Fuhrman and Azam,
1980) and corrected for the average degree of participation. The conversion of [3 H]thymidine incorporation
rates into carbon units was made by applying the factor
2.2 10 13 g C m 3 (Bratbak & Dundas, 1984).
Bacterial numbers and biomass were determined
on sediment and seston samples preserved with filtersterilized (0.2 m) formaldehyde (final concentration
4%). Epifluorescense microscopy (Nikon filters Combination B-2A [Dichroic mirror 510, Excitation filter
450–490 and Barrier filter 520]) was used after sonication in an ice bath at 100 W for 1 min, dilution
(final concentration of 0.005%), staining with acridine
orange (Merck) and filtration onto 0.2 m prestained
(Sudan black (Sigma)) Nucleopore filters. 400 cells
in at least 20 random fields were counted. Average
bacterial volumes were calculated from size measurements by eyepiece graticule of a least 100 cells from
each sample. Bacterial biomass was converted to carbon units by applying the factor 2.2 10 13 g m 3
of C (Bratbak & Dundas, 1984). Chlorophyll a in
sediments and seston was analysed spectrophotometrically after freezing, freeze-drying and extraction with
ethanol according to Hansson (1988).
Sediment water content was determined after drying at 105 ˚C for 24 h. Sediment densities used
in transformations into volume units were estimated using the equation: sediment density = 1.6174 0.0062 sediment water content (established for surface sediments at the sampling site). C- and Nconcentrations were determined on dry sediment using
a CHN-Elemental Analyser (Carlo-Erba). Tot-P of
sediment and seston was analysed as phosphate after
acid hydrolysis at high temperature (340 ˚C) followed
by Murphy and Riley procedure (1962). Sediment Pfractionation was performed using a modified Hieltjes
and Lijklema (1980) scheme (Figure 1). In addition to
the ordinary extraction scheme, digestion step was performed on the NaOH extractable P fraction in order to
separate it into a reactive and a non-reactive form. Altogether, sediment P was separated into the following
fractions (extractants and conditions are given within
parenthesis): (a) loosely bound or labile P (1 M NH4 Cl
at pH 7), (b) Fe- and Al-bound P (0.1 M NaOH) as
reactive P, and non-reactive P after digestion of the
extract (representing e.g. easily degradable organic P).
By subtracting the concentration of NaOH-rP (reactive
P) from NaOH-Tot P, NaOH-nrP (non reactive P) is
determined (Furumai and Ohgaki, 1982), (c) Ca-bound
P (0.5 M HCl) and (d) residual P (calculated by subtracting the sum of extracted P from the Tot-P, mainly
consisting of refractory org-P but also including the
inert P fraction). Extractions were made on the seston
suspension on day 0 and on wet sediment immediately
after sectioning of cores. Cpart and Npart , were captured
on precombusted glass fibre filters (Whatman GF/C)
and analysed using a CHN-Elemental Analyser (CarloErba). O-P was determined according to Murphy and
Riley (1962). P in the overlying water is expressed as
the net sediment retention (the difference between outand inflowing water).
hydr3811.tex; 7/06/1998; 17:00; v.5; p.3
58
Figure 1. The modified Hieltjes & Lijklema (1980) scheme used for phosphorus fractionation, including a digestion step to distinguish reactive
P (rP) and non-reactive P (nrP) in NaOH extracted P. Subtracting NaOH-rP from NaOH-Tot P gives NaOH-nrP.
Table 1. Initial (day 0) values of chlorophyll a, bacterial biomass (B.B.),
mean bacterial volume (MCV) and bacterial production (B.P.) in seston
and in the 0–0.5 cm sediment layer (values are means 1 s.d. of 4
replicate samples and cores).
Chl. a
(mg m
Seston
146
Sediment 89.2
2)
B.B.
(g m
2)
10.4 9.56 1.57
5.80 7.90 0.75
MCV
(m3 )
0.239
0.165
0.024
0.006
B.P.
(mg C m
54.61
22.97
2
d
12.54
6.06
Characterization of seston and sediment
The seston was dominated by large diatoms, a majority
(> 80%) was alive and in good condition (intact cells
with chloroplasts). Especially abundant were Aulacoseira islandica (Grun) O. Müller, Fragilaria crotonensis Kitt. and Asterionella formosa Hass. Seston
additions corresponded to an input of 17.9 2.3 g C,
2.52 0.30 g N and 0.31 0.03 g P m 2 ( SD, n = 4).
The chlorophyll a content of the added seston was 1.6
times higher than in the sediment (Table 1) or corresponding to the total sedimentation of seston in 13 m
water depth with a mean chlorophyll a concentration
of 11 g l 1 . Bacterial biomass was approximately
equal in seston and surface sediments. The mean bacterial cell size, however, was significantly higher in
the seston suspension (Mann-Whitney U-test p< 0.05)
owing to the occurrence of large rod-shaped bacteria.
Bacterial carbon in the seston suspension constituted
1)
Table 2. Temperature, redox potential (Eh) at 0.5 cm sediment depth
and sediment water content of the 0–0.5 cm sediment layer in seston
cores and controls (values are means 1 s.d. of 4 replicate cores).
Day Temp. Eh, seston Eh, control Water content Water content
(˚C) cores (mV) cores (mV) seston cores control cores
(%)
(%)
0
1
2
4
7
12
20
9.1
9.0
9.0
9.0
9.0
7.2
6.5
324
260
190
324
298
272
22
33
47
15
21
36
321
294
228
315
320
305
307
36
15
102
43
12
28
25
94.70
94.21
95.52
95.79
96.04
95.64
0.84
0.62
0.58
0.86
0.19
0.55
93.82
93.59
93.86
94.61
94.96
94.26
94.88
0.40
0.56
0.63
0.50
1.37
0.44
0.29
ca. 10% of the total seston C. Bacterial production was
approximately 2.4 times higher in the seston than in
the original 0–0.5 cm sediment layer.
Results
Seston additions caused an immediate increase in bacterial production, cell-specific bacterial activity and
total sediment metabolism (Figure 2). On day 1, bacterial production was 4.5 times higher in cores with seston
addition than in control cores. Bacterial productivity at
in situ temperature remained high the first week and
then decreased linearly to approximately the same level
as in the control cores on day 20. Cell-specific bacterial
hydr3811.tex; 7/06/1998; 17:00; v.5; p.4
59
Figure 3. Temporal changes in (a) bacterial biomass (open boxes)
and the NaOH-nrP phosphorus fraction (filled boxes) and (b) in the
NH4 Cl phosphorus fraction (open dots) in the 0–0.5 cm sediment
layers. Results are expressed as the differences between seston cores
and controls (values are means 1 S.D. of 4 replicate cores). Results
are expressed per g dry sediment. Also indicated is the C:P ratio
(filled circles) calculated from bacterial biomass C and P in the
NaOH-nrP fraction.
Figure 2. Temporal changes in (a) bacterial production, (b) bacterial
specific growth rate and (c) heat production in the 0–0.5 cm sediment
layers in seston cores (open symbols) and controls (filled symbols).
Dots show bacterial production at 9 ˚C on days 12 and 20. All values
are means of 4 replicate cores 1 S.D.
activity (specific growth rates) exhibited a slightly different pattern with maximum rates on day 12. Bacterial
production at 9 ˚C (additional incubations performed
due to decreasing experimental temperatures) on days
12 and 20 was low compared to values obtained at the
same temperature during the first week of the experiment. Sediment heat production integrated over the
experiment and assuming no diurnal variation could
explain the oxidation of 3.20 and 1.33 g m 2 of C in
the treatments with seston additions and in controls,
respectively. Consequently, approximately 1.9 g m 2
of C or 11% of the Csest originally added was oxidized.
The rapid increase in bacterial and overall sediment
activity (sediment heat production) caused an increase
in O2 consumption as shown by lower O2 concentrations in the overlying water on days 2, 4 and 7 (50–70%
saturation in seston cores and 75–85% in the controls).
A small decrease in sediment redox potentials was also
observed in both seston and control cores during the
first days of the experiment (Table 2). During the last
two weeks of the experiment, however, redox potentials at 0.5 cm depth varied around + 300 mV, indicating aerobic conditions in the top 5 mm of the sediment.
Redox potentials decreased slowly with increasing sediment depth. The + 200 mV redox cline was found at
approximately 1 cm sediment depth and the + 100 mV
cline at 7 cm sediment depth in both seston and control
cores.
Bacterial biomass in the sediment approximately
doubled after seston additions (Table 3), remained high
until day 7 and then decreased by a factor 2 to day
20. The decrease in biomass between day 7 and 12
was caused by a decrease in bacterial abundance, as
indicated by more or less constant mean bacterial volume over the first 12 days of the experiment (Table
3). Between day 12 and 20, however, the decrease in
bacterial biomass can mainly be explained by a shift
towards smaller cell sizes, shown as a decrease in volume from 0.24 to 0.17 m3 . A similar drop in bacterial
hydr3811.tex; 7/06/1998; 17:00; v.5; p.5
60
Table 3. Changes in bacterial biomass and mean bacterial cell volume in the 0–0.5 cm
sediment layer in seston cores and controls (values are means 1 s.d. of 4 replicate cores).
Day
Bacterial biomass (g m 2 )
Seston cores
Control cores
0
1
2
4
7
12
20
17.5
21.4
17.3
21.2
22.3
14.7
11.1
1.74
3.99
5.60
5.56
3.48
3.55
1.02
Mean bacterial cell volume (m3 )
Seston cores
Control cores
7.90 0.75
10.5 1.32
9.10 1.75
8.39 1.17
8.91 0.93
6.79 1.22
4.75 1.23
0.202
0.210
0.197
0.205
0.213
0.242
0.170
0.025
0.022
0.006
0.019
0.029
0.014
0.020
0.165
0.185
0.175
0.178
0.190
0.192
0.147
0.006
0.017
0.017
0.010
0.008
0.011
0.023
Table 4. The distribution of the different phosphorus pools in the added seston and in seston cores and contols
(values are means 1 s.d. of 4 replicate cores, all values are expressed as g P/g D.W.) (Day 0 values in seston
cores were caluclated as the weighted sum of sediment in control cores and the added seston).
Day
NH4 Cl-P
(g P/g)
NaOH-rP
(g P/g)
NaOH-nrP
(g P/g)
HCl-P
(g P/g)
Res-P
(g P/g)
Tot-P
(g P/g)
Seston
Seston cores
0
2
4
7
12
20
Control cores
0
2
4
7
12
20
260
1
260
2
1120
49
160
1
180
1
2000
178
172
97
219
150
115
154
9
33
42
25
16
55
497
497
625
540
587
629
21
15
86
60
52
235
863
641
774
800
795
806
23
15
76
92
96
111
297
350
395
380
412
368
31
16
44
24
15
29
312
526
379
545
416
661
63
274
130
121
209
132
2148
2110
2393
2415
2325
2618
78
225
146
94
214
322
117
85
108
90
103
93
15
25
30
19
13
6
645
612
575
562
750
574
34
123
127
51
113
2
702
541
553
522
627
650
23
90
134
34
59
69
382
361
358
402
405
381
51
27
99
11
36
15
394
746
546
556
452
761
103
153
197
137
94
271
2240
2345
2140
2133
2338
2460
28
67
237
172
99
306
biomass and volume was observed between day 12 and
20 in the control cores.
The NaOH-nrP and NH4 Cl-P fractions showed a
clear increase after seston additions (Table 4). The
NaOH-nrP fraction in the added seston suspension,
alone constituted more than 50% of the Psest . A significant relationship was found between mean values of the
NaOH-nrP fraction and bacterial biomass expressed
as the difference between seston supplied and control cores (linear regression, r2 = 0.858, p = 0.008).
Expressed over time, both the NaOH-nrP fraction and
bacterial biomass approximately doubled between day
0 and 7 and then decreased to the initial level by day 12
(Figure 3a). Assuming that 50% of the NaOH-nrP frac-
tion represented bacterial poly-P (Hupfer et al., 1995),
a bacterial C:P ratio (bacterial biomass in carbon unit:P
in the NaOH-nrP fraction) was calculated on a weight
basis. This C:P ratio peaked on day 2 (36:1) and then
decreased almost linearly to approximately 20:1 on
day 20 (Figure 3a). The NH4 Cl-P fraction (representing labile P in the sediment) also exhibited significant
changes during the experiment (Figure 3b). Between
day 0 and 4 this P fraction approximately doubled and
then quickly decreased from 110 g g 1 on day 4 to
approximately 10 g g 1 on day 12.
All phosphate in the overlying water of both seston
supplied and control cores was trapped in the sediments
during the first week of the experiment and released
hydr3811.tex; 7/06/1998; 17:00; v.5; p.6
61
Figure 4. Mean net sediment retention (mg m 2 d 1 ) of P, (mean
and 1 S.D. of 4 replicate cores) in seston cores (open bars) and
controls (filled bars) and the amounts of P (open circles) supplied
through the inflowing water during the different sampling intervals.
from day 12 on (Figure 4). Integrated over the whole
experiment, the net P-release from the sediments corresponded to a release of 14 and 9 mg m 2 for the
treatments with and without seston additions, respectively. Compared to the P-supply of 21 mg m 2 with
the inflowing water, however, a P-fixation occurred in
both treatments.
Discussion
Sediment bacteria have been suggested to play an
important role in the uptake, storage and release of
P by sediments through the incorporation into and
release from intracellular poly-P granules (Gächter et
al., 1988; Uhlmann & Bauer, 1988). This suggestion
is supported by the recent discovery of bacterial polyP in lake sediments by Hupfer et al. (1995) and that
bacterial poly-P was found to constitute between 31
and 50% of the NaOH-nrP sediment P fraction. The
NaOH extraction step used in our and the above mentioned studies, will also when present, extract phytate (de Groot & Golterman, 1993; Golterman et al.,
1998), a stable cyclic P-compound probably associated with bacterial metabolism. The NaOH-nrP fraction may therefore contain an unknown portion of
phytate-P. However, P bound in sediment bacteria may
be released through metabolic reactions, extracellular reactions and lysis of bacterial cells in response to
changing environmental conditions, for example shift
towards anoxia (Boström et al., 1988) and should therefore constitute an important and potentially mobile P
pool in sediments with a large NaOH-nrP fraction like
the ones investigated in this study.
The most noticeable result in this study is the
close coupling and the concomitant changes in bacterial activity, bacterial biomass, the NaOH-nrP and
labile P pools and the sediment uptake and subsequent release of phosphate as a result of the deposition
of seston on the sediment surface. Altogether, these
results illustrate an important role of bacteria in the
P-regeneration in the oxic surface layers of the sediment and in the P-exchange between sediment and
water. P-fractionations, normally performed to characterize sediments regarding the composition of the different P-pools were used to investigate possible shortterm changes within these pools. The results show that
significant changes in some sediment P-pools can be
detected within a time scale as short as one or a few
days.
C:P ratios of sediment bacteria, calculated from
bacterial biomass carbon and the NaOH-nrP fraction
assuming a 50% bacterial P content, decreased from
36 on day 2 to approximately 20 by the end of the
experiment, indicating an active bacterial uptake and
storage of P. In spite of the rough estimate of bacterial
P, the ratios are in agreement with literature values of
18.4 (Fenchel & Blackburn, 1979), 14.9 (Gächter et al.,
1988) and the 3 to 29 with a median value of 11 found
by Vadstein et al. (1988). Even though the estimated
bacterial C:P ratios were within the range described in
literature they may be understimates, since algal polyP and other organic P-forms in the seston suspension
may have been included in the NaOH-nrP fraction. The
large size of this fraction (approximately 50% of the
Tot-P) in the seston may indicate that non-bacterial
P forms were included, but could also be explained
by low C:P ratios of the seston-associated bacteria.
Moreover, the P fractionation techniques using NaOH
(0.1 M) early in the sequential extraction procedure,
in this study only preceded by NH4 Cl, have disadvantages. Org-P may to some extent be hydrolyzed by
NaOH (Golterman, 1988), resulting in an overestimation of NaOH-rP on behalf of Org-P (NaOH-nrP).
Bacterial mortality was high in both seston supplied and control cores. This was shown as a decrease
in bacterial biomass by a factor two between day 7 and
20. This increased bacterial mortality may directly and
indirectly have been responsible for the observed Prelease from the sediments. Firstly, a direct P-release
can be expected at bacterial lysis (Boström et al., 1988).
hydr3811.tex; 7/06/1998; 17:00; v.5; p.7
62
Secondly, the diminishing bacterial community should
have a decreasing ability of to take up and store and
P liberated from the diatoms and the underlying sediment. A redox-controlled P-release from the sediment
is less probable since oxic conditions were prevailing
during the last two weeks of the experiment. A diffusion of P from deeper sediment layer is probable since
the + 200 mV redox cline was at approximately 1 cm
depth. A diffusion of P from deeper sediment layer may
be an explanation to the tendency of increasing Tot-P
observed over the experiment. This should, however,
not affect our assumptions of an active role of bacteria
in the regeneration of P.
The uptake and release of P in control cores was
similar to that in seston supplied cores but less pronounced. Also bacterial biomass and production in the
control exhibited very similar but less pronounced patterns as in the seston treatment. These results can most
probably be explained by the fact that the sediment
cores were collected at the end of the autumn diatom
bloom (the seasonal maximum in chlorophyll a in profundal sediments of the lake was recorded only a few
days later) and it has to be assumed that an unknown
amount of fresh seston was already deposited when
sediment cores were collected.
High numbers of large rod-shaped bacteria with
a high cell-specific activity in the seston suggest
an important role of these bacteria in the observed
increase in bacterial activity after seston addition. A
co-sedimentation of actively growing pelagic bacteria
with settling diatoms has been observed in Lake Erken
both during spring and autumn and have been suggested to play an important role in the high bacterial activity normally observed in the sediments in response to
sedimentation events (Goedkoop & Törnblom, 1996).
Some indirect evidence for the significance of cosettled bacteria in our study is given by the observations
that mean bacterial cell volumes were higher in the seston than in the sediments and that mean bacterial cell
volumes in the treatment with seston addition remained
high or even increased until day 12. An active role of
bacteria originating from the sediments before seston
additions cannot be ruled out, however, since the high
and increasing mean bacterial cell volumes observed
also could be explained by an increased growth of
these bacteria. Bacterial production was affected by
the decrease in temperature as shown by bacterial production measurements at a higher temperature (9 ˚C) on
days 12 and 20. The difference in production between
9 ˚C and the lower experimental temperature, however, was too small to explain the more than threefold
decline in bacterial production between days 7 and 20.
Since the cell-specific bacterial activity remained high
at least until day 12, the strong decrease in bacterial
biomass can most probably be explained by grazing by
meio- and microfauna and/or viral infection. A similar rapid decline in bacterial abundance and bacterial
activity in Lake Erken sediments has been observed
within days of the deposition of a spring diatom bloom
(Goedkoop & Törnblom, 1996).
Sediment heat production could explain the oxidation of ca. 11% of the Csest originally added and
a comparison between sediment heat productions and
bacterial production expressed in carbon units, suggests that the total activity was largely bacterial. For
comparison, Goedkoop & Johnson (1996) showed that
between 1.9 and 12.4% of the seston C deposited during spring was mineralized by bacteria during the same
period. Further comparisons with natural conditions
are difficult to make since the amount of seston added
was too large to mimic a natural diatom sedimentation
event.
To summarize, the microbial sediment community
responded quickly to the deposition of natural seston
in terms of increased activity and biomass. Changes
in bacterial activity and biomass were directly related to changes in the NaOH-nrP and labile P-pools in
the sediment and to the uptake and subsequent release
of phosphate by the sediments. Decreasing bacterial
C:P ratios over time indicated an active uptake and
storage of P by the bacterial community. Althogether,
our results imply that uptake and storage of P by sediment bacteria and/or bacteria introduced with seston
assemblages can be of importance in the P-turnover
following the sedimentation of natural seston.
Acknowledgements
We would like to acknowledge Anu Toom for assistance in P-fractionations, Stefan Djurström for help
in the construction of the experimental setup, Jan
Johansson for laboratory assistance, Kjell Hellström
for counting and measuring bacteria and Kurt Petterson
for providing space at the Erken Laboratory. This work
was supported by grant nr 13416 from the Swedish
Environmental Protection Agency.
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