Assessing Aral Sea residual lake system: impact of fluctuating salinity on phytoplankton communities

Abstract

The Aral Sea was once the fourth-largest inland water body in the world. However, the lake rapidly shrank over the past six decades, mainly due to the loss of inflow from one of its tributaries, the Amu Darya River. Lakes and reservoirs are traditionally characterized by static chemical and morphological parameters, leaving untouched a dynamic impact of phytoplankton changes. We used an integrated approach combining traditional microscopy and FlowCam-based imaging flow cytometry to study phytoplankton communities during the 2018 and 2019 expeditions in the Aral Sea remnant lakes system. The residual Aral Sea water bodies experienced different environmental conditions, forming hypersaline South Aral, North Aral Sea that is constantly getting freshwater, and brackish Chernyshev Bay and Tushchybas Lake with 2-8 times amplitude of salinity changes attributed to the variability in the precipitation and periodical influx of freshwater. The salinity fluctuations had an impact on the phytoplankton communities in Chernyshev Bay, making it similar to the phytoplankton of North Aral in 2018 while resembling the hypersaline South Aral phytoplankton assemblages in 2019. Multivariate analysis revealed that salinity, water temperature, ammonium, and nitrates were major contributors to explaining the variance in the sampling data. We conclude that drastic phytoplankton fluctuations occur in the two brackish water bodies in the middle of the former Aral Sea, reflecting changes in salinity