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

The trophic interactions between bacteria, phytoplankton, nanoflagellates (2–20 m), microzooplankton (20–200 m) and mesozooplankton (200–2 000 m) 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 m) 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 m 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.