Mechanisms of phosphorus released from the sediment–water interface in Xiamen Bay, Fujian, China

The concentrations of DRP (dissolved reactive phosphorus) in the estuary harbour waters along the coast of China are generally low, and the N/P ratios are much higher than the Redfield ratio. From Dalian Bay to the Zhujiang Estuary these ratios are as high as n·10–n·100. Field observations at the Jiulong Estuary and Xiamen Harbour in Fujian in recent years revealed that the high values of N/P ratios are 50-80 during April–October and 90-200 during November–March, respectively. A red tide of Skeletonema costatum developed in Dalian Bay waters in the summer of 1988, where the N/P ratio was increased rapidly from 50 to 190, and reached as high a value as 340 at the end of the rampant algal propagation. This clearly indicates that phosphorus plays an important role in controlling eutrophication and red tide in China's marine coastal waters. The annual average of DRP concentration in the waters of the inlet of Xiamen Bay is 0.4-0.48 μmol/l. The adsorption/desorption of phosphorus between the sediment–water interface is the main mechanism of controlling the buffering capacity of DRP in water. It is verified by the determination of the following three processes.(i)Released from the early diagenesis process in surface sediment. the flux of DRP is Fp = 1.03 μmol/m2 d, which is only 1/1600 of the ammonia released.(ii)Released from the adsorption/desorption process across the sediment-water interface. At normal temperature, the amount of TDP released in the long-term is the function of major environmental parameters, such as:[TDP] = 7.92 + 92.1 DO - 20.8 COD - 889 pH (sea water S = 27.2%)[TDP] = -38.8 - 1.66 DO + 23.3 COD + 25.1 pH (waste-sea water S = 22.5%o).The maximum flux of TDP is 3.84·104 μmol/m2 d, and has a half-release time of 5.5 days (corresponding to the half-renewal time of the water body). The equilibrium concentration in the water phase of adsorption/desorption system is (0.65 + 0.03) μmol/l. This is a rapid process, which plays a significant role in buffering phosphorus content in overlying water, Xiamen Bay.(iii)Released from the mineralization process of biotic detritus. Based on the determination of SODR (sediment oxygen demand rate) in the overlying water, we suppose the average depth of water is Hm, the residence time is tR d, then the maximum flux of phosphorus can be estimated as:[DRP]max = 2.65 • SODRmax •H/tR We obtained this flux of 6.61·104 μmol/m2 d, being the same order magnitude of adsorption/desorption. Meanwhile, it was found that in