Seasonal Changes in Zooplankton Biomass and Grazing in a Temperate Estuary, South Africa

Abstract

Seasonal changes in size-fractionated chlorophylla (chla) and primary production, zooplankton biomass and zooplankton community grazing were investigated at six stations in the temperate Kariega Estuary, South Africa. Variations in chla, primary production and zooplankton biomass during the study demonstrated a seasonal pattern with minimum values recorded in winter and maximum values in summer. Total chla and primary production during the study ranged from 0·11 to 2·01μg l−1and between 13·9 and 37·7mg Cm−3d−1, respectively. Throughout the study, picophytoplankton (<2μm) dominated total chla and primary production. Zooplankton biomass during the study ranged from 7·98 to 59·31mg Dwt m−3. Pearson correlation analysis indicated that total zooplankton biomass was significantly correlated to total chla concentration (P<0·05). Community grazing rates of the zooplankton also demonstrated a seasonal pattern and ranged from 0·91 to 6·93ml l−1h−1. The elevated grazing rates recorded during summer could be related to higher seawater temperatures and zooplankton biomass. Grazing impact of zooplankton community corresponded to a loss of between 1·4 and 57·8% (mean=9·8%) of the total daily phytoplankton production. Specific ingestion rates of the two dominant copepod species, Acartia longipatella and Pseudodiaptomus hessei, indicated that carbon derived from the consumption of phytoplankton alone was sufficient to meet the basic metabolic requirements of the two copepods during summer and during spring for A. longipatella. The low contribution of phytoplankton to the total daily ration of the two copepods appears to be related to the size structure of the local phytoplankton which is too small to be grazed efficiently by the two copepod species.