A large artificial dyke greatly alters partitioning of sulfate and iron reduction and resultant phosphorus dynamics in sediments of the Yeongsan River estuary, Yellow Sea

Highlights: • A large artificial dyke alters C_org oxidation pathways via sulfate and iron reduction in estuarine and limnetic sediments. • High sulfate reduction in estuarine sediment was responsible for high benthic P release. • High benthic P release may support primary production in water column via benthic-pelagic coupling in coastal ecosystem. • In limnetic sediments, Fe-P and Al-P suppressed benthic P release, which makes limnetic sediments significant P sinks. -- Abstract: We investigated sediment geochemistry, partitioning of organic carbon (C_org) oxidation by iron reduction (FeR) and sulfate reduction (SR), and benthic phosphorus (P) release, together with the P speciation in the sediments to elucidate the P dynamics in two contrasting sediments (i.e., estuarine vs. limnetic) separated by a large dyke in the Yeongsan River estuary of the Yellow Sea. In the sediments of the Yeongsan River estuary (St. YE), SR dominated the C_org oxidation pathway, accounting for 81.7% of total anaerobic C_org oxidation. Under the SR-dominated condition, H₂S derived from SR reacts quickly with iron oxides to form iron sulfides, which ultimately release the P bound to Fe(III) into the pore water. The enhanced benthic P flux (0.24 mmol m⁻² d⁻¹) at the YE site accounted for 80% of the P required for primary production in the water column. In contrast, in the limnetic sediments of the Yeongsan Lake (St. YL), where high levels of CH₄ accumulated, most P was bound to Fe and Al, which resulted in a low benthic P flux (0.03 mmol m⁻² d⁻¹). The results suggest that the frequent discharge of relatively P-depleted freshwater into the estuary via the artificial dyke may result in relatively P-limiting conditions in estuarine ecosystems. As a result, benthic P release from the SR-dominated estuarine sediment is a significant internal source of P in the coastal ecosystem. Our results indicate that the construction of a large dyke at a river mouth greatly alters C_org oxidation pathways and P dynamics in coastal ecosystems.