Regional-scale assessment of eutrophic condition indicators in subtropical temporarily closed estuaries

Abstract

The adoption of a catchment-to-coast management approach is critical to prevent the continued loss of ecosystem functionality in small temporarily closed estuaries (TCEs). This study investigated the impact of anthropogenic pressures on the water quality (i.e., inorganic N and P, dissolved oxygen) and phytoplankton community dynamics of 55 TCEs situated along the subtropical coastline of KwaZulu-Natal, South Africa. Extensive spatial surveys were conducted in summer (high flow) and winter (low flow) to capture variability associated with open (84%; n = 46) and closed (73%; n = 40) mouth conditions, respectively. The health of the TCEs in this region are severely threatened (‘High’ or ‘Very High’ ratings) by pollution (67%) and habitat loss (53%) pressures. Wastewater effluent discharges are a key source of pollution, with 25 of the study estuaries receiving substantial combined annual inflow volumes (44 × 106 m3) and inorganic nutrient loads (352,900 kg DIN; 222,200 kg DIP) from upstream wastewater treatment plants. The loss of habitat in these small TCEs stems from high levels of catchment land-use modifications (∼50% transformed), with agricultural (e.g., sugarcane farming, commercial forestry) and development (e.g., built-up, informal) activities accounting for 35% and 12% of overall land-use, respectively. The impact of these pressures is highlighted by the relatively high proportion of TCEs exhibiting eutrophic levels of inorganic nutrients during the high (DIN: 25%, n = 14; DIP: 18%, n = 10) and low (DIN: 31%, n = 17; DIP: 26%, n = 14) inflow periods. Despite this, estuarine phytoplankton bloom conditions (>20 μg Chl-a l−1) are sporadic due to the limited tidal exchange (perched mouths) and low water residency (augmented baseflows) that characterise these TCEs. As such, anthropogenic perturbations serve to dampen the role of estuarine primary producers as nutrient buffers in subtropical TCEs, thus promoting the export of excessive nutrient loads to the adjacent coastal waters. Catchment-scale interventions should be geared towards effective governance, infrastructure maintenance, and implementation of restoration activities (e.g., diversion and/or reuse of wastewater discharges) to improve the health and biodiversity of small, microtidal TCEs.