Relationships between zoo- and phytoplankton in a warm-temperate, semi-permanently closed estuary, South Africa

Seasonal surveys were carried out in the shallow, well-conserved temporaril y open Nyara Estuary in the Eastern Cape, South Africa. Although temporaril y open estuaries constitute over 70% of estuaries in South Africa, few data are available on the structure and functioning of these systems in the reg ion. The main aim of the study was to test the hypothesis that owing to irr egular nutrient input, temporarily open estuaries may exhibit poorly-develo ped pelagic food webs, with low phytoplankton and pelagic biomass in genera l. Results from the investigation indicate that phytoplankton biomass is ge nerally low, and dominated by pico and nanophytoplankton, with almost total absence of diatoms. This may be explained in terms of the regenerated, rat her than new, nutrient pool that is available to phytoplankton as a result of the semipermanently closed nature of the estuary. Chlorophyll a levels n ever exceeded 4.1 mg m(-3). However, low levels of phytoplankton biomass we re in contrast to relatively large stocks of zooplankton, which attained ma ximum levels of about 2 g (dry weight) m(-3). Thus, there is an imbalance b etween the biomass of the primary producers and that of the consumers. In o rder to satisfy the zooplankton energy budget, either phytoplankton product ion rates are extremely high or a substantial proportion of their food dema nds must be met through utilization of alternative sources, such as detritu s, protozoans and microphytobenthos. It is suggested that microphytobenthos in particular may play a major role in this regard because of the prevaili ng good conditions for its growth in this type of ecosystem. Microbenthic c hlorophyll a concentrations in the Nyara Estuary are in the upper range of values measured in South African estuaries; with an average of congruent to 190 mg m(-2). Further studies are needed to investigate the ability of the dominant species of zooplankton to ingest and assimilate benthic microalga e, particularly during the day when most species remain in close associatio n with the substratum. (C) 2000 Academic Press.