Long-term seasonal nutrient limiting patterns at Meiliang Bay in a large, shallow and subtropical Lake Taihu, China

Lake Taihu has undergone severe eutrophication in the past three decades, and harmful cyanobacteria blooms occur nearly every year in Meiliang Bay at the north end of the lake. To elucidate the potential relationship between seasonal nutrient limitation and phytoplankton proliferation, a 20-year (1991-2012) time series of nutrient limitation in Meiliang Bay was analyzed for deviations between trophic state index (TSI) parameters. Results showed that patterns of nutrient limitation in Meiliang Bay were distinctly seasonal, where phytoplankton growth was generally phosphorus (P)-limited in winter and spring, but nitrogen (N)-limited mainly occurred in summer and fall. This general pattern, however, shifted into N limitation across the four seasons during the mid-1990s because a rapid increase in industrialization led to a significant rise in the input of N and P from inflowing tributaries. The initial patterns were restored by environmental regulation in the end of 1990s, including the Zero Actions plan. Using routine monitoring data, a generalised additive model (GAM) with time and deviations between trophic state indexes for nitrogen and phosphorus (TSIN-TSIP) as explanatory variables was used to explore which nutrient was responsible for limitation of phytoplankton chlorophyll-a (Chl-a) in different seasons. Surprisingly, the model revealed a weak N limitation (TSIN-TSIP = -10) corresponded to peak values of Chl-a in summer-autumn season, which is probably because the phytoplankton community is co-limited by N & P during the period. The shift of nutrition limitation during winter-spring would partially explain high values of Chl-a throughout 1996. This study suggests that seasonal patterns of nutrient limitation must be considered to develop effective management measures to control cyanobacterial blooms.