Limnol.
hemop.,
40(2),
1995, 299-305
0 1995, by the American Society of Limnology and Oceanography, Inc.
Dynamics of dissolved organic carbon in a coastal ecosystem
U/la Li Zwe$el,l Johan Wikner, and ike Hagstriim
Department
of Microbiology,
University
of Umea, 90 1 87 Umea, Sweden
Erik Lundberg and Bosse Norrman
Umea Marine
Sciences Center, POB 3 124, 903 04 Ume%, Sweden
Abstract
.
In the Bothnian Sea, there was a marked seasonal variation of dissolved organic C (DOC) in 1990-1992,
with a large increase in DOC concentrations in summer at two stations. The accumulation of DOC at the
coastal station persisted for 5 months, reaching peak values 24-3 1% above the mean winter value (288 PM).
At the offshore station DOC concentrations were elevated throughout the water column in July, reaching
14% above the mean winter value (29 1 PM). The DOC concentration at the coastal station was significantly
correlated to water flow in an adjacent river, suggesting that the source of the summer DOC increase was
largely explained by riverine input. Bioassays indicated that a large portion (22-99%) of the introduced DOC
was degradable by bacteria after inorganic nutrients were added. A negative correlation between DOC and
phosphate concentration was also found, suggesting that the system was P deficient in summer. The accumulation of DOC in summer was thus possibly caused by slow bacterial degradation due to phosphate
deficiency and transient accumulation of refractory DOC. An annual C.balance at the coastal station indicated
an insufficient supply of C from phytoplankton
production to support the C demand of the system; at the
offshore station the budget was close to balanced. The results suggest that riverine DOC had a major impact
on coastal DOC dynamics and that it was partly used in the microbial food web in the bay.
The contribution
of organic C transported via rivers is
a potential energy source for the coastal ecosystem. In the
Elorn estuary, Aminot et al. (1990) found a large seasonal
variation in DOC (dissolved organic C) concentrations
with minimum concentration in spring and maximum at
midsummer. The net flux of river DOC contributed almost equivalent amounts of organic C as primary production. Aminot et al. concluded that the summer accumulation of DOC was due to accumulation of refractory material; however, there is substantial evidence that
a fraction of terrestrially derived DOC is used in aquatic
environments
(Wetzel 1992). Comparisons of sedimentation with coastal respiration show that terrestrial C is
actively metabolized in coastal ecosystems (Berner 1982).
On a global scale, -50% of the land-derived C can be
respired annually in the coastal zone (Smith and Mackenzie 1987). Also, bioassays have shown that fluvial
DOC can be used by marine bacteria (Moran and Hodson
1990; Tranvik 1990). These findings argue against the
earlier view of land-derived organic C as a refractory pool
with low bioavailability
and turnover times of millennia
(Gagosian and Lee 198 1).
The Bothnian Sea (northern Baltic Sea) has a large annual influx of freshwater. From the extensive drainage
area, large amounts of allochthonous
DOM (dissolved
’ To whom correspondence
Acknowledgments
This study was supported
Science Research Council
Environmental
Protection
and 802-586-90).
The use of the facilities
gratefully acknowledged.
should be addressed.
by grants from the Swedish Natural
(B-BU9054-306)
and the Swedish
Agency (802- 109 l-9 1, 802-83 l-9 1,
at Umea Marine
Science Center is
299
organic matter) enter the sea via rivers (Wulff and Stigebrandt 1989). We investigated the annual dynamics of
DOC at a coastal and an offshore station in the Bothnian
Sea and monitored temporal variation in DOC concentration in the water column and measured primary production, bacterial production, inorganic nutrient concentrations, and sedimentation. We discuss the mechanism
for DOC accumulation.
Material
and methods
Sampling- Samples came from the Bothnian Sea, which
has a volume of about 4,340 km3 and an annual river
inflow of 105 km3 (Wulff and Stigebrandt 1989). The
percentage of direct freshwater input is -2%. Water samples were taken at coastal station NB 1 (63”30.5’N,
19’48.0’E) in the Ore bay (mean depth, 16 m); the water
depth at NB 1 is 25 m and salinity ranges from 2 to 6
(PSU). Samples were taken from eight depths with a polycarbonate water sampler; subsamples were taken for the
respective analyses. Samples were also taken at depths
from 0 to 225 m at offshore station US 5b (62”35.2’N,
19’58.1 ‘E) in the Bothnian basin (mean depth, 66 m). At
both stations, samples were collected about biweekly.
Sampling was performed as a part of two programs: the
Swedish-Finnish
joint assessment of the Gulf of Bothnia
(199 1) and the Marine Environmental
Monitoring
Program funded by the Swedish EPA. Primary data are stored
in a database at the Umea Marine Science Center. Temperature and salinity were recorded with a Sensortec (formerly Simtronix)
UCM $0 Mk II probe. Flow data
(monthly means) for the Ore River were obtained from
the Swedish meteorological
institute (SMHI, Norrkoping). This river is not used for hydroelectric power gen-
300
Zweifel et al.
eration and thus has a natural seasonal flow pattern, with
a pronounced spring flow peak. Nutrients were analyzed
with a Technicon T 800 autoanalyzer by standard analytical methods (Grasshoff et al. 1983).
Pelagic primary production-Primary
production was
determined with the 14C-uptake method (Parsons et al.
1984). Whole-water samples were taken at eight depths
between 0 and 14 m in the photic layer at the coastal
station and from 0 to 20 m at the offshore station. The
incoming light at these depths is 5 3 hquanta mu2 s-l.
Polycarbonate sample bottles (75 ml) were used and 120
hl(8 PCi) NaH14C03 was added to each bottle; the bottles
were subsequently incubated in situ for 3 h at the respective sampling depths. After incubation whole-water
samples were acidified and sparged with air. Radioactiv,
ity was counted in a Beckman scintillation
counter after
addition of LKB Highsafe II scintillant.
Determination of bacterial production-Bacterial
production was measured with the tritiated thymidine uptake
method (Fuhrman and Azam 1982). Samples (5 ml) were
incubated with 10 nM tritiated thymidine in glass vials
for l-2 h. Incubation was terminated by adding l/20
volume of 5 M NaOH. The cells were precipitated with
equal volumes of 10% ice-cold TCA, and the precipitate
was collected on 0.2~pm polycarbonate filters (MST), after
which the radioactivity
was counted in a Beckman scintillation counter. Incorporation
of label was converted to
cell numbers with a conversion factor of 1.O x 1Ols cells
mol-l thymidine. The conversion factor has been experimentally determined with bacterial batch cultures growing in seawater from station NB 1. The same conversion
factor was used to calculate bacterial production at the
offshore site because Heinanen and Kuparinen (1992)
found similar values offshore in the adjacent Baltic proper.
DOC determinations-Water samples were transferred
to acid-rinsed polycarbonate or polypropylene bottles and
subsequently passed through 0.2~pm Supor filters (Gelman) using 20-ml disposable syringes (Terumo) and 25mm polycarbonate filter holders (Sartorius), essentially
as described elsewhere (Norrman 1993). The plasticware
was acid washed in 1 M HCl and rinsed extensively in
Milli-Q water. Samples (7.5 ml) were collected in polypropylene tubes (15 ml Falcon). The samples were immediately acidified with 100 ~1 of 1.2 M HCl and kept
at + 4°C until analysis. DOC was measured in a Shimadzu
TOC 5000 analyzer. Two or three injections were made
for each sample with an auto injector. The injected volume was 150 ~1 and the DOC concentration was calculated with the instrument software and a 4-point calibration curve with potassium hydrogenphtalate as standard.
Triplicate injections showed good precision with standard
deviations of 0. I- 1%. Reference water from the Bothnian
Sea (0.2~pm filtered and acidified water stored in a polycarbonate bottle in the dark) was measured on all analysis
occasions (48 occasions from August 199 1 to January
1992) showing a standard deviation of 2.6 PM around
the mean value of 335 PM. Blanks were in the lo-15 PM
range and were not subtracted from the data.
Sedimentation-Sedimentation
was determined with
moored sedimentation
traps. Sedimented material was
collected in two polypropylene cylinders (104-mm diam)
that were gyroscopically attached to the buoy (Larsson et
al. 1986). At deployment, 2 ml of chloroform was added
to the cylinders to avoid bacterial degradation of the collected material. The traps were moored at 14-m depth at
NB 1 and 30-m depth at US 5b for periods of 2 weeks
during the productive season. The collected material was
allowed to settle at +4”C, transferred to centrifuge tubes,
and precipitated by centrifugation
at 17,700 x g. After
removal of the supernatant, the centrifuge vials were
weighed and the content lyophilized. The dry weight was
determined and the chemical composition analyzed with
a CHN analyzer (Carlo Erba 1106).
Seawater cultures- Bacterial batch cultures using seawater from station NB 1 were performed in summer 1992.
Predator-free batch cultures were inoculated with a mixed
marine bacterial assemblage. Cultures were prepared with
and without nutrient additions. P and N were added initially to give an enrichment of 0.6 PM P04-P (Na,HPO,)
and 2 PM NH4-N (NH,Cl). These concentrations were
always nonlimiting
for bacterial growth. Bacterial numbers, DOC concentration,
and inorganic nutrients were
monitored until stationary phase was reached (4-7 d).
Bacterial growth rates were estimated from the increase
in bacterial numbers during exponential growth phase.
The procedure has been described in detail elsewhere
(Zweifel et al. 1993).
Results
DOC measurements-The DOC concentration in the
water column of the Bothnian Sea ranged between 250
and 400 PM. At station NB 1, higher values were found
in the surface layer along with a large increase in DOC
concentration in summer (Fig. 1A). The mean DOC concentration in the water column (O-l 6 m) was calculated,
giving a condensed view of the annual dynamics (Fig.
LB). For three consecutive years the mean DOC concentration in winter was 288 PM. In early summer 199 1, the
increase in DOC concentration was 68 PM or 24% above
the mean winter value, resulting in an increase of 1.1 mol
C me2 in the water column (Table 1). In 1992 the corresponding increase was 89 PM or 3 1% above the mean
winter value, equivalent to 1.4 mol C m-2.
At station US 5b (monitored during 199 1 only), an
increase in DOC was found in summer, but with a shorter
duration than at station NB 1 and with high DOC concentrations throughout the water column (Fig. 2A). The
increase was 42 PM or 14% above the mean winter value
(291 PM), resulting in an increase of 2.7 mol C m-2 in
the water column (O-66 m) (Fig. 2B, Table 2).
Links between DOC and river flow-Station
&e
bay, 5 km south of the &e
River
NB 1 is in
outflow in the
301
Seasonal variation in DOC
Bothnian Sea. The monthly flow in this river was compared with the seasonal variation of DOC concentration
in the bay (Fig. 1B). In 199 1 and 1992, the seasonal
pattern of changes in DOC concentration covaried with
flow in the ore River. The monthly average of DOC
concentration
from June 1990 to December 1992 was
calculated and plotted against river flow the previous
month; significant correlation was found (r = 0.74, P <
0.001) (Fig. 3). In addition, the concentration of DOC
appeared to increase with decreasing surface salinity (O8 m) at the coastal station (r = -0.48, P < 0.001). At
the offshore station, the correlation between surface salinity and DOC was weaker (r = -0.39, P < 0.001).
Links between DOCand nutrient availability-The
seasonal variation in mean phosphate concentration in the
water column varied inversely with mean DOC concentrations at the coastal station, with low phosphate values
at the time of maximal DOC values (Fig. 1C). A significant negative correlation was found between discrete values of DOC and phosphate (r = -0.52, P < 0.001) (Fig.
4A). Above a concentration of -0.1 PM P04, variation
in DOC was low. At the offshore station, we also found
a negative correlation between DOC and phosphate in
the upper O-l 6 m but with a lower correlation coefficient
(r = -0.24, P < 0.04) (Figs. 2C, 4B). For integrated
nitrate, there was a significant negative correlation at both
stations, but the correlation was lower than for phosphate
(r = -0.36, P < 0.001 at NB 1 and r = -0.16, P < 0.03
at US 5b, data not shown). For ammonium, we found no
significant relationship.
Consumption of DOC in seawater cultures- Bacterial
seawater cultures were used to determine the fraction of
utilizable DOC. The degree of DOC consumption varied
during summer and consumption of DOC was consistently higher in nutrient-enriched
cultures than in the
controls (Fig. 5). In the controls, 8-21 PM DOC was
degraded, whereas in the nitrogen- and phosphorus-enriched cultures 1 l-34 PM DOC was consumed (Table 3).
Also, bacterial growth rates increased by 43-66% in nutrient-enriched
cultures. During winter, the DOC concentration remained constant, and thus we have chosen
0
5
25
375
120
u^
350
100
z
3.
c
325
80
3
n
300
60
w:
c
40
275
20
25C
0
0.2
F
.s
a
‘b
g
0.i
0.:
0.:
0.
)-I
(
Jun 90
Dee 91
Jun 91
Dee 90
Jun 92
Fig. 1. A. DOC concentration in the water column at coastal
station NB 1 1990-1992. Discrete samples indicated by dots.
B. Mean DOC concentration (9) in the water column at NB 1
(O-l 6 m) and flow (0) in the Ore River. Mean DOC concentrations were calculated by integrating discrete measurements
and then dividing the sum by the length of the water column.
Flow rates are monthly averages. C. Mean PO, concentrations
(calculated as in panel B) in the water column at NB 1.
the mean winter value as a reference when evaluating
bacterial utilization
of the DOC introduced in summer.
In the control cultures, the bacteria consumed 1 l-55%
of the DOC accumulated above the winter concentration,
and as much as 22-99% was consumed in nutrient-enriched cultures. The total amount of DOC consumed was
Table 1. Carbon balance at station NB 1. All numbers given in mol C m-2.
Period of DOC
increase
NB 1
Bacterial production
Bacterial respiration
Sedimentation
Primary production
DOC pool change
* 25 April-28 June.
t 22 April-30 June.
$29 June-19 September.
0 1 July-22 September.
Dee 92
Period of DOC
decrease
1991*
1992T
1991#
1992§
0.4
1.2
2.6
1.3
+1.1
0.7
1.9
0.9
1.0
+1.4
0.5
1.4
1.3
1.4
-1.1
0.6
1.7
1.3
0.9
-1.4
Annually
1991
1992
1.3
3.6
5.4
4.2
0
1.7
4.7
5.4
4.0
0
302
Zweifel et al.
cI-
Table 2. Carbon balance at station US 5b. All numbers given
in mol C m-2.
)-
Period of
DOC
increase
1991*
)-
US 5b
)-
Bacterial production
Bacterial respiration
Sedimentation
I-
I
25CI--
I
I
I
I
I
I
I
I
I
375
u^
350 l-
3
u”
325
Pi
300 l-
L
250 ,--
o.4
0.3
&
0.2
0.6
1.7
0.2
2.6
-2.7
1.8
4.9
2.0
7.7
0
sedimentation from the trophic layer was 5.4 mol C m-2
for both years.
At the offshore station, carbon fixation was considerably higher with an annual value of 7.7 mol C mm2. Bacterial C demand was 6.7 mol C m-2, and sedimentation
was 2.0 mol C m-2.
C
-
8
a”
‘b
Primary production
Annually
1991
* 25 April-28 June.
t 29 June-19 September.
275
0.5
0.5
1.4
0.8
3.5
+2.7
DOC pool change
B
Period of
DOC
decrease
1991t
Discussion
0.1
0I-_
Jun 90
Dee 90
Jun 91
Dee 91
Jun 92
Dee 92
Fig. 2. A. As Fig. 1A, but at offshore station US 5b. B, C.
As Fig. lB, C but at US 5b (O-100 m).
similar to but somewhat lower than that found in oceanic
waters (Kirchman et al. 1991).
Pelagic carbon Jixation and carbon demand-The pelagic carbon flux through the system was estimated and
compiled in Tables 1 and 2. To compare the coastal and
offshore station, we integrated primary production over
the euphotic zones for the two systems (O-14 m in the
Ore estuary and O-20 m in the Bothnian Sea basin), and
bacterial carbon demand was integrated ovec the mean
depth in the respective system (16 m in the Ore estuary
and 66 m in the Bothnian Sea basin). In addition to annual
values, the data are presented for two periods: the period
of DOC increase and the period of DOC decrease. The
annual primary production at the coastal station was 4.2
mol C m-2 in 199 1 and 4.0 in 1992. The bacterial production during 199 1 ranged between 2 x lo7 and 6 x
lo8 cells liter-l d-l during the time of DOC increase. In
1992, the corresponding numbers were 1 x lo7 to 7 x
1OScells liter- * d-r. Bacterial production and respiration
were calculated using a bacterial carbon content of 32 fg
cell-l and a growth efficiency of 27%. These numbers
were obtained from seawater cultures collected at the study
site in summer 1992 (Zweifel et al. 1993). The total bacterial C demand (biomass production + respiration) was
4.9 mol C mm2 in 199 1 and 6.4 in 1992; the carbon
Importance of riverine DOC input-At station NB 1,
we found marked annual variations in DOC concentrations with a pronounced increase in summer. The increase
in DOC could be due to in situ biological activity, input
from external sources, or both. Biological activity as a
dominant factor can be excluded because all the carbon
fixed in early summer would have to be released as DOC
without any concomitant uptake to explain the observed
increase in DOC (Table 1). Input of external C as a major
explanation of DOC variation was supported by the significant correlation between DOC and flow in the &e
River and the negative correlation between DOC and
salinity at NB 1. The decline in salinity and the increase
in DOC in summer can be used to calculate the DOC
400
3750
350G
0
3
u'
8
0
20
40
60
80
100
120
140
160
Flow (m3 s-l)
Fig. 3. Mean DOC concentration at NB 1 vs. time-adjusted
flow in the Ore River. The best-fit linear regression is shown.
Seasonal variation in DOC
303
400
375
3
3
i
n
350
325
300
250
450
G
2
Y
Ei
275
B
250
0
400
350
0
i
4.
300
.
25C
1
oh5
011
oh5
012
PO, w
o.i5
of the potential
c
5
p>
Fig. 4. Plot of discrete measurements
stations NB 1 (A) and US 5b (B).
concentration
013
source. Assumand conservative mixing,
of input DOC to be on
This estimate is in good
(total organic C) concen-
freshwater
ing a Oo/oo
salinity of input water
we calculated the concentration
average 930 PM (SD+26 1pM).
agreement with measured TOC
tration in the Ore River, which
ranges between 670 and
1,250 PM DOC over the year (Forsgren and Jansson 1993;
Ivarsson and Jansson 1994).
At the study site, bacterial production
and bacterial
predation are balanced (Wikner and Hagstriim 199 1). After flagellate grazing, the remaining particulate
organic C
(POC) represents < 15% of the carbon consumed by the
bacteria, assuming a bacterial and flagellate growth yield
of 30 and 50% (Caron et al. 1985; Zweifel et al. 1993).
As a first approximation,
total
bacterial
C demand
will
Temp.
(“C)
4May
21 May
17 Jun
30 Jun
21 Jul
11 Aug
4
4
8
15
15
15
8
13
21
10
11
10
10
15
28
18
15
12
11
17
34
21
21
16
SeP
of the respiration
in the system. With
this assumption, the carbon balance at the coastal station
was negative and primary production met only 41 and
34% of the C demand by bacteria and sedimentation in
199 1 and 1992. To validate this result, we used a known
range of input data and conservative estimates to compile
the carbon balance. If the carbon balance is calculated
with the range of values found in this area for growth
efficiency (1 O-50%) and bacterial C content (20-50 fg C
cell-l), the contribution
of primary production to the C
demand was 25-60% in 1991 and 20-54% in 1992.
Part of the sedimentation at the coastal station could
originate from resuspension or particles transported via
the Ore River and not from biological activity at the
station itself (Forsgren and Jansson 1993). However, if
the carbon balance is calculated with bacterial demand
as the only sink for C at NB 1, the budget will still be
negative and the primary production would explain 88
and 63% of the carbon demand in 199 1 and 1992.
Table 3. Effects of nutrient additions on the consumption
rates in seawater cultures at station NB 1 (1992).
DOC consumption
(PM C)*
Control
P
N + P
A%
Fig. 5. Degradation of DOC by marine bacteria in predatorfree seawater batch cultures (1992): O-in situ DOC concentration at NB 1 4 m (start of cultures); O-DOC
concentrations at
the end of the incubation (4-7 d) in control cultures (no nutrients
added); A-DOC
concentrations at the end of the incubation
(4-7 d) in cultures enriched with ammonium and phosphate;
dotted line- mean DOC concentration at station NB 1 in winter.
give a good estimate
of DOC vs. PO, at
Jul
Jun
May
of DOC and bacterial growth
Growth rate
(h-l)
Control
P
N+P
0.043
0.055
0.084
0.073
0.078
0.067
0.06 1
0.080
0.115
0.112
0.119
0.096
* DOC consumption over 4-7-d incubations.
t [(Growth rate in enriched culture - growth rate in control)/growth
0.063
0.080
0.121
0.119
0.129
0.102
% stimulation
of growth rate t
P
N+P
44
44
37
53
53
45
48
45
43
63
66
54
rate in control]
x 100.
304
Zweifel et al.
At the offshore station, the period of DOC increase was
shorter than at the coastal station and the influence of
riverine water was less obvious because the correlation
between salinity and DOC concentration was weaker. The
increase likely was not caused by biological activity, considering that the increase in DOC concentration was evenly spread throughout the water column with traces remaining in the deep water for a month. We suggest that
the increase may have been caused by resuspension from
sediments by advective mixing. C production and demand were obviously more balanced at the offshore station and the primary production met 89% of the total C
demand by bacteria and sedimentation. Varying the C
content and growth efficiency as in the balance for NB 1,
primary production could support 32-l 90% of the C demand. When comparing the two stations in 199 1, we
found that the bacterial production in the Ore estuary
was 27% lower than in the Bothnian Sea basin, while
primary production was 44% lower. This difference also
indicates that a C source other than primary production
provides supplementary energy at the coastal station.
Considering the data, we infer that phytoplankton production could neither explain the observed increase in
DOC concentration nor the carbon demand at the coastal
station, that the dynamic variation in the DOC pool at
the coastal station was primarily a consequence of Gverine inflow, and that input of riverine carbon to the Ore
estuary presumably supports a fraction of the carbon demand of heterotrophic bacteria in the bay.
Mechanisms for accumulating DOC-At
the coastal
station, the DOC concentration
increased over a 2.5month period and then decreased during the succeeding
2.5 months. If the input and degradation of DOC were
balanced there should be no observable variation in DOC
concentration over the year. The accumulation of DOC
may be caused by different mechanisms, of which we have
considered the following three alternatives: that the DOC
was refractory and the measured increase and decrease
were simply a consequence of mixing, that an essential
nutrient other than C was limiting the growth of heterotrophic bacteria and thereby delaying the degradation
process, and that the bacterial growth rate was temperature limited and too low to instantaneously consume the
excess C. Based on the data presented, we drew the following conclusions.
The coupling between DOC concentration and flow in
the &e River, in addition to the correlation between
DOC and salinity, indicates that the buildup and disappearance of DOC at NB 1 could be caused by mixing
processes. However, the results from the seawater cultures
suggested that a substantial fraction of the accumulated
DOC was usable (1 l-55% in controls, 22-99% in nutrient-enriched cultures). Seawater cultures give a measure
of the bioavailable DOC at any given time, but do not
elucidate the origin of the consumed C. At NB 1, the
average primary production in summer was 1.l PM C
liter - l d- I, and 1 l-34 PM DOC was available for growth
of heterotrophic bacteria at any time as measured by the
seawater culture bioassay. Therefore no more than a small
fraction of the consumed DOC in the seawater cultures
consisted of newly produced carbon; otherwise several
weeks of accumulated total primary production would be
present at all times. The winter concentration of DOC
was similar at both stations (NB 1 = 288 PM C and US
5b = 291 PM C). Hence, during the period of low production and low riverine input, a balance between DOC
input and slow degradation of recalcitrant DOC seems to
have been reached. In our calculations, we thus assumed
that there is a pool averaging 288 PM DOC (present all
year) that is stale (nondegradable) and that the DOC consumed in the cultures is a fraction of the DOC that accumulates in summer. Our assumption, however, may
lead to an overestimate of the fraction of degradable DOC
contributed by the rivers.
At the coastal station, phosphate is the least available
element for growth of heterotrophic bacteria, followed by
inorganic N and C (Zweifel et al. 1993). At both stations,
a significant negative correlation between DOC and phosphate was found with a higher degree of explanation at
the coastal station than at the offshore station (r = -0.52
compared with r = -0.24). Moreover, the results from
the seawater cultures showed that nutrient addition enhanced the degradation of DOC. This suggests that low
PO, concentration in summer may partly limit the heterotrophic use of DOC, at least at the coastal station. We
found no significant difference in phosphate concentration between the two stations in the 0-20-m layer (mean
concn, NB 1 = 0.10 PM and US 5b = 0.12 PM). At US
5b, however, the deep-water phosphate concentration (20100 m) was, on average, 0.26 PM (data not shown) and
phosphate may have become available to the surface layer
by mixing. Thus, the bacterial P limitation was possibly
less severe at the offshore station. Nitrogen dynamics may
also be important for DOC degradation, and we found a
significant negative correlation between DOC and nitrate;
however, the degree of explanation was lower than for
phosphate.
The accumulated DOC corresponded to 32% of the
bacterial C demand in summer at NB 1 in 199 1 and 28%
in 1992. If the accumulated DOC was usable, an increase
in bacterial production equivalent to these amounts would
allow consumption of the excess C. When nutrients were
added to the seawater cultures, the bacterial growth rates
increased 43-66% when incubated at in situ temperature.
This increase in growth rate indicates that there was no
temperature-dependent
delay in bacterial degradation of
DOC during nonlimiting
concentrations of inorganic nutrients.
In conclusion, we infer that the process of DOC accumulation at the coastal station was caused by partially
refractory properties of the riverine-introduced
DOC and
by inorganic nutrient limitations
delaying the bacterial
degradation process.
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Submitted: 27 January 1994
Accepted: 23 September 1994
Amended: 27 September 1994