Homogenization of reservoir eukaryotic algal and cyanobacterial communities is accelerated by dam construction and eutrophication

Abstract

Extensive construction of dams and increased human activities have greatly accelerated the homogenization of regional biotic communities. The cumulative effects caused by dams and anthropogenic nutrient loading on the temporal turnover of phytoplankton communities are largely unexplored, but of critical conservation importance for understanding reservoir ecosystem functioning. Here, biodiversity patterns in eukaryotic algal and cyanobacterial communities were investigated in a large reservoir over the last ~80 years by sequencing DNA preserved in sediment cores. The succession of eukaryotic algal and cyanobacterial communities recovered by sequencing of rRNA gene markers exhibited a high degree of consistency, with both showing evidence for significant changes in the 1970’s and 2000’s based on univariate and multivariate analyses. Temporal turnover indicated a marked loss of the taxonomic and phylogenetic structures of communities, which became more similar through time due to the homogenous reservoir environment and concomitant ecological pressures from human activities. Specifically, temporal homogenization of algal communities could be attributed to the damming of the reservoir in the 1960s that then affected the hydrological conditions of the reservoir and nutrient retention, with increased anthropogenic nutrient loading favoring the growth of specific taxa like Microcystis, as evidenced from Random Forest machine learning and ecological network analyses. Thus, these results demonstrate accelerated homogenization of algal communities in the reservoir due to the impacts of damming and nutrient enrichment. These observations also call for a need to develop conservation strategies to maintain ecosystem resilience and prevent harmful blooms in reservoir systems.