Relationships between Zoo- and Phytoplankton in a Warm-temperate, Semi-permanently Closed Estuary, South Africa

Abstract

Seasonal surveys were carried out in the shallow, well-conserved temporarily open Nyara Estuary in the Eastern Cape, South Africa. Although temporarily open estuaries constitute over 70% of estuaries in South Africa, few data are available on the structure and functioning of these systems in the region. The main aim of the study was to test the hypothesis that owing to irregular nutrient input, temporarily open estuaries may exhibit poorly-developed pelagic food webs, with low phytoplankton and pelagic biomass in general. Results from the investigation indicate that phytoplankton biomass is generally low, and dominated by pico and nanophytoplankton, with almost total absence of diatoms. This may be explained in terms of the regenerated, rather than new, nutrient pool that is available to phytoplankton as a result of the semipermanently closed nature of the estuary. Chlorophyll a levels never exceeded 4·1mgm−3. However, low levels of phytoplankton biomass were in contrast to relatively large stocks of zooplankton, which attained maximum levels of about 2g (dry weight) m−3. Thus, there is an imbalance between the biomass of the primary producers and that of the consumers. In order to satisfy the zooplankton energy budget, either phytoplankton production rates are extremely high or a substantial proportion of their food demands must be met through utilization of alternative sources, such as detritus, protozoans and microphytobenthos. It is suggested that microphytobenthos in particular may play a major role in this regard because of the prevailing good conditions for its growth in this type of ecosystem. Microbenthic chlorophyll a concentrations in the Nyara Estuary are in the upper range of values measured in South African estuaries, with an average of ≅190mgm−2. Further studies are needed to investigate the ability of the dominant species of zooplankton to ingest and assimilate benthic microalgae, particularly during the day when most species remain in close association with the substratum.