The Influence Of Silver Carps And Floating Plant Rafts (Hawaii Grass) On Phytoplankton Community Dominated By Cyanobacteria: A Large Enclosure Experiment

Silver carp(Hypophthalmichthys molitrix) is the most important cultured species in tropic reservoirs in China.At present,silver carp and floating plant rafts have been applied to improve water quality and to control phytoplankton biomass in reservoir for drinking water supply. However,to what extent these methods improve the water quality is unclear.In order to understand the environmental effects of silver carp culture in tropical reservoir and the possibility of applying floating plant rafts to improve water quality,we conducted an experiment to know the influence of silver carp and floating plant rafts(Paspalum vaginatum) on water quality and phytoplankton community in dry season when the Cyanobacteria biomass was predominant..The large enclosure experiment was conducted in a typical reservoir in Guangdong province from Nov.to Dec.in 2008.In the experiment,there are two groups of enclosures.In the first group,silver carps were cultured in two densities(30g/m~3,50g/m~3),each with two repeats, no fish in a control enclosure;,and in the second group,the silver carps were cultured with three densities(0,30g/m~3,50g/m~3) in three enclosures with floating rafts of 30%coverage,each with one repeat.Variables of water quality such as nutrients,chlorophyll a concentration and phytoplankton were sampled every week.The TN concentration in the two enclosures with silver carp of 30g/m~3 rise from 0.60mg/L and 0.87mg/L to 1.04mg/L and 0.83rng/L in the second week after the experiment began,while the TN concentration in the two enclosures with silver carp of 30g/m~3 rise from 0.42mg/L and 0.70mg/L to 0.84mg/L and 0.89mg/L.It means that silver carps can enhance TN loading into the surface water by release.Floating plant rafts prove to improve water transparency.The SD in the enclosure with low density of silver carps and rafts increased more than in the enclosures only with fishes,but it is not markedly improved in the 50 g/m~3 of silver carps.The SD in the enclosure only with 30g/m~3 of silver carps increased from 1.4m to 1.5m.The SD in the enclosure with 30g/m~3 of silver carps and plant rafts increased from 1.35m to 1.5m.The SD in the enclosure with 50g/m~3 of silver carp and plant rafts increased 5cm.The TN concentration in the enclosure with plant rafts or both plant rafts and silver carps were reduced markedly,it decreased from 1.13mg/L to 0.88mg/L in the enclosure only with plant rafts,it decreased from 1.08mg/L to 0.75mg/L in the enclosure with 30 g/m~3 of silver carps and plant rafts.The TP concentration was reduced in the enclosures with both plant rafts and silver carps.Application of plant rafts can reduce the dominance of Microcystis.The correlation efficiency between Microcystis and phytoplankton was much less in the enclosures with both silver carps and higher plant rafts than in the other enclosures.