ABSTRACT: Microzooplankton grazing impact (<200 µm 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.

KEY WORDS: Microzooplankton grazing · Herbivory impact · Phytoplankton distribution · Estuaries

Published in AME Vol. 14, No. 3 (1998) on April 6
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 1998