Effect of algal bloom on phosphorus exchange at the sediment–water interface in Meiliang Bay of Taihu Lake, China

Taihu Lake, the third largest freshwater lake, is typical of many eutrophic lakes in China. In recent years, Taihu Lake is frequently suffering from extensive algal blooms which dramatically change the environmental conditions (e.g., oxygen concentration and pH) and thereafter may influence phosphorus (P) exchanges at the sediment–water interface. Here, we used the repeated ANOVA to compare the water P concentration either with or without algae addition by a mimic experiment. Over a period of 20 days of incubation, chlorophyll a (Chl-a), water temperature, dissolved oxygen (DO), oxidation–reduction potential (ORP), pH, nitrate (NO₃ ⁻), as well as the P fractions of overlying water and sediments were monitored. Results showed that compared with the Control treatment algal bloom significantly decreased DO, ORP, and pH and increased P concentration of overlying water. The first 7 days of incubation could be considered as a period of algal decomposition for the Algae treatment based on the decreased Chl-a concentration, as well as the black and smelly water. The increased total P (TP), dissolved total P (DTP), and soluble reactive P (SRP) during the initial 7 days mainly resulted from the P release from the decomposing algae when DO concentration was lower than 0.5 mg L⁻¹, and NO₃ ⁻ having a potential to suppress iron (Fe) reduction was up to 1.1 mg L⁻¹. Subsequently, Chl-a concentration dramatically increased and reached at a maximum value on September 17, which indicated that algal growth became the dominant process. The increased P level during algal growth might result from the sediment Fe-bound P release when DO and NO₃ ⁻ concentration was kept at low levels, which could be further confirmed by the lowest NaOH-extractable P (NaOH-P) in the sediments on September 19. The results indicate that influence of algal blooms on sediments P release cannot be neglected during algal decomposition as well as the followed period of algal growth in Meiliang Bay, the highly polluted region of Taihu Lake.