Wind-driven shear stress on sediment phosphorus release: Implications for water quality, and harmful algal blooms in...

Internal phosphorus loading from sediments can act as a persistent nutrient source that fuels eutrophic conditions and harmful algal blooms (HABs), two global water quality issues. While most studies estimate the concentration gradient-driven diffusive flux, the role of wind-driven sediment disturbance remains less explored in shallow, coastal systems. This study quantified soluble reactive phosphorus (SRP) flux from sediments in a shallow sub-tropical estuary, quantifying both the diffusive and advective pathways. Sediment mapping at 160 sites identified strong west-east gradients in total phosphorus (TP), with four representative sites selected for laboratory incubations. Aerobic diffusive flux was measured over two weeks in intact cores, while advective flux was determined under increasing bedload shear stress (0.01-0.45 Pa) at 20-minute intervals using a Gust Erosion Microcosm. Diffusive SRP flux averaged 0.13 mg P m^-2 d^-1 (∼0.21 Mt d^-1 estuary-wide) and was below 0 mg P m^-2 d^-1 at half the sites, markedly lower than previous estimates. Advective flux averaged 30.8 mg P m^-2 d^-1 (∼49.3 Mt d^-1), representing up to a ∼ 240-fold increase over diffusive flux. The advective flux scaled linearly with shear stress, suggesting that even moderate winds can mobilize substantial sediment SRP pools. Seasonal autumn and winter cold fronts may deplete sediment SRP, reducing availability during summer bloom-favorable conditions. In contrast, tropical storms in the summer months could trigger significant P releases under optimal HAB growth conditions. These findings underscore the importance of incorporating episodic, wind-driven advective flux into nutrient budgets and resource management strategies for shallow water bodies influenced by strong seasonal wind regimes.