Rapidly increasing sulfate concentration: a hidden promoter of eutrophication in shallow lakes

Abstract

Except for excessive nutrient input and climate warming, the rapidly rising SO42- concentration is considered as a crucial contributor to the eutrophication in shallow lakes; however, the driving process and mechanism are still far from clear. In this study, we constructed a series of microcosms with initial SO42- concentrations of 0, 30, 60, 90, 120, and 150 mg L-1 to simulate the rapid SO42- increase in Lake Taihu, China, subjected to cyanobacteria blooms. Results showed that the sulfate reduction rate was stimulated by the increase in initial SO42- concentrations and cyanobacteria-derived organic matter, with the maximal sulfate reduction rate of 39.68 mg (L d)-1 in the treatment of 150 mg L-1 SO42- concentration. During the sulfate reduction, the produced maximal ΣS2- concentration in the overlying water and acid volatile sulfate (AVS) in the sediments were 3.15 mg L-1 and 11.11 mg kg-1, respectively, and both of them were positively correlated with initial SO42- concentrations (R2=0.97; R2=0.92). The increasing abundance of sulfate reduction bacteria (SRB) was also linearly correlated with initial SO42- concentrations (R2=0.96), ranging from 6.65×107 to 1.97×108 copies g-1. However, the Fe2+ concentrations displayed a negative correlation with initial SO42- concentrations, and the final Fe2+ concentrations were 9.68, 7.07, 6.5, 5.57, 4.42, and 3.46 mg L-1, respectively. As a result, the released total phosphorus (TP) in the overlying water, to promote the eutrophication, was up to 1.4 mg L-1 in the treatment of 150 mg L-1 SO42- concentration. Therefore, it is necessary to consider the effect of rapidly increasing SO42- concentrations on the release of endogenous phosphorus and the eutrophication in lakes.