Trophic cascading in an oligotrophic temperate estuary, South Africa

Abstract

Trophic cascading resulting from the coupling between components of the microbial food comprising bacteria, phytoplankton and protozooplankton [nano- (2-20 μm) and microzooplankton (20-200 μm)] and mesozooplankton (copepods) (200-2000 μm) was investigated at three stations in the oligotrophic temperate Kariega estuary during the summer (February) and winter (May) 2001. Total chlorophyll a (Chl a) was dominated by picophytoplankton (<2 μm) throughout the study comprising between 55 and 74% of the total pigment during summer and between 64 and 78% of the total during winter. Among the heterotrophic components of the pelagic food web, nanozooplankton (HNAN) were identified as the most important consumers of bacteria and microzooplankton (MICRO) the primary consumers of Chl a. The low impact of the mesozooplankton (MESO) on the Chl a appears to be related to the size structure of the phytoplankton community in the estuary which is too small to be grazed efficiently by the larger zooplankton. In response to the unfavourable size structure of the phytoplankton community in the estuary, it is suggested that the MESO consume HNAN, MICRO and/or detritus. The negative impact of the MESO on the net growth rates of the HNAN and MICRO resulted in a decrease in the feeding impact of these organisms on the Chl a < 2 μm and bacteria. This result is consistent with the expectations of predator-prey cascades.