Seasonal changes in microzooplankton feeding behavior under varying eutrophication level in the Bahía Blanca estuary (SW Atlantic Ocean)

Dilution experiments were conducted during a two-year survey in the Bahía Blanca estuary (SW Atlantic Ocean) to evaluate seasonal variations of microzooplankton grazing (m) and phytoplankton growth (μ). The trophic interactions between these groups were investigated in relation to anthropogenic eutrophication. For this purpose, two sites exposed differently to sewage input in the estuary were selected. The percentage of primary production grazed by microzooplankton averaged 79%, although this percentage showed high seasonal variability. The lowest consumer control was found in winter and summer, and was associated with high density of chain-forming diatoms. Conversely, the microzooplankton grazing exerted heavy control on phytoplankton biomass during spring and fall, in concurrence with the highest density of nanoflagellates. Results showed average differences between μ and m close to zero during most of the annual cycle, suggesting microzooplankton control of autotrophic prey in the two sites. Under high exposure to sewage, however, experimental results evidenced the prevalence of non-linear feeding response, poor trophic phasing at low phytoplankton growth rate and lower reactivity of microzooplankton grazing. Regardless their exposure to sewage input, microzooplankton was an important loss term of phytoplankton in the estuarine system and appear as a primary factor defining phytoplankton biomass accumulation.