Microzooplankton grazing in the estuary of Mundaka, Spain, and its impact on phytoplankton distribution along the salinity gradient

Microzooplankton grazing impact (<200 mu m fraction) on phytoplankton along a salinity gradient in the Estuary of Mundaka (Bay of Biscay, Spain) was analyzed during summer (August 1990) using a serial dilution method. Both microzooplankton grazing rate and phytoplankton growth rate were high (mean g = 0.75 d(-1), mean k = 1.90 d(-1) respectively), and well correlated, denoting an optimization in the efficiency of energy transport through the pelagic food web. Microzooplankton herbivory represented a mean loss of 38% of initial standing stock of chlorophyll and 46% of potential phytoplankton primary production in the inner zone of the estuary, where changes in phytoplankton biomass were mainly a function of initial biomass. In the intermediate and outer zones, microzooplankton grazed around 54% of initial standing stock of chlorophyll and more than 60% of potential phytoplankton primary production. In these zones, changes in phytoplankton biomass were mainly a function of phytoplankton growth. In spite of the importance of microzooplankton grazing, this factor alone cannot be responsible for the observed phytoplankton distribution along the salinity gradient.