Modified hydrological regime from the Three Gorges Dam increases the risk of food shortages for wintering waterbirds in Poyang Lake

Poyang Lake is an internationally important wintering ground for waterbirds. Since the Three Gorges Dam (TGD) began operations, the lake’s hydrological regime has been changed, which may influence the growth of submerged plants and increase the risk of food shortages for wintering waterbirds. We explored the effect of hydrological regime change in Poyang Lake on food abundance for herbivorous waterbirds using a 19-year (1999–2017) monitoring dataset of densities of Vallisneria and their tubers. Overall, food abundance decreased significantly and fluctuated considerably after the TGD started full operations in 2009. The change was significantly correlated with the autumn drought and summer flood. Densities of plants and their tubers in early winter significantly increased with higher water levels in the previous October, and significantly decreased with higher June water levels. Autumn drought magnified the negative effect of average June water levels, and the combined effect explained 86.9% of the deviance in Vallisneria density. The optimal generalized additive model (GAM) for tuber abundance (explaining 86.2% of the deviance) contained the average June water levels, tuber density two years before and the average water level of the previous October. Average water levels in June and the previous October were important predictors of food abundance. However, TGD operations have made droughts in October a norm since 2006 and resulted in more frequent June floods. It is concluded that the risk of food shortages probably has become a new norm in Poyang Lake. The results offer new information for waterbird conservation and hydrological regime management for Poyang Lake, as well as for the middle and lower reaches of the Yangtze River as a whole.