SPATIO-TEMPORAL DISTRIBUTION OF PLANKTON AND DRIVING ENVIRONMENTAL FACTORS IN THE TUOJIANG RIVER

To investigate the spatio-temporal pattern of plankton in the Tuojiang River and explore the relevant envi-ronmental factors regulating it, we conducted seasonal surveys and collected samples from 10 cross sections along the Tuojiang River in 2013. We identified 13 crustacean zooplankton species mostly composed of small-sized cladocerans and cyclopoida. Although there was no significant difference between the densities of the two zooplankton groups, copedoda dominated the total biomass of the whole zooplankton community because of a larger body size than cladocera. A total of 95 species of phytoplankton were recorded, and 7 genera dominated the total biomass of the phytoplankton community. Both zooplankton and phytoplankton were abundant in the upstream Tuojiang River crossing the urban area of Ziyang City but scarce in the lower reaches flowing through rural areas. These two plankton groups had a similar seasonal pattern in total biomass that peaked in the wet season (August) and reached its lowest level in the dry season (February). Compared with the previous studies in 1980s, there were a decline in crustacean zooplankton richness and an increase in phytoplankton abundance. Phytoplankton density was up to 2.7107 cells/L dominated by dia-tom all the year round. Results from Structure Equation Modeling (SEM) and Redundancy Analysis (RDA) indicate that crustacean zooplankton was enriched in habitats with high phytoplankton production and dissolved oxygen, and that copepoda had a higher requirement for oxygen than cladocera. Phytoplankton, especially some dominant taxa, preferred to live in an environment with high temperature, nutrient concentration and pH. Phytoplankton population was at its highest level (2.7107 cells/L) in urban sections with slow flow and high contamination, and was weakly restrained by zooplankton grazers despite their selective predation. Our study suggests substantial risks of algae blooms in Tuojiang River posed by high pollutants and therefore appeals efforts to reduce pollution in this river.