The copepods Pseudodiaptomus stuhlmanni, Acartia natalensis and the mysid Mesopodopsis africana are the three dominant zooplankton taxa in the St Lucia Estuary, Africa's largest estuarine lake. This system is currently undergoing severe desiccation, owing to freshwater deprivation. Carbon and nitrogen stable isotopes of the main primary and secondary producers were analysed temporally in two contrasting habitats, Charters Creek, heavily affected by desiccation, and the Mouth, virtually under unchanged conditions. This study aimed to compare the diet composition of these species under different environmental conditions. The mixed model SIAR (Stable Isotope Analysis in R) v 4.0 was used to determine the contribution of each food item to the diet of these taxa. Copepods made a substantial contribution to the diet of M. africana in the Mouth region, while those at Charters Creek utilized an even mixture of the available sources. At the Mouth, P. stuhlmanni utilized mostly particulate organic matter (POM), while feeding on an even mixture of available sources at Charters Creek. Limited data were available for A. natalensis, due to its short seasonal occurrence. Results do, however, indicate that this species feeds primarily on POM at the Mouth. Overall, diet composition of all three species is more selective at the Mouth. This can be attributed either to the harsh environmental conditions prevailing at Charters Creek or to the poor separation of Î13C signatures of primary producers in this region. The food partitioning abilities shown by these species may facilitate the utilization of resources available in the estuary.
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