Trophic interactions amongst the plankton in the temporarily open/closed East Kleinemonde Estuary, South Africa

Abstract

The trophic interactions between bacteria, phytoplankton, nanoflagellates (2–20 μm), microzooplankton (20–200 μm) and mesozooplankton (200–2 000 μm) were investigated in winter and summer in the temporarily open/closed East Kleinemonde Estuary, Eastern Cape. During winter the phytoplankton community was dominated by microphytoplankton (20–200 μm) and during summer by nano- and picophytoplankton. Among the heterotrophic components of the plankton, the nanoflagellates were identified as the most important consumers of bacteria and small <20 μm phytoplankton cells. During both winter and summer the larger heterotrophic organisms, particularly mesozooplankton, resulted in a reduced impact of the nanoflagellates on both the bacteria and phytoplankton, indicating that the larger heterotrophs were preying upon the nanoflagellates. This resulted in a decrease in the predation impact of these organisms on the bacteria during both winter and summer, and on the phytoplankton in summer. This result is consistent with expectations of predator-prey cascades. The presence of the larger heterotrophs therefore mediates interactions between primary bacterivores, nanoflagellates, and bacteria in this estuary.