Impact of turbidity on pseudodiaptomus stuhlmanni, a dominant copepod in Lake St Lucia, iSimangaliso Wetland Park.

Abstract

Although the St Lucia Estuary is a Ramsar Wetland of International Importance, it has historically been subjected to human-accelerated ecological stressors. One of these is high turbidity resulting from excessive sediment inputs. Laboratory-based studies have revealed a negative turbidity effect on the feeding and mortality rate of two dominant zooplankton species, the mysid Mesopodopsis africana and the calanoid copepod Acartiella natalensis. The first aim of this study was to determine the effect of turbidity on the feeding, respiration, and mortality rate of another important calanoid, Pseudodiaptomus stuhlmanni. Although this species was negatively affected by high turbidity, it was substantially more tolerant than M. africana and A. natalensis. The second aim was to test the field response of the dominant St Lucia zooplankton to a silt plume-causing flood event that occurred in March 2014. As M. africana was not abundant in the system prior to this event, attention was paid to the copepods. The field response of A. natalensis and P. stuhlmanni were in good agreement with the findings from the laboratory-based experiments. The population of A. natalensis underwent an immediate, and sharp decline, whereas that of P. stuhlmanni only declined in April 2014, after a month of surviving in highly turbid waters. However, P. stuhlmanni also took longer to recover, but this may be attributed to the attachment of parasitic epibiotic ciliates to this species. Therefore, although to different degrees, turbidity negatively impacted the dominant St Lucia zooplankton species. Through its observed positive correlation with the parasitic ciliates, turbidity further suppressed the abundance of the most turbid-water tolerant species, P. stuhlmanni. The importance of carefully managing sediment loading in St Lucia is stressed, as the effect of turbidity on zooplankton likely has food web-wide consequences.