Environmental Significance of Phosphorus Fractions of Phytoplankton-and Macrophyte-Dominated Zones in Taihu Lake

Abstract

It is of great importance to study the environmental significance of phosphorus fractions in overlying water and sediments of typical phytoplankton-and macrophyte-dominated zones. It will help to clarify the process of phosphorus migration and transformation in the sediment-water interface, and has practical significance for understanding the eutrophication process and its treatment in different regions of Taihu Lake. The investigation was conducted within typical phytoplankton-and macrophyte-dominated zones of Taihu Lake over four seasons to analyze the spatial and temporal differences between phosphorus fractions in water and sediments, and reveal their environmental significance. The results showed that:① Total phosphorus (TP), total soluble phosphorus (DTP), dissolved inorganic phosphorus (DIP), and particulate phosphorus (PP) in the overlying water of phytoplankton-dominated zones were much higher than those in macrophyte-dominated zones. Most of them showed seasonal characteristics, which were higher in summer and autumn than in winter and spring. PP is the main component of TP, accounting for 71.8% to 89.6%. A similar distribution character was found in the content of chlorophyll (Chl-a) compared with phosphorus concentration in overlying water. ② The concentration of TP in the surface sediments of phytoplankton-dominated zones was 372.38-529.64 mg·kg-1, and that in macrophyte-dominated zones was 304.29-454.27 mg·kg-1. In surface sediments, concentrations of TP in phytoplankton-dominated zones were significantly higher than in phytoplankton-dominated zones. The highest TP concentrations appeared in winter, and the lowest in summer. These were owing to the input of exogenous pollution, and the migration and transformation of internal phosphorus between sediments and overlying water under different environmental conditions. ③ The order of the mass fraction of phosphorus in sediments was:NH4Cl-P < Fe-P < Org-P < Res-P < Al-P < Ca-P. Mobile-P=NH4Cl-P+Fe-P+Org-P, accounting for 9.10%-16.93% of TP in phytoplankton-dominated zones, and slightly higher in macrophyte-dominated zones, where it was 8.11%-13.50%. Res-P accounted for 10.06%-14.97% of TP in phytoplankton-dominated zones, and 11.02%-20.28% in macrophyte-dominated zones. The risk of internal phosphorus release in phytoplankton-dominated zones is high, which is not conducive to the fixation and burial of phosphorus. The eutrophication degree of different regions in Taihu Lake is obviously different, and different characteristics of phosphorus release and burial are showed. The phytoplankton-dominated zones deserve special attention because of their high internal phosphorus load and release potential.