Herbivory of nanozooplankton in polyhaline and euhaline zones of a small temperate estuarine system (estuary of Mundaka): seasonal variations

The grazing activity of nanozooplankton natural assemblages on phytoplankton was analysed in the polyhaline and euhaline zones of the estuary of Mundaka throughout the spring-summer-autumn period. Grazing experiments were conducted in situ using the dilution method. Incubations occurred within a relatively wide range of initial chlorophyll concentration (1-10 mu g chl l(-1), but saturated feeding kinetics were not observed in most experiments. The heterotrophic nanoplankton grazed on average between 43 and 51% (mean values from all dilutions data and the 3 more diluted data, respectively) of the phytoplankton stock in the polyhaline zone, and between 36 and 43% in the euhaline. The grazing impact on primary production averaged around 65% in both zones, but showed higher variability in the euhaline (0-223%) than in the polyhaline (15-117%). Herbivore pressure was found to be higher in polyhaline waters on average, but between-zone differences were net significant. The nanozooplankton herbivory was not clearly related to water temperature, nor to phytoplankton biomass (expresed as chlorophyll concentration), and the coupling between production and consumption was weak, being more unbalanced in the euhaline zone. Nanozooplankton grazing was sometimes found to cause phytoplankton decline, while at other times its impact was negligible. In addition, indirect evidence suggests that nanozooplankton grazing activity may be affected by predation pressure from mesozooplankton in the polyhaline zone. Seasonal patterns of phytoplankton exploitation by heterotrophic nanoplankton differed among saline zones, suggesting that the fate of phytogenic carbon in the estuary of Mundaka may be highly variable both spatially and seasonally. (C) 1998 Elsevier Science B.V. All rights reserved.