Phytoplankton responses to adaptive management interventions in eutrophic urban estuaries

Upstream anthropogenic perturbations can dramatically affect estuarine water quality, especially in small systems where water retention times are high. Management of these modified ecosystems often requires direct interventions to prevent detrimental long-term effects. The heavily urbanized and temporarily closed Zandvlei Estuary in South Africa has a long history of anthropogenic manipulation, and therefore acts as an ideal case-study to gauge reactive management practices. Continued deterioration of estuarine water quality and ecological functioning prompted the adoption of an adaptive management approach. Subsequent management interventions to increase salinity variability and nutrient removal included the (1) adoption of a mouth management plan, (2) regular harvesting of submerged vegetation, and (3) removal of flow obstructions. Physico-chemical and nutrient monitoring data (2010-2018) were assessed to document the response of phytoplankton to these interventions. Time-series analysis indicated an upward trend in salinity throughout the estuary. This corresponded with a temporal decline in phytoplankton biomass levels and was supported by model results that highlighted a strongly inverse relationship with salinity. The frequency of high-biomass events (>80 μg Chl-a l^-1) also declined in each of the designated estuarine zones. In April 2012, an extensive harmful algal bloom (HAB) of Prymnesium parvum (Prymnesiophyceae) was recorded (~530 μg Chl-a l^-1) in the estuary, culminating in hypoxic conditions (O₂ < 2 mg l^-1) and fish kills. However, the subsequent mechanical breaching of the mouth has prevented reoccurrences of P. parvum. Similarly, more saline conditions arising from increased marine connectivity reduced the abundance of the previously dominant Bacillariophyceae and Chlorophyceae classes. The overall improvement in water quality validates the efficacy of management interventions aimed at restoring and conserving ecosystem functionality. Yet, these efforts are a deviation from natural estuarine functioning and thus a 'catchment to coast' approach that incorporates upstream nutrient mitigation measures is needed to achieve sustainable long-term management objectives.