Turbidity effects on the feeding, respiration and mortality of the copepod Pseudodiaptomus stuhlmanni in the St Lucia Estuary, South Africa

• Pseudodiaptomus stuhlmanni is a key prey item of higher trophic level taxa in the St Lucia Estuary.
• The system is threatened by external silt inputs.
• High (1500–2500 NTU) turbidities negatively affect the fitness of P. stuhlmanni.
• Intermediate (500–1000 NTU) turbidities appear to be beneficial to this copepod.
• Measures need to be taken to reduce the threat of silt loading into this estuary.

The St Lucia Estuary is the largest estuarine lake in Africa and due to anthropogenic activities it has since the 1950s been kept isolated from its main source of freshwater, the Mfolozi River. Stakeholders are increasingly calling for a full reconnection of the historical linkage in order to initiate system restoration. Studies conducted to test the effects of turbidity on the copepod Acartiella natalensis and the mysid Mesopodopsis africana have shown that turbidity reduces the fitness of these species. The turbidity tolerance of the dominant copepod Pseudodiaptomus stuhlmanni has not yet been documented. The aim of this study was to determine the effects of turbidity on the feeding, respiration and mortality rates of P. stuhlmanni. Although chlorophyll a and phaeopigment consumption was negatively affected by inorganic silt, the use of natural silt resulted in more variable trends. In contrast, a positive relationship was found between total organic carbon (TOC) consumption and turbidity. The high ingestion rates in the highest turbidity treatments of the TOC experiments are attributed to gut loading of organically-enriched silt. High turbidity also resulted in increased respiration rates of P. stuhlmanni, highlighting the energetic demand of turbid environments. Compared with other previously studied St Lucia zooplankters, P. stuhlmanni appears to be relatively tolerant of high turbidity. However, as the overall effect on this species is also negative, it is concluded that the St Lucia Estuary needs to be protected from excessive silt inputs if its ecological integrity is to be sustained.