Impacts of a large river-to-lake water diversion project on lacustrine phytoplankton communities

Abstract

Allochthonous inputs of species and nutrients and hydrodynamic disturbance induced by water diversion projects are two critical factors of ecohydrological effects in eutrophic lakes. However, identification and quantification of potential contributions of allochthonous inputs and physicochemical habitat shifts to variations in phytoplankton communities remain challenging. The present study addresses this issue with a study of the Water Diversion from the Yangtze River to Lake Taihu in China. To explore the effects and contributions of seasonal water diversion activities on lacustrine phytoplankton communities, the comparative analysis was conducted to compare the biotic and abiotic variables between the water diversion and non-diversion periods in different seasons. The results showed that seasonal water diversion activities, in addition to significantly reducing organic pollutant concentrations, definitely increased the average concentrations of nitrate and phosphorus in the Gonghu Bay. Compared with the results in the Gonghu Bay on the non-diversion days, phytoplankton diversities increased and the community compositions were altered, with the Bacillariophyta species and non-Microcystis Cyanophyta species dominating in the Gonghu Bay on the water diversion days in different seasons. The venn diagram analysis showed that the highest potential contribution of the allochthonous species to the increase in phytoplankton diversity in the Gonghu Bay on the water diversion days was about 15.8%. The physicochemical habitat disturbance induced by the water diversion contributed about 12% to 31.3% of the phytoplankton diversity and 23.3% to 31.3% of the phytoplankton community variations in the Gonghu Bay. The allochthonous phytoplankton species may contribute directly to the lacustrine phytoplankton community variations. However, due to the high loads of nutrients from inflow rivers, positive effects of water diversion shaping the phytoplankton communities were always short-term. Pollutant control and multi-objective operation, considering flood control, water supply, and water environment improvement, are indispensable for the long-term management of water diversion projects.