Grazing rates and feeding preferences of the mysid shrimp Gastrosaccus brevifissura in a temporarily open estuary in South Africa

Gastrosaccus brevifissura is a key crustacean species in most South African estuaries. The biomass, distribution, grazing rates and feeding preferences of this species were investigated in the Mpenjati Estuary, on the east coast of South Africa. The species exhibits a very marked diurnal migration behaviour, with biomass varying within the range 0.005 to 0.26 g m -3 dry wt (DW) during the daytime and 0.002 to 1.53 g m -3 DW during the night. The estimated total population grazing impact of the mysid ranged between 22 and 32% of the available phytoplankton in summer and win- ter, respectively. This suggests that G. brevifissura may not meet all its metabolic demands by con- suming phytoplankton alone. Other possible food sources available in the estuary include plant detri- tus (DTR), microheterotrophs and benthic microalgae (BMA). In particular, the vertical migration behaviour of G. brevifissura allows close spatial association with the benthic microalgae during most of the day. Indeed, feeding experiments suggest that G. brevifissura is able to feed efficiently on settled as well as on resuspended benthic microalgae. This is supported by results from stable isotope analysis (δ 13 C and δ 15 N), which show that benthic microalgae contribute 68 and 24% to the total diet of G. brevifissura in winter and summer, respectively. This also suggests that other food sources are needed by this species to meet all its energetic demands. Carbon rations obtained from in situ graz- ing methods are 35 to 44% (winter) and 1.6 to 3.9% (summer). When these values are compared to the previously estimated basal metabolic requirements for the mysid, it follows that G. brevifissura is able to meet all its energetic requirements from a pure autotrophic diet only during the winter. On the other hand, during summer, these values are barely enough to meet the basal metabolic rate of this species.