Effects Of Tilapia (Oreochromis Niloticus) On Phytoplankton Community Structure And Water Quality

Tilapia（Oreochromis niloticus）,as highly invasive omnivorous fish species,is widely distributed into rivers,lakes and reservoirs globally and even dominated in many freshwater systems causing serious ecological problems.Phytoplankton are generally the main primary producers in aquatic ecosystems,and its biomass and community structure will ultimately affect water quality.Mesocosm experiment with and without tilapia was designed to evaluate the effect of tilapia on phytoplankton community assemblage and water quality.Nutrients,biomass as chlorophyll a（Chl a）of phytoplankton,micro-phytoplankton（>20μm）,nano-phytoplankton（2-20μm）,pico-phytoplankton（0.2-2μm）and periphytic algae and total suspended solids（TSS）were measured.Comparing with the controls,the total nitrogen（TN）,total dissolved nitrogen（TDN）,ammonia nitrogen（NH₄⁺-N）and TSS concentrations in the tilapia treatments increased by 54%、18%、21%and 72%,respectively,deteriorating the water quality.In addition,tilapia increased the total biomass of phytoplankton and micro-phytoplankton and nano-phytoplankton biomass with increasing the proportion of micro-phytoplankton from 31%to 63%,decreased the proportion of pico-phytoplankton from 60%to 29%,thereby modifying the phytoplankton community assemblage.Moreover,tilapia reduced the biomass of periphyton by 60%.In addition,experiments with submerged macrophytes in mesocosms with and without tilapia were also conducted to evaluate the effect of tilapia in aquatic ecosystem with submerged macrophytes.Comparing with the submerged macrophyte group,the nutrients of total phosphorus（TP）and TN,TDN,NH₄⁺-N and TSS concentrations in the tilapia with submerged macrophyte group increased by 79%,52%,60%,27%and 268%,respectively,deteriorating the water quality.In addition,under presence of tilapia,the biomass of phytoplankton,as well as that of micro-,nano-and pico-phytoplankton,increased leading to an increase in micro-phytoplankton from 54%to 67%,but a decrease in pico-phytoplankton from 38%to 25%,changing the structure of the phytoplankton assemblage.The water transparency of the tilapia with submerged macrophyte group was lower than that of the submerged macrophyte group,and tilapia deteriorated the light conditions on sediment surface.Thus,tilapia increased nutrients concentrations and phytoplankton biomass,changed phytoplankton community assemblage,increased suspended particles and reduced water transparency,and ultimately deteriorated the water quality.Removal of tilapia may be an effective measure to improve the water quality.