Nitrogen and phosphorus limitation of phytoplankton growth in different areas of Lake Taihu, China

Lake Taihu has attracted attention worldwide because of its extensive cyanobacterial blooms in the warm season. A better understanding of nutrient controls over phytoplankton is critical if the blooms are to be controlled. In the present study, we investigated the effects of nutrient addition on the growth of phytoplankton collected from Meiliang Bay (Station 1), Lake Center (Station 2), and Xukou Bay (Station 3) in July and September 2011. Cyanophyta was dominant in all water samples. Nevertheless, we observed higher proportions of other phyla and lower concentrations of microcystin-LR in September than in July. Further, Station 3 possessed many fewer Cyanophyta, but disproportionately higher concentrations of microcystin-LR than the other two stations. The overall phytoplankton biomass decreased in the order Station 1 > Station 2 > Station 3 and July > September. More interestingly, phosphorus-induced phytoplankton growth coincided with a distinct drop in particulate organic carbon (or nitrogen) to phosphorus ratio at Stations 2 and 3. By contrast, nitrogen, applied either alone or together with phosphorus, had negligible effect on phytoplankton growth. These results suggest that phytoplankton at Stations 2 and 3 were limited by phosphorus and no nutrient limitation occurred at Station 1. Therefore, nutrient limitation in Lake Taihu may be more dynamic and sporadic than previously thought and has implications for management of phytoplankton blooms.