| In recent decades, eutrophication, harmful algal blooms, and seasonal hypoxia in bottom water have been frequently reported in Chinese coastal waters due to excessive human-induced nutrients input. Phosphorus (P) is an essential biogenic element for marine phytoplankton and plays a key role in the eutrophication and harmful algal blooms of the estuarine and marine ecosystems. Sediment not only has a buffering effect on the P concentration in the overlying water, but also is an important source of P for sustaining pelagic primary production. In addtion, the cycling and release of P in sediments play a significant role in maintaining the water trophic status. Thus, the biogeochemical cycle of P in sediments is one of the hot topics in marine science around the world, and the information on behaviors of P adsorption and desorption in sediments is necessary to understand P cycling and estimate its release risk in estuarine and coastal settings.
Maowei Sea is a typical tropical bay with high density oyster aquaculture where industrialization and urbanization have synchronously altered the natural ecosystem structure and marine ecological environments in northern Beibu Gulf. It is in the Silk Road Economic Belt and the 21st-Century Maritime Silk Road, the development which has also become an important part of the national developmental strategy of China. The ecological environment of Maowei Sea has been affected by human activities, especially in the main estuaries of the Maowei Sea. Consequently, the average N/P molar ratios are much higher than the Redfield ratio of 16, P has become the limiting element for phytoplankton growth in the study area. The adsorption and desorption of P in sediments play an important role in the dynamic cycling of P in aquatic ecosystems. However, compared to other coastal and estuarine regions around the world, geochemical information about P adsorption behaviour characteristics in surface and core sediments in the main estuaries of the Maowei Sea, have been largely ignored.
The sediment acts as the “sink” or “source” of phosphorus (P) in water through the behaviors of P adsorption/desorption, which has a significant impact on marine primary productivity and water eutrophication. However, up to date, the characteristics of P adsorption behavior of core sediments in the estuaries of the subtropical bay are not clear. The core sediments of the two main estuarine regions in the Maowei Sea to study the effects of different sediment particle sizes and salinity on sedimentary P adsorption behavior through adsorption kinetics and isothermal adsorption experiments. P speciation in the sediments before and after adsorption experiments was quantified using the improved sequential extraction (SEDEX) method, and the P adsorption mechanisms in sediments were explored. The results showed that the adsorption kinetics of P in sediments could be described by a fast and slow two-stage first-order kinetic equation, and the adsorption isotherms fitted the modified Langmuir-crossover model. The adsorption capacity of P in sediments at different depths of the same station was different to some extent, and the desorption behavior existed at both sites when the initial P concentration was low. Sediments with smaller particle size had a high adsorption capacity for P. The increase of salinity will reduce the adsorption capacity of P in sediments, indicating that low salinity was conducive for P adsorption in sediments. The contents of exchangeable P (Ex-P) and iron-bound P (Fe-P) increased significantly in sediments after adsorption. The adsorption processes of P in sediments included physical and chemical adsorptions, and physical adsorption was the main processes. The results may provide useful information for further research on P biogeochemical cycle and ecological effect, as well as the beautiful bay construction and sustainable development of marine economy. |
