| In order to study the impact of carp on water quality and plankton in a subtropical eutrophic shallow lake, enclosure experiments were carried out in Ming Lake of Jinan university from June 23 to August 18 in 2009. There were three treatments with triplicates:(1) Fish group:carps were stocked in the enclosure. (2)Fish-net group:a plastic net was put in the enclosure and carps were stocked above the net by which prevent sediment disturbance by carps. (3) Control group:there was no fish and no plastic net in enclosure.Results showed that carp did not changed the water pH and dissolved oxygen (P>0.05) whether the sediment was disturbed or not. Carp increased suspended solids and reduced transparency significantly (P＜0.05) in fish group. The suspended solids also increased' significantly (P＜0.05), but the transparency did not (P>0.05) in fish-net group. When the carp stocked, the total nitrogen and total phosphorus in water increased significantly (P<0.05). But the effect of the carp on the ammonia nitrogen, nitrate nitrogen and orthophosphate phosphorus were not significant (P>0.05). The carp changed the community structure of the phytoplankton. The Pyrrophyta and the Crytophyta were predominant in the control group, but in the fish-net group the Pyrrophyta and the Chlorophyta were predominant. In the fish group, the Pyrrophyta were predominant. The abundance of the phytoplankton increased no obvious difference (P>0.05). The carp had no significant effect on the abundance of the zooplankton, but in fish group, the abandance of thermcyclops taihokuensis increased significantly (P<0.05). Our study suggests that carp can affect water quality significantly which is likely induced through fish excretion not sediment resuspension. The effect of carp on lake ecosystems varies with density and size of carp, types of sediments and phytoplankton community structures needs further studies.
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