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 phytopl ankton was analysed in the polyhaline and euhaline zones of the estuar y of Mundaka throughout the spring-summer-autumn period. Grazing exper iments were conducted in situ using the dilution method. Incubations o ccurred within a relatively wide range of initial chlorophyll concentr ation (1-10 mu g chl l(-1), but saturated feeding kinetics were not ob served in most experiments. The heterotrophic nanoplankton grazed on a verage 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 i mpact on primary production averaged around 65% in both zones, but sho wed higher variability in the euhaline (0-223%) than in the polyhaline (15-117%). Herbivore pressure was found to be higher in polyhaline wa ters on average, but between-zone differences were net significant. Th e nanozooplankton herbivory was not clearly related to water temperatu re, nor to phytoplankton biomass (expresed as chlorophyll concentratio n), and the coupling between production and consumption was weak, bein g more unbalanced in the euhaline zone. Nanozooplankton grazing was so metimes found to cause phytoplankton decline, while at other times its impact was negligible. In addition, indirect evidence suggests that n anozooplankton grazing activity may be affected by predation pressure from mesozooplankton in the polyhaline zone. Seasonal patterns of phyt oplankton exploitation by heterotrophic nanoplankton differed among sa line zones, suggesting that the fate of phytogenic carbon in the estua ry of Mundaka may be highly variable both spatially and seasonally. (C ) 1998 Elsevier Science B.V. All rights reserved.