Phosphorus Bioavailability and Release Potential Risk of the Sediments in the Coastal Wetland: A Case Study of Rongcheng Swan Lake, Shandong, China

Rongcheng Swan Lake, a natural coastal wetland, is connected to Rongcheng Bay of Yellow Sea, China. In this lagoon, water quality has deteriorated gradually in recent 30 years and filamentous Chaetomorpha linum have bloomed during the past five years. In present study, the concentration, fractions, and bioavailability of phosphorus (P) in surface sediments were investigated throughout the lake. In order to evaluate P retention ability of the wetland, six sediment samples from different lake regions were collected to study P adsortion kinetics, adsortion capacity and the relation to sediment properties. In Swan Lake, total P concentrations in sediments varied from 79 to 616 mg/kg, which were significantly higher in northwest region than those in southeast region. Sediment grain size and external inputs were the main factors affecting P distribution. Inorganic P concentration was obviously higher than that of organic P, and Ca-P was the important form in the sediments. Fe/Al-P showed a large spatial variation, with relatively high concentrations occurring at the northeastern corner and the western tip strongly affected by the discharge of wastewater. The adsorption isotherms were well fitted by Linear equation at low P conditions and by Langmuir equation at high initial P concentrations, respectively. The sediments in the northern and central lake had higher P adsorption capacity, while in southern lake the degree of P saturation of the sediments was high. Ammonium oxalate extractable Al, organic matter, and fine particle were important factors influencing P adsorption. In Swan Lake, algae available P (AAP) was the largest form of bioavailable P, followed by Olsen-P; moreover, AAP and Olsen-P showed significant correlations with Fe/Al-P. As a whole, TP concentration in sediments from Swan Lake was low, and the sediments at most regions had buffering ability for P in overlying water; while at the western tip of lake where macroalgal blooms frequently breakout, AAP concentration was relatively high and the sediments had higher potential for releasing P into water column.