Harmful algal blooms as a sink for inorganic nutrients in a eutrophic estuary

Phytoplankton-mediated nutrient fluxes typically provide only pulsed relief to adjacent coastal waters during the productive period, with nutrient export increasing in the absence of substantial phytoplankton biomass. On the warm temperate coastline of South Africa, the Sundays Estuary is characterised by highly regulated freshwater inflow patterns, nutrient-enriched conditions, and resident harmful algal blooms (HABs). Given these attributes, the study objective was to investigate the effect of these phytoplankton blooms on fluvial inorganic nutrient dynamics. To assess uptake, we analysed inorganic nutrient (phosphate, ammonium, NOx) and phytoplankton concentrations in relation to salinity using data from 17 surveys. Property-salinity mixing diagrams and statistical analyses indicated a positive association between increasing phytoplankton biomass and decreasing NOx flux (p < 0.001), and to a lesser degree phosphate flux (p = 0.22), along the gradient from low-salinity inner estuary to high salinity outer estuary. High biomass HAB accumulations of Heterosigma akashiwo (> 100 mu g chl a l(-1)) represent significant removal of available NOx (similar to 100%) and phosphate (> 75%) during warmer conditions (> 20 degrees C). These events, together with continuous inorganic nutrient uptake during less severe bloom conditions, remove a substantial portion of annual NOx and phosphate loads (36.5 and 36.4% flux, respectively). Although this buffers inorganic nutrient loading to adjacent coastal waters, it also represents an emerging legacy pollution issue in the form of a benthic accumulation of organic material in bottom waters subject to recurrent hypoxia. Future management efforts should adopt an ecosystem-based approach centred around simultaneous restoration of hydrological variability and dual nutrient reduction strategies (N and P).

Automated summary

The study in the Sundays Estuary of South Africa found that increasing phytoplankton biomass led to decreased flux of NOx and phosphate. High biomass harmful algal blooms were able to significantly remove available NOx and phosphate, while continual inorganic nutrient uptake also removed a substantial portion of annual NOx and phosphate loads. These findings highlight the importance of ecosystem-based management strategies to address nutrient fluxes and emerging pollution issues.