Environmental variables influencing phytoplankton communities in hydrologically connected aquatic habitats in the Lake Xingkai basin

Hydrological connectivity can strongly influence phytoplankton communities and ecological status through the interchange of nutrients and organisms in interconnected aquatic habitats. However, there is little consensus regarding the distinction of environmental variables influencing phytoplankton communities related to hydrological connectivity in different aquatic habitats. In this two-year (2013-2014) study, we investigated phytoplankton communities and their relationships with environmental variables in four hydrologically connected aquatic habitats (from upstream to downstream: the Nur Dongbei wetland, Lake Xiaoxingkai, Lake Xingkai, and the Song′acha River) in the Lake Xingkai basin in northeast China. Our aim was to study the effects of hydrological connectivity on phytoplankton communities, and to identify potential indicators of water quality for the basin. We observed that environmental variables and phytoplankton communities showed spatial and temporal heterogeneity in the four habitats, with higher phytoplankton metrics in Lake Xiaoxingkai and lower phytoplankton metrics in the Song′acha River, including species richness, abundance, biomass, and diversity. Increased nutrient concentrations occurred in those habitats connected to the eutrophic Lake Xiaoxingkai during the hydrologically connected period. Lake Xiaoxingkai, Lake Xingkai, and the Song′acha River were dominated by Chlorophyta, whereas Bacillariophyta was the major component of phytoplankton composition in the Nur Dongbei wetland. Environmental variables influencing phytoplankton communities differed during connected and disconnected periods across the four habitats. Results of Pearson correlation analysis and canonical correspondence analysis (CCA) showed that phytoplankton community was mainly affected by concentrations of iron (Fe2+), total nitrogen (TN), and total phosphorus (TP) in the lakes; by water depth and transparency in the river; and by pH and silicate concentrations in the wetland. Based on the results of non-metric multidimensional scaling (NMDS) analysis, hydrological connectivity increased phytoplankton community similarity among different habitats during the connected period. Cyclotella meneghiniana and Anabaena azotica could be used as useful indicators to determine water quality conditions due to their strong sensitivity to nutrient concentrations in freshwater bodies. Our findings provide a scientific basis for water quality protection and management at the basin scale.