Effects Of Silver Carp(Hypophthalmichthys Molitrix) On The Distributions Of Phosphorus: An Enclosure Experiment

In order to understand the effects of silver carp(Hypophthalmichthys molitrix) on the distributions of phosphorus, an in situ enclosure experiment was performed from March to May in 2015. Four treatments were set as follow: black control group(C), low density fish group(L, 0.5g/ m3, 0.05 ind./m3), middle density fish group(M, 5g/ m3, 0.31 ind./m3) and high density fish group(L, 20g/ m3, 2.94 ind./m3), each group have three repeat. Sampling once a week for six weeks. The initial TP(total phosphorus) concentration was 46±3μg/L.The average of TP during last three weeks in C treatments were significantly higher than M, but was not significant with L and H. The amount of PP > 200μm(mesoplankton) were significantly higher in enclosures without silver carp than with fish, L was also significantly higher than H and M, whereas the amount of PP in nanoplankton(< 20μm) and PP micoplankton(20~200μm) were not significant. The phytoplankton biomass,the ratio between chlorophyll a and TP were significantly higher in enclosures with silver carp than without fish, whereas the zooplankton biomass was significantly lower. In the absence of fish and low density of fish treatments, Daphnia galeata dominated the zooplankton community, whereas calanoids were dominant in the middle and high fish treatments. The biomass of fish increased in L(+260%) and M(+7%) treatments, but H(-7%) treatments showed opposite results. The amount of phosphorous excretion by fish and zooplankton was significantly lower in enclosures with silver carp than in enclosures without fish.Our experiments indicated silver carp increased the biomass of phytoplankton and the ratio between chlorophyll a and TP, decreased the biomass of zooplankton, result in more phosphorous distribution in phytoplankton. Silver carp were P sinks when their density is low and the food is enough to support the biomass increased, while fish were P source when their density is high and the food isn’t enough to support the biomass increased or even drop.