dynamics of dissolved organic carbon in a coastal ecosystem

Dynamics of Dissolved Organic Carbon in a Coastal EcosystemAuthor(s): Ulla Li Zweifel, Johan Wikner, Ake Hagstrom, Erik Lundberg and Bosse NorrmanSource: Limnology and Oceanography, Vol. 40, No. 2 (Mar., 1995), pp. 299-305Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2838224 .

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Limnol. Oceanogr., 40(2), 1995, 299-305 ? 1995, by the American Society of Limnology and Oceanography, Inc.

Dynamics of dissolved organic carbon in a coastal ecosystem

Ulla Li Zweifel, 1 Johan Wikner, and Ake Hagstrom Department of Microbiology, University of Umea, 901 87 Umea, Sweden

Erik Lundberg and Bosse Norrman Ume'a Marine Sciences Center, POB 3124, 903 04 UmeA, 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 24-31% above the mean winter value (288 ,uM). At the offshore station DOC concentrations were elevated throughout the water column in July, reaching 14% above the mean winter value (291 ,uM). 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 Elom 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 al- most 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 Mac- kenzie 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 1981).

The Bothnian Sea (northern Baltic Sea) has a large annual influx of freshwater. From the extensive drainage area, large amounts of allochthonous DOM (dissolved

I To whom correspondence should be addressed. Acknowledgments

This study was supported by grants from the Swedish Natural Science Research Council (B-BU9054-306) and the Swedish Environmental Protection Agency (802-1091-91, 802-831-91, and 802-586-90).

The use of the facilities at Umea Marine Science Center is gratefully acknowledged.

organic matter) enter the sea via rivers (Wulff and Sti- gebrandt 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, 19058.1 '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 (1991) and the Marine Environmental Monitoring Pro- gram funded by the Swedish EPA. Primary data are stored in a database at the Ume'a Marine Science Center. Tem- perature and salinity were recorded with a Sensortec (for- merly Simtronix) UCM 40 Mk II probe. Flow data (monthly means) for the Ore River were obtained from the Swedish meteorological institute (SMHI, Norrk6- ping). This river is not used for hydroelectric power gen-

299



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 ana- lytical methods (Grasshoff et al. 1983).

Pelagic primary production -Primary production was determined with the '4C-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 ?3 Aquanta m-2 S-'.

Polycarbonate sample bottles (75 ml) were used and 120 Al (8 ,Ci) NaH'4C03 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 1-2 h. Incubation was terminated by adding 1/20 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-Am polycarbonate filters (MSI), after which the radioactivity was counted in a Beckman scintillation counter. Incorporation of label was converted to cell numbers with a conversion factor of 1.0 x 1018 cells mol'-I thymidine. The conversion factor has been exper- imentally determined with bacterial batch cultures grow- ing in seawater from station NB 1. The same conversion factor was used to calculate bacterial production at the offshore site because Heiniinen and Kuparinen (1992) found similar values offshore in the adjacent Baltic prop- er.

DOC determinations -Water samples were transferred to acid-rinsed polycarbonate or polypropylene bottles and subsequently passed through 0.2-Am Supor filters (Gel- man) using 20-ml disposable syringes (Terumo) and 25- mm polycarbonate filter holders (Sartorius), essentially as described elsewhere (Norrman 1993). The plasticware was acid washed in 1 M HCI and rinsed extensively in Milli-Q water. Samples (7.5 ml) were collected in polypropylene tubes (15 ml Falcon). The samples were im- mediately acidified with 100 Al of 1.2 M HCI 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 Al and the DOC concentration was calculated with the instrument software and a 4-point calibra- tion curve with potassium hydrogenphtalate as standard. Triplicate injections showed good precision with standard deviations of 0.1-1 %. Reference water from the Bothnian Sea (0.2-,m filtered and acidified water stored in a polycarbonate bottle in the dark) was measured on all analysis occasions (48 occasions from August 1991 to January 1992) showing a standard deviation of 2.6 AM around

the mean value of 335 AM. Blanks were in the 10-15 AM 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 supematant, 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 ini- tially to give an enrichment of 0.6 AM P04-P (Na2HPO4) and 2 AM NH4-N (NH4Cl). 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 AM. At station NB 1, higher values were found in the surface layer along with a large increase in DOC concentration in summer (Fig. lA). The mean DOC concentration in the water column (0-16 m) was calculated, giving a condensed view of the annual dynamics (Fig. 1 B). For three consecutive years the mean DOC concentration in winter was 288 AM. In early summer 1991, the increase in DOC concentration was 68 AM or 24% above the mean winter value, resulting in an increase of 1.1 mol C m-2 in the water column (Table 1). In 1992 the corresponding increase was 89 AM or 31% above the mean winter value, equivalent to 1.4 mol C m-2.

At station US 5b (monitored during 1991 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 AM or 14% above the mean winter value (291 AM), resulting in an increase of 2.7 mol C m-2 in the water column (0-66 m) (Fig. 2B, Table 2).

Links between DOC and river flow- Station NB 1 is in Ore bay, 5 km south of the Ore River outflow in the



Seasonal variation in DOC 301

Bothnian Sea. The monthly flow in this river was compared with the seasonal variation of DOC concentration in the bay (Fig. iB). In 1991 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 (0- 8 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 DOC and 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 ,uM P04, variation in DOC was low. At the offshore station, we also found a negative correlation between DOC and phosphate in the upper 0-16 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 consis- tently higher in nutrient-enriched cultures than in the controls (Fig. 5). In the controls, 8-21 ,uM DOC was degraded, whereas in the nitrogen- and phosphorus-enriched cultures 11-34 ,uM 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

?- 360)pu .32'tJ7It4 i:' 3t\2t7

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1 0 1 - Jun3902 Dec 90 Jun 91 Dec 91 Jun 928Dec092

0~~~~~~~~2

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128

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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 (0) in the water column at NB 1 (0-i16 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 P04 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 11-55% 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 Period of DOC increase decrease Annually

NB 1 1991* 1992t 1991t 1992? 1991 1992

Bacterial production 0.4 0.7 0.5 0.6 1.3 1.7 Bacterial respiration 1.2 1.9 1.4 1.7 3.6 4.7 Sedimentation 2.6 0.9 1.3 1.3 5.4 5.4 Primary production 1.3 1.0 1.4 0.9 4.2 4.0 DOC pool change +1.1 +1.4 -1.1 -1.4 0 0 * 25 April-28 June. t 22 April-30 June. : 29 June-i 9 September. ? 1 July-22 September.



302 Zweifel et al.

0- A \ 300v0

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Fig. 2. A. As Fig. IA, but at offshore station US 5b. B, C. As Fig. IB, C but at US 5b (0-100l m).

similar to but somewhat lower than that found in oceanic waters (Kirchman et al. 1991).

Pelagic carbon fixation 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 (0-14 m in the Ore estuary and 0-20 m in the Bothnian Sea basin), and bacterial carbon demand was integrated over 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 1991 and 4.0 in 1992. The bacterial production during 1991 ranged between 2 x 107 and 6 x 108 cells liter-1 d-l during the time of DOC increase. In 1992, the corresponding numbers were 1 X 107 to 7 x 108 cells liter-1 d-1. Bacterial production and respiration were calculated using a bacterial carbon content of 32 fg cell-1 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 m-2 in 1991 and 6.4 in 1992; the carbon

Table 2. Carbon balance at station US 5b. All numbers given in mol C m-2.

Period of Period of DOC DOC

increase decrease Annually US 5b 1991* 1991t 1991

Bacterial production 0.5 0.6 1.8 Bacterial respiration 1.4 1.7 4.9 Sedimentation 0.8 0.2 2.0 Primary production 3.5 2.6 7.7 DOC pool change +2.7 -2.7 0 * 25 April-28 June. t 29 June-19 September.

sedimentation from the trophic layer was 5.4 mol C m-2 for both years.

At the offshore station, carbon fixation was consider- ably higher with an annual value of 7.7 mol C m-2. Bac- terial C demand was 6.7 mol C m-2, and sedimentation was 2.0 mol C m-2.

Discussion

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 Ore 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-

375 -

350- u~~~~~~

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0 300-

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250- 0 20 40 60 80 100 120 140 160

Flow (m3 s-1)

Fig. 3. Mean DOC concentration at NB 1 vs. time-adjusted flow in the Ore River. The best-fit linear regression is shown.




500-

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P04 (pM P)

Fig. 4. Plot of discrete measurements of DOC vs. P04 at stations NB 1 (A) and US 5b (B).

concentration of the potential freshwater source. Assum- ing a 0%oo salinity of input water and conservative mixing, we calculated the concentration of input DOC to be on average 930 uM (SD?261,gM). This estimate is in good agreement with measured TOC (total organic C) concentration in the Ore River, which ranges between 670 and 1,250uM 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 Hagstrom 1991). Af- ter 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

400

375 -

350-

325-

0

275-

May Jun Jul Aug Sep

Fig. 5. Degradation of DOC by marine bacteria in predator- free seawater batch cultures (1992): O-in situ DOC concentration at NB 1 4 m (start of cultures); *-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 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 1991 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 (10-50%) and bacterial C content (20-50 fg C cell-'), 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 1991 and 1992.

Table 3. Effects of nutrient additions on the consumption of DOC and bacterial growth rates in seawater cultures at station NB 1 (1992).

DOC consumption Growth rate % stimulation

Temp. (,uM C)* (h- l) of growth rate t (?C) Control P N + P Control P N + P P N + P

4 May 4 8 10 11 0.043 0.061 0.063 44 48 21 May 4 13 15 17 0.055 0.080 0.080 44 45 17 Jun 8 21 28 34 0.084 0.115 0.121 37 43 30 Jun 15 10 18 21 0.073 0.112 0.119 53 63 21 Jul 15 11 15 21 0.078 0.119 0.129 53 66 11 Aug 15 10 12 16 0.067 0.096 0.102 45 54 * DOC consumption over 4-7-d incubations. t [(Growth rate in enriched culture - growth rate in control)/growth 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 even- ly 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 32-190% of the C demand. When comparing the two stations in 1991, 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 riverine 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 2.5- month 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 Ore River, in addition to the correlation between DOC and salinity, indicates that the buildup and disap- pearance 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 (11-55% 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 1.1 uM C liter-1 d-1, and 11-34 ,uM 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 ,uM C and US Sb = 291 uM 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 uM DOC (present all year) that is stale (nondegradable) and that the DOC consumed in the cultures is a fraction of the DOC that ac- cumulates 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 en- hanced the degradation of DOC. This suggests that low P04 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 uM and US Sb = 0.12 uM). At US Sb, however, the deep-water phosphate concentration (20- 100 m) was, on average, 0.26 uM (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 1991 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



dynamics of dissolved organic carbon in a coastal ecosystem

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