Responses of bacterioplankton and phytoplankton to organic carbon and inorganic nutrient additions in contrasting oceanic ecosystems

Experiments were carried out on Georges Bank, a productive coastal region i n the northwestern sector of the North Atlantic Ocean, and in the oligotrop hic western Sargasso Sea to examine the effects of nutrient (inorganic nitr ogen and phosphorus) and organic carbon (glucose) additions on bacterial an d phytoplankton growth. Four experiments were conducted in each environment . Phytoplankton growth was monitored over a 36 h period by following change s in the concentration of chlorophyll in unfiltered seawater and in seawate r prefiltered through 5 pm screening to reduce grazing pressure. Bacterial production was estimated initially and after 24 h using the H-3-thymidine ( TdR) method in unfiltered seawater and in 1 pm filtrate. Phytoplankton biom ass increased significantly in response to nutrient additions in all but 1 experiment, whereas chlorophyll concentrations remained unchanged or decrea sed in all of the unamended (control) treatments or treatments supplemented with glucose. Responses of the phytoplankton community were similar for th e <5 um and unfiltered treatments. Bacterial production increased after 24 h in all of the treatments on Georges Bank, and there was Little effect of nutrient or glucose addition in unfiltered seawater relative to unamended c ontrols. However, glucose addition to the <1 lm filtrate caused substantial increases in bacterial production relative to controls and N/P-amended tre atments in 2 of the experiments from this environment. Glucose had no stimu latory effect (relative to unamended treatments) in 3 of the 4 Sargasso Sea experiments; and only a marginal effect in the fourth. However, the additi on of inorganic nitrogen and phosphorus in the latter ecosystem resulted in higher bacterial production (relative to unamended treatments or glucose a ddition) in 2 of the experiments with unfiltered seawater, and very large i ncreases in 3 of the experiments with 1 pm filtrate. The magnitude of the c hanges in bacterial production differed greatly between unfiltered and filt ered seawater in both ecosystems, indicating an important role for bacteria l grazers in controlling bacterial population growth. The results of this s tudy indicate different nutritional restraints on bacterial production in t hese contrasting environments.