Diatom evidence of ecological change in a deep subtropical reservoir (southeastern China) over the past 60 years

Understanding the ecosystem history of reservoirs threatened by water-quality degradation is of utmost relevance for public health and ecosystem services. In this study, we examined the responses of diatom composition and diversity (richness and evenness) to long-term climate change and anthropogenic activities since 1960 CE in a deep subtropical reservoir (Qiandao Lake) in southeastern China. Besides, we detected structure changes in diatom community with F statistic test, and disclosed early warning signals of critical transition with a composite index comprising three resilience metrics, variance, skewness, and lag-1 autocorrelation (AR1). Our results showed that annual mean water level (MWL), annual mean temperature (Tann), ratio of total nitrogen to total phosphorus (N:P), and fish catch (FC) significantly affected diatom compositional change. Diatom richness was regulated primarily by MWL and Tann, whereas evenness was affected by MWL and FC. In general, human activities (most notably, water-level regulation) exerted the greatest influence on diatom composition and diversity changes since the impoundment. In maintaining the service function of Qiandao Lake, assessing the ecological effects of water-level regulation is important. F statistic test indicated striking structure changes of diatom assemblage in 1980 and 2002 CE. According to the composite resilience index, the early warning signals appeared a decade before the regime shift in 2002 CE. This study demonstrated that resource competition was probably the critical mechanism triggering regime shifts in the diatom community during the past 60 years. For management practice, our work suggested the necessity of monitoring water temperature and nutrient stoichiometry regularly, and implementing more stringent measures for constraining nutrient loads are necessary in preventing catastrophic shifts in Qiandao Lake ecosystem.