Turbidity effects on feeding and mortality of the copepod Acartiella natalensis (Connell and Grindley, 1974) in the St Lucia Estuary, South Africa Nicola K. Carrasco ⁎, Renzo Perissinotto, Salome Jones School of Life Sciences, University of KwaZulu-Natal, Westville Campus, P. Bag 54001, Durban 4000, South Africa abstract article info Article history: Received 15 January 2013 Received in revised form 18 April 2013 Accepted 19 April 2013 Available online 21 May 2013 Keywords: Acartiella natalensis Feeding Mortality Silt St Lucia Estuary Turbidity The St Lucia estuarine lake is the largest in Africa and has recently experienced a freshwater-deprivation cri- sis, 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 cope- pod 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 deter- mine 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. Re- sults 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Estuaries are dynamic ecosystems characterized by extreme vari- ability in physico-chemical factors (Carrasco and Perissinotto, 2011; Cyrus et al., 2011). These fluctuations often result in the resident biota being categorised by strong spatial and seasonal variability (McLusky and Elliott, 2004). The St Lucia Estuary is the largest estuarine lake sys- tem in Africa and is of great significance for southern Africa (Wallace, 1975; Wright and Mason, 1993), as it forms part of the iSimangaliso (previously known as the Greater St Lucia) Wetland Park (Carrasco and Perissinotto, 2011), South Africa's first World Heritage Site (Begg, 1978). The St Lucia Estuary is also classified as a Ramsar Wetland of In- ternational Importance. The system is regulated by cyclical wet and dry phases, with each period lasting between 4 and 10 years at a time (Begg, 1978). The lake system experienced below average rainfall be- tween 2002 and 2011, with drought conditions intensified by the Mfolozi River canalization and the successive freshwater diversion away from the St Lucia Estuary. Prior to 1920, the Mfolozi River discharged into the St Lucia Estuary and would buffer water loss during periods of drought, but in the 1930s a canal was excavated through the Mfolozi flats for agricultural purposes (Begg, 1978; Whitfield and Taylor, 2009). The natural filtration system of the swamps was, there- fore, destroyed and the two systems have since been artificially maintained separate in an attempt to avoid siltation from the Mfolozi into the already naturally turbid St Lucia system. Copepods belonging to the genus Acartiella are common constituents of zooplankton communities in many South African estuaries, where they occur in relatively high densities (Carrasco and Perissinotto, 2011; Jerling et al., 2010; Montoya-Maya and Strydom, 2009; Wooldridge, 1999). In the St Lucia system, Acartiella natalensis is one of three key zooplankton species that play a vital role in the ecosystem. With the dramatic salinity changes and prevalent drought spells caused by the canalization, the relevance of the St Lucia Estuary as an established nurs- ery and spawning site has increasingly become compromised (Cyrus and Vivier, 2006). The resident zooplankton taxa are important indicators of change in systems such as St Lucia, that experience erratic changes in en- vironmental conditions, as they have a faster physiological response than most other larger organisms (Hays et al., 2005). The detrimental effects of the Mfolozi canalization on the estuarine functioning of St Lucia has led to various stakeholders questioning the decision to maintain the two systems separate, with management eventually calling for their re-connection (iSimangaliso Wetland Park Authority, 2012). The plan to build a beach spillway between the mouths of the Mfolozi River and St Lucia Estuary was initiated in the early months of 2012, with the channel becoming fully functional Journal of Experimental Marine Biology and Ecology 446 (2013) 45–51 ⁎ Corresponding author. Tel.: +27 73 171 8406. E-mail address: nicola.carrasco@gmail.com (N.K. Carrasco). 0022-0981/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jembe.2013.04.016 Contents lists available at SciVerse ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe