Effects of Seasonal Inundation on Phosphorus Cycling in Freshwater Lake Sediments

Phosphorus (P) geochemical processes in lake sediments significantly affect the P cycling and eutrophication in lake, and are also research hotspots. However, the mechanisms of phosphorus seasonal cycle in lake sediments have rarely been investigated. We have conducted a geochemical investigation during both summer and winter in seasonal inundation zone (SIZ) of Dongting Lake, the second largest freshwater lake in China, and the water regimes of the lake undergoes drastic changes because of the construction of dams (such as the Three Gorges Dam) and climate changes. Sediment cores (30~40 cm of sediment) were taken from SIZ in both summer and winter and sliced at 2 cm resolution, and the lake water and pore water hydrochemistry, sediments bulk composition and iron and P speciation were examined. The results showed that the P content in the sediments was 720~1050µg/g and 900~1040µg/g in winter and summer, respectively, and it is significantly higher in summer than in winter. In individual seasons, P cycle was dominated by mineralization of organic matters, the reductive dissolution of iron (Fe) (oxyhydr)oxides and conversion between authigenic P (Ca-P) and detrital P. While from winter to summer, the increase of organic P (Org-P) and Fe bound P (Fe-P) resulted in total P increase. With increasing water level, the sampling site was inundated and herbaceous plants decayed after being submerged in the wet season, leading to increase of Org-P. Then, the degradation of increased organic matter can release P to the pore water or overlying water, which can be absorbed on the surface of sediments or Fe (oxyhydr)oxides leading to the increase of exchangeable-P (Ex-P) and Fe-P. Moreover, the increased water level from winter to summer also reduced the content of dissolved oxygen and promoted the reductive dissolution of Fe (oxyhydr)oxides. Therefore, seasonal flooding nature of the sediments can increase the P content and further increase the risk of phosphorus release from sediments to lake water. Thus, increasing fluctuations of lake water level induced by human activities and the increasingly frequent occurrence of climate change events may pose considerable risks to the productivity and eutrophication of lakes.