Phytoplankton biomass and size structure in two South African eutrophic, temporarily open/closed estuaries

The biomass and size structure of phytoplankton were compared in the temporarily open/closed Mdloti and Mhlanga estuaries, in KwaZulu-Natal, South Africa. Samples and measurements were taken between March 2002 and March 2003, to assess the effects of differing mouth breaching regimes, freshwater inflow and nutrient loading on the dynamics of phytoplankton. The Mdloti Estuary exhibits a larger catchment area (z500 km2), substantial upstream impoundment (Hazelmere Dam) and little discharge of treated sewage waters in its upper reaches (8 Ml d 1). Conversely, the Mhlanga Estuary has a smaller catchment area (z100 km2), no upstream dams and a large inflow of treated sewage waters (20 Ml d 1), resulting in a capping flow of 0.23 m3 s 1. DIN and DIP concentrations were usually well above the accepted threshold levels (2 mM for DIN and 0.5 mM for DIP) for nutrient-limited primary production and were also much higher in the Mhlanga than in the Mdloti. This led to episodes of eutrophication, with high levels of mean chl-a occurring in the Mhlanga Estuary in particular. In both estuaries, highest chla concentrations occurred during the closed phase, while the lowest occurred during the open phase. Phytoplankton biomass ranged from 0.869 to 111 mg chl-am 3 in the Mdloti, and from 0.732 to 303 mg chl-am 3 in the Mhlanga. The maximum value of 303 mgm 3 of chl-a, recorded at the Mhlanga in October 2002, is the highest chl-a concentration ever recorded in any South African estuary. In both estuaries, phytoplankton biomass was dominated by the nanophytoplankton fraction (2e20 mm) during the entire study period. This accounted for an average of 80.6% and 79.6% of the total phytoplankton biomass in the Mdloti and the Mhlanga, respectively. This was followed by the picophytoplankton (!2 mm), with 12.9% at the Mdloti and 11.9% at the Mhlanga and the microphytoplankton (O20 mm) with 6.53% at the Mdloti and 8.65% at the Mhlanga. Due to the large volumes of treated sewage released into the two estuaries, particularly the Mhlanga, they breached more frequently than they would have under natural circumstances. Thus, despite their high nutrient concentrations, eutrophic waters within the estuaries exhibited a short residence time, thereby restricting the time available for phytoplankton to take up enough nutrients to form more regular and intense blooms.