Thermocline Stratification Favors Phytoplankton Spatial Overlap and Species Diversity in a Subtropical Deep Reservoir

Abstract

Though algal blooms have become more prevalent, monitoring phytoplankton at the surface and depth become critically important in freshwater ecosystems. To fully understand the effects of thermal stratification on phytoplankton species packing across the whole water column, we collected high-frequency depth profile data of chlorophyll fluorescence (ChlF), water temperature, and phytoplankton structure in oligotrophic Lake Qiandaohu. Significant linear relationships were observed between the thermal depth (TD) and deep chlorophyll maxima (DCM) parameters (e.g., depth, concentration, and thickness). In addition, spatial overlap (SO) amongst major phytoplankton groups increased with thermal stratification becaming strong. SO was also significantly positive to phytoplankton species diversity through community evenness. The results suggested that strong stratification favored the shallower, thinner DCMs formation, leading to large phytoplankton SO and high species diversity mainly through a preclusion of dominance. Interestingly, our findings showed cyanobacteria were not influenced by thermal stratification. Other phytoplankton (such as Chlorophyta and Bacillariophyta) may coexist with Cyanobacteria at the surface and has the risk of forming persistent blooms in the future under climate warming. Our results support the theoretical prediction of species packing that should occur in thin layers where the light environment co-limits with nutrients.

Keywords: Vertical distribution of phytoplankton, Deep chlorophyll maxima, Spatial overlap, Species diversity, Thermal depth