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Key factors that drive phytoplankton biomass and community composition in the urbanised Nahoon Estuary, South Africa

Estuaries are under intense pressure, because of urban developments and water abstraction. Water column characteristics of the Nahoon Estuary were examined in 2014–2015 to identify factors that influenced phytoplankton dynamics. The estuary was found to be saline (∼33) and remained well-oxygenated (∼6 mg l−1) throughout the current study. During this period, freshwater inflow into the estuary was minimal and chlorophyll a biomass was low (3.6 ± 0.3 µg l−1). Nutrient concentration at Nahoon Estuary showed a linear relationship with salinity, indicative of a conservative behaviour. Phytoplankton dynamics were altered by fluctuations in nutrient levels, not necessarily in response to freshwater inflow. The impact of Nahoon Dam and destructive land-use alongside the Nahoon River decreases freshwater inflow and increases nutrient loading into the estuary. In terms of nutrient loading, the current study showed the importance of microalgae as indicators of the nutrient status of an estuary. The presence of cyanobacteria and high phytoplankton biomass in the upper reaches of the estuary confirms the input of excessive nutrients. The results of the current study clearly show that better management of both the Nahoon River and its catchment is vital to ensure the health of the system.