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Marine Ecology Progress Series

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MEPS 663:63-76 (2021)  -  DOI: https://doi.org/10.3354/meps13655

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

Daniel A. Lemley1,2,*, Janine B. Adams1,2, John L. Largier3,4

1Botany Department and the Institute for Coastal and Marine Research, Nelson Mandela University, Port Elizabeth 6031, South Africa
2DSI/NRF Research Chair in Shallow Water Ecosystems, Nelson Mandela University, Port Elizabeth 6031, South Africa
3Department of Environmental Science and Policy, University of California, Davis, California 95616, USA
4Coastal and Marine Sciences Institute, University of California, Davis, Bodega Bay, California 94923, USA
*Corresponding author:

ABSTRACT: 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 µg chl a l-1) represent significant removal of available NOx (~100%) and phosphate (>75%) during warmer conditions (>20°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).


KEY WORDS: Coastal filter · Eutrophication · Heterosigma akashiwo · Nutrient transformation · Phytoplankton


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Cite this article as: Lemley DA, Adams JB, Largier JL (2021) Harmful algal blooms as a sink for inorganic nutrients in a eutrophic estuary. Mar Ecol Prog Ser 663:63-76. https://doi.org/10.3354/meps13655

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