Plankton community respiration was examined in a shallow, very eutrophic Danish estuary during April to September. Maximum rates (3.98 g O2 m-3 d-l) were measured during the phytoplankton spring bloom in the most eutrophicated inner parts. Community respiration was significantly related to biomass and gross productivity of phytoplankton and net production of bacterioplankton. The relationship was weak to temperature and bacterial biomass. Regression analysis suggested that phytoplankton and bacteria, on average, accounted for 64 to 83 % of the community respiration at the 2 most eutrophic localities. The regressions also provided estimates of mean phytoplankton respiration (0.76 mg O2 mg-l chlorophyll a h-', 6.0 % of gross productivity), mean bacterial respiration (1.4 times net production) and mean bacterial growth yield (42 %). Pelagic respiration exceeded benthic respiration and became proportionally larger with increasing eutrophication and phytoplankton productivity in the inner parts of the estuary. These results stress the importance of phytoplankton blooms for pelagic respiration, overall oxygen balances and oxygen depletion phenomena.

The abundance and relative importance of picoalgae (autotrophic picoplankton, APP) were surveyed in Danish marine waters during summer stratification to test the hypothesis of high APP importance under oligotrophy. A picoalgal community dominated by Synechococcus was found at 28 of 29 stations visited. Abundance in the mixed surface layers ranged from 6 X lo6 cells 1-I in the eutrophic German Bight to 4.5 X lo8 cells l 1 in the Baltic. Differences in cell sizes among different areas were also found. Total APP biovolumes and < 2 pm fractionated chlorophyll concentrations showed no relationship; consequently the APP biomass as indicated by chlorophyll was calculated from the biovolume estimates. High relative APP importance was negatively related to total chlorophyll, A power function with an exponent close to -1 showed the best model of the relative importance. However, an analysis of the APP chlorophyll values in the mixed surface waters showed a constant biomass of 0.5 to 0.6 pg chl 1 1 , although with a trend for higher values in the Baltic and lower values in the German Bight. Thus, picoalgal biomass could be considered constant over the entire range of total chlorophyll, and increases in total autotrophic biomass were mainly due to increases in the nanoplankton

We measured phytoplankton and bacterial biomass and production at weekly intervals during summer in water samples from 2 sites in the shallow, eutrophic Roskilde Fjord, Denmark. In addition we measured Oz-uptake on unfiltered water and size-fractions < 100 pm and < 1 pm. Phytoplankton gross production in the water column was 6.2 g O2 m 2 d ' at Stn 1 and was balanced by pelagic community respiration (Rc, 3.8 g 0; m 2 d ' ) and sediment respiration (2.5 g 0; m"' d-l).Phytoplankton gross respiration (3.0 g O2 m 2 d l ) was temporarily exceeded by pelagic community respiration (3.1 g O2 m 2 d ' ) plus sediment respiration (2.0 g 0; m 2 d l ) at Stn 2 where there is additional production by Littoral plant communities. Phytoplankters and bacteria together accounted for 72 to 85 % of Re and zooplankters for the remainder Phytoplankters respired a large proportion (ca 30%) of their gross production and were apparently mainly grazed by benthic suspension feeders. Phytoplankters dominated pelagic respiration (50% of Re) at Stn 1, which has most phytoplankton, while bacteria dominated (44 % of Re) at Stn 2. We ascribe the relatively larger respiratory activity of bacteria at Stn 2 to additional supply of organic matter from littoral plant communities and frequent sediment resuspension. The importance of bacteria in the pelagic food web was supported by other findings. Bacterial biomass approached phytoplankton carbon biomass at Stn 2 and bacterial net production in the water column was 10 % (Stn 1) and 35 " % (Stn 2) of phytoplankton gross production. Bacterial net production and respiration were linearly related in the bacterial size-fraction (< 1 pm) with a bacterial growth yield of 47 %. We argue that the conversion factors applied to calculate bacterial net production and the mean growth yield attained are reasonable values considering the other measurements of pelagic carbon pools and processes.