Seawater cultures were used to study seasonal and diel variations in bacterial growth and nutrient availability. In both the Baltic Sea and the Northeast Mediterranean, the least available component for bacterial growth was phosphorus. In the Baltic Sea, carbon was available in excess for bacterial growth on all sampling occasions. Compared to the controls, additions of nonlimiting concentrations of inorganic nitrogen and phosphorus increased the yield of bacteria compared to the control with 156% and the degradation of dissolved organic carbon (DOC) by 64 % (as determined by high temperature catalytic oxidation). Analogous, bacterial growth yield increased along with an accumulation of inorganic nutrients in diel experiments with an intact foodweb (microcosm). The concentration of uthzable carbon (UC) was determined from bacterial consumption of DOC in seawater cultures during non-limiting nutrient conditions. Utilizable phosphorus (UP) and utilizable nitrogen (UN) were calculated by converting the bacterial biomass in the cultures into phosphorus and nitrogen equivalents.In the Baltic the average concentrations of UC, UN and UP were found to be 23 FM C. 0.6 pM N and 0.03 pM P respectively. Heterotrophic bacteria preferentially utilized inorganic nitrogen and phosphorus to support growth on a short time scale (days). Bacterial carbon content decreased as a result of nutrient additions from 51 k 7 to 32 f 5 fg C cell-' Growth efficiencies varied from 11 to 54 % in untreated cultures compared to 14 to 58% in cultures supplemented with nitrogen and phosphorus.

Stable isotope measurements of carbon were used to study the transfer of carbon from phytoplankton to bacteria during an experimental diatom bloom. A large-volume tank was filled with 900 liters of lo-pmfiltered seawater from Woods Hole Harbor, and N, P, and Si nutrients as well as 13C-enriched NaHCO, were added. This resulted in a high nutrient system with an isotope label of +85?& in the dissolved inorganic carbon pool. Algal growth depleted nutrients to near-zero concentrations midway through the 2-week experiment with little net dissolved organic C (DOC) accumulation; algal chlorophyll concentrations peaked at 38 pg liter-l near the time of nutrient depletion. Decline of the algal peak was associated with diatom sedimentation to the bottom of the tank and rapid accumulation of DOC. In 3 d, DOC increased from -130 to 250 PM C. The C : N ratio of this new DOC was >22 vs. 14 in background harbor water. 13C labeling showed that bacteria preferentially assimilated the newly produced DOC in both the rapid growth and postbloom phases of the experiment, while DOC measurements indicated that very little if any of the background DOC from Woods Hole Harbor was metabolized by bacteria during the experiment. Overall, the experiment showed rapid DOC release following algal blooms, selective use of newly produced DOC by bacteria, and that DOC released during blooms is not rapidly or completely turned over following algal blooms.Heterotrophic bacteria growth in aquatic ecosystems encounter a diverse suite of organic substrates of varying origin, chemical composition, and nutrient content. Some of these substrates are readily assimilated and incorporated into bacterial biomass, some are rapidly respired to CO2 and not incorporated into biomass (Ducklow et al. 1986), and others are extremely refractory and resistant to bacterial attack for thousands of years (Williams and Druffel 1987). Bulk dissolved organic C (DOC) measurements mostly reflect concentrations of refractory components. The ongoing slow metabolism of refractory substrates is unfortunately

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.

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