Diatom assemblage responses to multiple environmental stressors in a deep brackish plateau lake, SW China

Eutrophication, climate change, water level fall, fish introduction, and salinity have been widely recorded to impact lake ecosystems around the world. However, the combined responding pattern in the lake ecosystem to the above multiple environmental stressors is not well understood. Here, we present diatom assemblage and geochemical indicators (total organic carbon (TOC), total nitrogen (TN), and total phosphorous (TP)) in lake sediment to investigate the long-term trends in the aquatic ecosystem in response to multiple environmental stressors in Lake Chenghai, a deep brackish lake located on Yunnan-Guizhou Plateau, SW China, during the past 80 years. We identified 8 genera and 15 species of diatom reaching a relative abundance of ≥ 2% in at least one sample, with the dominant taxa such as Cyclotella rhomboideo-elliptica and Aulacoseira alpigena through the sediment core. There was a clear shift in the diatom community from oligotrophic species of C. rhomboideo-elliptica to eutrophic species such as Cyclotella meneghiniana and Cyclostephanos dubius becoming dominant since ca. 1998 CE. In addition, the changes in the fish introduction, water level, temperature, and salinity also resulted in the variation in abundance of planktonic and benthic diatoms. The increase in the abundance of diatoms of C. rhomboideo-elliptica since ca. 1986 CE may be related to the decline in water level and increasing fish production. For one thing, the decline in water level reduces the concentrations of benthic diatoms but increases the relative biomass of planktonic diatoms. For another, the increasing fish production results in the decrease in the zooplankton biomass, thus reducing the predation pressure on planktonic algae. Besides, some specific species such as C. meneghiniana may respond to the increased nutrient release and increased salinity since ca. 1998 CE. Combined, our findings demonstrate that trophic level is the main driver of diatom assemblage changes, and other environmental variables including water level, fish introduction, and climate warming also contribute to diatom community variation in this brackish plateau lake during the last 80 years.