Nitrate limitation and accumulation of dissolved organic carbon during a spring-summer cyanobacterial bloom in Lake Taihu (China)

Lake Taihu, high-molecular-weight dissolved organic matter (HMW-DOM), with sizes between 1 kDa and 0.2 μm, were collected using cross-flow ultrafiltration, from three different eutrophic regions. The DOC, and HMW-DOC concentrations, as well as environmental factors, including water temperature, nitrate, phosphate, and Chlorophyll-a (Chl-a), were analyzed. A significant negative relationship was observed between Chl-a and nitrate concentration, suggesting that cyanobacterial bloom was limited by nitrate. The high phosphate concentration can probably be attributed to phosphorus released from the sediment or can be a result of the accumulation of bloom. Furthermore, DOC concentration significantly increased with water temperature, and was negatively related to nitrate concentration, indicating that these two environmental factors are well correlated to DOC dynamics. DOC concentration did not significantly corresponded with the Chl-a concentration in Lake Taihu. Moreover, the relationship between HMW-DOC and Chl-a concentration was only observed in Gonghu Bay, suggesting that DOC and HMW-DOC are not solely derived from cyanobacterial bloom. Other organic carbon origins, such as terrestrial input, were also assumed to play an important role in Lake Taihu.