Forms and releasing flux of phosphorus in the surface sediments in Haizhou Bay marine ranching

In order to understand the impact of Haizhou Bay marine ranching construction on the phosphate exchange at sediment-water interface, multiple surface sediments, overlying water and interstitial water samples were collected from the Haizhou Bay ocean ranch area in May 2017. The different forms of phosphorus were extracted by SMT (Standard Measurement Test), including exchangeable phosphorous (Ex-P), iron-aluminum bound phosphorous (Fe/Al-P), calcium-bound phosphate (Ca-P), then their amounts were determined. The phosphorus exchange flux at the sediment-water interface was estimated under the phosphorus flux simulation incubation experiment. The results showed that the contents of the total phosphorus (TP), inorganic phosphorus (IP) and Ca-P in the surface sediments of Haizhou Bay marine ranching ranged from 392.44 to 463.46 μg/g, 219.21 to 282 μg/g and 117.73 to 130.07 μg/g, respectively. The IP was found as the major form of phosphorus. The order of the contents of different forms of phosphorus was Ca-P> Ex-P> Fe/Al-P. The contents of phosphorus in the control area were greater than that in the reef area. The concentrations of various forms of phosphorus in the overlying water were higher than the sediment pore water. Laboratory phosphorus flux simulation experiments showed that the content of various forms of phosphorus in sediments were higher after the incubation process, and IP still the main form of phosphorus. The exchange fluxes of TP, TDP, and \rmPO_4^3 - at the sediment-water interface were estimated at −0.53—−0.05 mmol/(m²·d), −0.15—−0.01 mmol/(m²·d) and −0.03—−0.29 mmol/(m²·d), respectively. The exchange rate was faster at the initial stage of the incubation experiment, and then tended to stabilize. Compared with the control area, the exchange flux of phosphate in the marine ranching was slightly greater, but both showed the tendency that phosphorus migrate from the overlying water to sediment. It suggests that phosphorus in surface sediments acts as a sink of overlying water. These results are expected to provide scientific basis for the reef construction and restoration projects of Haizhou Bay Marine Ranch.