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

摘要: Abstract The biomass and size structure of phytoplankton were compared in the temporarily open/closed Mdloti Mhlanga estuaries, KwaZulu-Natal, South Africa. Samples measurements taken between March 2002 2003, to assess effects differing mouth breaching regimes, freshwater inflow nutrient loading on dynamics phytoplankton. Estuary exhibits a larger catchment area (≈500 km 2 ), substantial upstream impoundment (Hazelmere Dam) little discharge treated sewage waters its upper reaches (8 Ml d −1 ). Conversely, has smaller (≈100 km no dams large (20 Ml d resulting capping flow 0.23 m 3  s . DIN DIP concentrations usually well above accepted threshold levels (2 μM for 0.5 μM DIP) nutrient-limited primary production also much higher than Mdloti. This led episodes eutrophication, with high mean chl- occurring particular. In both highest occurred during closed phase, while lowest open phase. Phytoplankton ranged from 0.869 111 mg chl-  m −3 Mdloti, 0.732 303 mg chl- Mhlanga. maximum value 303 mg m , recorded at October 2002, is concentration ever any African estuary. was dominated by nanophytoplankton fraction (2–20 μm) entire study period. accounted an average 80.6% 79.6% total Mhlanga, respectively. followed picophytoplankton ( 20 μm) 6.53% 8.65% Due volumes released into two particularly they breached more frequently would have under natural circumstances. Thus, despite their concentrations, eutrophic within estuaries exhibited short residence time, thereby restricting time available take up enough nutrients form regular intense blooms.