Top-down effects of Anodonta woodiana on nutrient concentration & phytoplankton community composition in a microcosm ecosystem

Abstract

Microcosm experiments were conducted to study the impacts of intensive cultivation of Anodonta woodiana on nutrient concentration(different forms of phosphorus and nitrogen)and phytoplankton community structure.The results showed that the bivalve increased the concentration of total soluble phosphorus,total soluble nitrogen,and water transparency,but had decreased the concentration of particle nitrogen.There were,however,no significant influences on the total phosphorus,total nitrogen,and ortho-phosphorus.The intense filter feeding of Anodonta woodiana strongly restrained the growth of phytoplankton.Apparent changes of phytoplankton community structure were observed.The percentage of Cyanobacteria sp.decreased greatly,while the percentage of Chlorophyte sp.increased during the first 30 days'experiment.There was a slight increase of Bacillariophyte sp. percentage,but no change of Cryptophyte sp.The bottom-up effect of nutrient(i.e.phosphorus,nitrogen)on phytoplankton,which was significant in the control(no feeding),had been shifted to the strong top-down effects based on the Spearman correlation analysis between nutrient concentration and biomass of phytoplankton.With the decreasing of phytoplankton abundance,a sort of filamentous alga,Ulothrix sp.,and some attached algae had gradually inhabited in the microcosm system.This study suggested that the use of intensive bivalve for bio-manipulation may not be a good method to changenge the eutrophication status significantly,although its suppression of phytoplankton and improving of water transparency were observed in the microcosms.