Planktonic food webs and benthic-pelagic coupling in three South African temporarily-open estuaries

Seventy-three percent of South African estuaries are temporarily-open and close off from the sea during the dry season, under low river inflow. Following periods of high rainfall, the water level rises gradually until breaching of the sandbar at the mouth occurs. These ecosystems often exhibit high zooplankton (0.008-2.6 g DW m(-3)) but low phytoplankton biomass (0.01-11 mgChl-a m(-3)). Benthic microalgal biomass is on average 1-3 orders of magnitudes higher than that of phytoplankton. The zooplankton community is dominated during much of the year by the mysid Gastrosaccus brevifissura and the copepods Pseudodiaptomus hessei and Acartia natalensis. In situ grazing of these zooplankton species on estuarine microalgae shows that their potential impact on water-column chlorophyll-a exceeds its availability at times. Thus, phytoplankton alone may not be able to sustain the entire energetic demands of the consumers throughout the year. Measurements of delta(13)C and delta(15)N ratios in the three dominant zooplankton species and in their possible food sources (particulate organic matter, detritus and microphytobenthos) show that each grazer derives most of its energetic requirements from a specific and unique food source within the same trophic level. This strategy may minimize inter-specific competition and hence improve the utilization of the food sources available in these estuaries. Benthic microalgae constitute the staple food item of the mysid G. brevifissura, and results of the stable isotope analysis suggest that all other major pelagic grazers are able to utilize this rich food source in temporarily-open estuaries. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.