Feeding Studies on Selected Zooplankton in a Temperate Estuary, South Africa

Abstract

Total chlorophyll- a (chla), primary production and grazing impact of selected zooplankton were estimated at six stations in the Kariega Estuary in summer 1999. Total surface chla and production ranged from 1·13 to 2·12mg chla m−3and from 18·1 to 37·7mg C m−3d−1, respectively. At all stations both chla and production were dominated by small picophytoplankton (<2μm). Total zooplankton abundance and biomass ranged from 126 to 16468ind m−3and from 4·7 to 58·2mg m−3, respectively. Throughout the study zooplankton (>200μm) were dominated by the copepods Acartia longipatella and Pseudodiaptomus hessei which comprised >75% of total biomass. Also well represented in the zooplankton assemblages were representatives of the genus Halicyclops and the mysid, Mesopodopsis wooldridgei which generally comprised <5% of total abundance. At all stations, the highest biomass and abundance values were recorded at night which could be related to the distinct vertical migrations of the zooplankton. Individual gut pigment concentrations forP. hessei ranged from 0·2 to 1·3ng (pigm) ind−1and between 0·1 and 0·5ng (pigm) ind−1for A. longipatella. For M. wooldridgei, gut pigment concentrations ranged from 0·8 to 1·3 ng (pigm) ind−1. Gut pigment destruction rates for A. longipatella, P. hessei and M. wooldridgei were estimated at 92 (±5), 79 (±7) and 53 (±9)%, respectively. Ingestion rates of the two copepods ranged between 17·6 and 31·1ng (pigm) ind−1d−1for P. hessei and between 19·7 and 38·4ng (pigm) ind−1d−1for A. longipatella. These rates correspond to a carbon ingestion rate of between 0·9 and 1·6μg C ind−1d−1for P. hessei and between 1·0 and 1·9μg C ind−1d−1for A. longipatella. Total daily ingestion rate of M. wooldridgei ranged between 21·1 and 35·2ng (pigm) ind−1. This corresponds to a carbon ingestion rate of between 1·2 and 1·8μg C ind−1d−1. Carbon derived from the consumption of phytoplankton was sufficient to meet the basic metabolic requirements of the two copepod species. The combined grazing impact of the zooplankton corresponded to between 4·3 and 20·8% of the total phytoplankton production.