Effects of phosphorus recycling in sediments on the primary productivity of phytoplankton in the northern South China Sea

The coupling relationship between sediment phosphorus (P) recycle processes and phytoplankton primary productivity (PP) on the spatial scale was investigated by characterizing the distributions of algal PP and P species in a total of 38 surface sediments from the Pearl River Estuary (PRE) to the outer shelf, northern South China Sea. Through analyzing the relationships among algal PP, phosphate concentrations (surface water and bottom water), and sediment bioavailable P species (Ex-P, OP, and Fe-P), our results revealed P exchange among sediment-bottom water-surface water, influencing the local PP. Specifically, in the PRE, an increased input of terrigenous nutrients resulted in high OP concentration and its subsequent accumulation in sediments. However, our results revealed that the PP in PRE was lower than that in the western coast, probably due to light restriction. In the western coast, different relationships between PP and P species were found, with a strong correlation in the upwelling zone, indicating the potential intensified recycling of P species, such as OP and Fe-P within the sediments released into the water column and further fertilize PP. In the outer shelf, the low correlations among Ex-P, OP, Fe-P, and PP suggest that P may no longer be a significant limiting factor. Our results highlight the influence of hydrodynamic conditions on the P recycle processes in the water column, which has important implications for understanding organic carbon deposition and P cycle under ocean anoxia in geological history.