Pulses outweigh cumulative effects of water diversion from river to lake on lacustrine phytoplankton communities.

Due to the allochthonous input of nutrients and species, the cumulative effects of water diversion on water-receiving lakes deserve attention. Taking the water diversion project from the Yangtze River to Lake Taihu (WDYT) as an example, we explored the temporal effects of WDYT on the phytoplankton community and physicochemical habitat of Lake Taihu in autumn and winter from 2013 to 2018. Although the short-term diversion significantly increased the risk of importing nutrients, the relatively high quality of the diversion water compared with other inflow rivers had improved the water quality of the water-receiving lake region. The seasonal water diversion significantly increased phytoplankton diversity and community network complexity and reshaped the lacustrine community to be diatom-dominated with their relative proportions of 24.1-64.9% during water diversion periods. The contributions of physicochemical habitat changes induced by water diversion to variations in phytoplankton communities were 24.0-28.0%. The differences in phytoplankton diversity, community composition and physicochemical habitat in the water-receiving lake region between the diversion and non-diversion years were more evident than those between the non-diversion years in the same season, when comparing the multivariate dispersion indices among them. However, the lacustrine phytoplankton community during non-diversion periods still has not been essentially altered after several years of diversion, so the pulse effects of short-term water diversion were more obvious than the long-term cumulative impacts. Better control of allochthonous nutrients, appropriate increase in inflow water, adhering to the long-term operation, should be effective to enhance ecological benefits of such water diversion projects.

Lacustrine micro-ecosystem responses to the inflow discharge gradient of water diversion from Yangtze River to Lake Taihu.

Water diversion project is always taken as the emergency and effective engineering measure to deal with the cyanobacterial blooms in eutrophic lakes. The inflow discharge and duration are the critical parameters influencing the effects and costs of the water diversion activities. Due to the impacts of meteorological and hydrological factors such as precipitation and wind-wave currents, the environmental influence of water diversion on shallow eutrophic lakes is always unclear. To explore the quantitative relationships among inflow discharges, duration and ecological parameters in water-receiving lakes, the typical water diversion engineering-Water Diversion Project from Yangtze River to Lake Taihu was taken as an example and the mesocosm experiment modeling the micro-ecosystem of the water-receiving Meiliang Bay in Lake Taihu was conducted with five groups of inflow discharges according to the practical discharges of the main river channel-Wangyu River. Each micro-ecosystem had a volume of 15 L and was studied for a period of 30 days (25 days for the water diversion period and 5 days for the stop period). The results showed that the inflow discharges had different extents of impact on the physicochemical and biological characteristics of the micro-ecosystems. The concentrations of total dissolved solids, total nitrogen, nitrate, active silicate and bacterial abundance in the experimental groups (inflow discharges > 100 m3/s) were all decreased compared with the control group, with the lowest values in the period of 10-15 days. During the stop period, the concentrations of sensitive biotic and abiotic parameters were all recovered with different extents and different from the initial state of this experiment, which revealed that the effects of the short-term water diversion on lake ecosystems were resilient and durable. There were quantitative relationships among the inflow discharge, content interpolation and variation in water nutrients, with different relationships in different periods of the water diversion. The influence of water diversion on lake ecosystems was not only related to the direct impacts of allochthonous inputs, but also with the indirect effects of internal habitat variation in lakes.