Effects Of Tilapia, Mud Carp And Macrophytes On Phytoplankton Communities

We performed a laboratory experiment to test the effects of tilapian and mud carp (Cirrhinus molitorella) on phytoplankton communities of freshwater microcosms. Through comparative field investigation, we also studied the effect of macrophytes on phytoplankton, in an urban shallow Huizhou West Lake.To assess the effects of the two species of fish on the phytoplankton communities, we setup the laboratory experiments three treatments: tilapia groups,mud carp groups and the controls(no fish). Phytoplankton and the water were sampled 6 day intervals for 31 days, and at the end of the experiment we sampled the algae (periphyton) attached on the tanks. It showed that: there were significant differences in the species,quantities,biomass,community structure and diversity of phytoplankton among tilapia,mud carp and control groups. Fish presence increased cell density of phytoplankton, chlorophyll a concentration, biomass, but the changes were more prominent for mud carp groups than tilapia groups. Mud carp presence made the diversity of phytoplankton community drop notably; At the end of the experiment Chlorophyta became dominant in the mud carp groups and Bacillariophyta in the tilapia groups. Two main factors may be attributed to the observed results: nutrient excretion and grazing. It was found that chlorophyll a concentration was correlated with inorganic nitrogen concentration and total phosphorus concentration in mud carp groups(r=0.65, r=0.65); In controls chlorophyll a concentration was also significantly correlated to total nitrogen concentration and total phosphorus concentrations (r=0.74, r=0.96); It showed negative correlation in tilapia groups. According the upper factor and the algae attached to the tanks and the feeding, we concluded that phytoplankton of mud group groups was effected mainly by nutrient excretion, while the influence is greater to phytoplankton for grazing pressure in tilapia groups. Both species of fish make the phytoplankton communities succession towards different directions.In Huizhou West Lake, we compared the difference of phytoplankton communities of Pinghu (a tropical eutrophic lake) and its ecological restoration demonstration area (ERDA) where the higher aquatic plants were planked in 2005 from April 2006 to March 2007. 130 species in 7 phylum of phytoplankton were observed in ERDA, 116 species in 7 phylum were observed in Pinghu. The number of phytoplankton in Pinghu was 30.8 times higher than that in ERDA. The biomass was also much higher Pnghu. The peak of algal growth was from June to September in the ERDA, while Pinghu from June until November, the biomass reached the highest value in November. The ERDA didn't appeare an absolute dominant species in a whole year. A large number of Synedra sp.and Achnanthes sp. appeared in spring and summer. Cryptomonas ovata and Cryptomona eiosa appeared more abundant in autumn, Cryptomona spp. and Synedra sp. in winter. While Chroococcus minutus and Merismipedia sp dominanted most of the year in Pinghu. In short, aquatic higher plants are able to exert a significant effected phytoplankton community structures in tropical shallow lakes.