Allometric releases of nitrogen and phosphorus from sediments mediated by bacteria determines water eutrophication in coastal river basins of Bohai Bay.

Although the Chinese government has conducted much work in recent years to reduce land-based pollutant discharge, eutrophication continues to occur frequently in many rivers, estuaries, and coastal waters. This may indicate that sediment is a major source rather than a sink for nitrogen (N) and phosphorus (P). To clarify the endogenous mechanisms of eutrophication in coastal river basins, the eutrophication status, physicochemical properties, and bacterial parameters of overlying waters and sediments in the catchment (CA), estuarine (EA), and offshore (OA) areas in the Duliujian River Basin of Bohai Bay were investigated. The results showed that the eutrophication index (EI) of CA, EA, and OA were 62.71, 57.86, and 36.51, respectively. The EI was more sensitive to increases in P (slope = 3.887) than to increases in N (slope = 0.734) of the overlying water, indicating that P is the main factor driving eutrophication in the coastal river basin. However, a nonlinear relationship was found between P in sediments and overlying waters, suggesting that bacterial mediation may occur during P release. As speculated in this study, P in the overlying water increased more quickly than N with increasing bacterial diversity and metabolic abundance, indicating that the allometric release of N and P mediated by bacteria increases the risk of eutrophication. Redundancy analysis showed that organic matter and total N in sediment have positive effects on bacterial communities, which explains 21.8% and 23.7% variation in bacterial diversity, and explains 31.3% and 7.1% variation in bacterial metabolism. This also suggests that the accumulation of N in the sediment promotes the release of P and further aggravates the eutrophication of water. Therefore, simultaneous control of N and P is necessary to control water eutrophication in coastal river basins.