Plankton community respiration along a nutrient gradient in a shallow Danish estuary

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⁻¹) 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⁻¹ chlorophyll a h⁻¹, 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.