This project had three main aims. The first aim was to investigate the metabolic physiology of the mysid Mesopodopsis wooldridgei. Secondly, the study examined the contribution of the mysid to the zooplankton community. Finally the study attempted to clarify the primary sources of carbon utilised by the mysid using stable carbon isotope analysis. The study was conducted in the freshwater dominated permanently open Great Fish Estuary (33°30’S; 27°08’E) located along the south-eastern coastline of southern Africa. Mass specific oxygen consumption rates of Mesopodopsis wooldridgei ranged between 0.11 and 8.38 ml O2 mg ww-1 h-1 and increased with an increase in water temperature. There were no significant differences in the respiration rates between the male and female mysids (P > 0.05). At a constant temperature (20°C), the oxygen consumption rate of M. wooldridgei decreased with an increase in salinity. Salinity tolerance of the mysid was strongly modified by temperature. At 5‰ and 30°C, mysids exhibited mass mortality. The Q10 value for M. wooldridgei at 15, 25 and 35‰ was estimated at 2.34, 1.44 and 2.14, respectively. Results of the study suggest that M. wooldridgei is well adapted to surviving in environments characterised by variations in temperature and salinity. Within the Great Fish Estuary, total chl. a concentration ranged between 2.68 μg L-1 and 31.12 μg L-1 and was always dominated by large phytoplankton cells (>5 μm). Average zooplankton abundance ranged between 62 and 28 917 ind. m-3 and biomass between 10 and 203 mg Dwt m-3. The zooplankton community was numerically dominated by the calanoid copepod Pseudodiaptomus hessei, which comprised up to 100% (range between 12 and 100%) of the total zooplankton counted. Total zooplankton biomass during the day was dominated by copepods and by mysids during the nighttime. Among the mysids, M. wooldridgei was the most numerically abundant mysid and comprised <10% of the total zooplankton abundance. Numerical analysis found no seasonal pattern in the total zooplankton abundance (including mysids). The lack of any seasonality could be attributed to the continuous freshwater inflow into the estuary due to the inter-basin transfer of water from the Gariep Dam to the Fish River system. A distinct spatial pattern in the zooplankton community structure was evident with the upper stations almost entirely dominated by the copepod P. hessei, while at stations occupied in the lower reaches of the estuary, the zooplankton community comprised a mixture of freshwater, estuarine and marine breeding zooplankton species. The mysids also demonstrated a distinct spatial pattern in their distribution. Mysids were generally absent from the upper reaches, while in the middle reaches Rhopalophthalmus terranatalis and M. wooldridgei were numerically dominant. Gastrosaccus brevifissura dominated in the lower regions of the estuary. Stable isotope analysis (δ13C) indicated that the dominant source of carbon utilised by the numerically dominant copepods and juvenile M. wooldridgei within the estuary was derived from the extensive phytoplankton stocks within the system. In contrast, sub-adult and adult mysids (R. terranatalis, M. wooldridgei and G. brevifissura) appeared to consume a combination of phytoplankton and copepods. The contribution of the various sources of carbon to the total carbon intake of the mysid remains unknown.