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

We investigated sediment geochemistry, partitioning of organic carbon (Corg) 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 Corg oxidation pathway, accounting for 81.7% of total anaerobic Corg 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 Corg oxidation pathways and P dynamics in coastal ecosystems.