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, Sp ain) was analyzed during summer (August 1990) using a serial dilution method. Both microzooplankton grazing rate and phytoplankton growth ra te 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 ene rgy transport through the pelagic food web. Microzooplankton herbivory represented a mean loss of 38% of initial standing stock of chlorophy ll and 46% of potential phytoplankton primary production in the inner zone of the estuary, where changes in phytoplankton biomass were mainl y a function of initial biomass. In the intermediate and outer zones, microzooplankton grazed around 54% of initial standing stock of chloro phyll and more than 60% of potential phytoplankton primary production. In these zones, changes in phytoplankton biomass were mainly a functi on of phytoplankton growth. In spite of the importance of microzooplan kton grazing, this factor alone cannot be responsible for the observed phytoplankton distribution along the salinity gradient.