Turbidity effects on feeding and mortality of the copepod Acartiella natalensis (Connell and Grindley, 1974) in the St Lucia Estuary, South Africa

• Acartiella natalensis is a key species in the St Lucia and other estuaries.
• The system is threatened by siltation due to the recent reconnection with the Mfolozi.
• High turbidities significantly affect ingestion and mortality rates of this copepod.
• While high silt loads interfere with its physiology, some sediment may be beneficial.
• The estuarine system needs to be carefully managed to avoid high silt load inputs.

The St Lucia estuarine lake is the largest in Africa and has recently experienced a freshwater-deprivation crisis, partly because its connection with the large Mfolozi catchment has been discontinued. This is due to the extremely high silt load carried by the Mfolozi waters. A study was undertaken during 2012 with the aim of determining the effect of high silt loads on the mortality and feeding rates of Acartiella natalensis, a key copepod species in the St Lucia Estuary. Two different types of experiments were conducted: one using natural organic silt to determine the effect of turbidity on mortality rate; and the other with inorganic silt to determine feeding rates. For the mortality experiments, the copepods were subjected to six different turbidity levels, ranging from ambient to 2500 NTU, with survival monitored at 0, 4, 24, 48, and 72 hour time intervals. For the feeding experiments, copepods were incubated in the same turbidity levels for a 24 hour period. Results revealed a significant effect of turbidity on both feeding and mortality, with lowest ingestion rates and survival recorded in the high turbidity treatments. These findings indicate that overall, this copepod species is negatively affected by high turbidities, but an unexpectedly high mortality rate was recorded under control conditions (6-38 NTU). The lowest mortality rate occurred at 500 NTU, suggesting that while very high silt loads (> 1000 NTU) clearly interfere with the physiological functioning of A. natalensis, some sediment may be advantageous, perhaps as a supplementary source of nutrition.