The Impact And Mechanism Of Crucian Carp On Shallow Lake Ecosystem

Crucian carp(Carassius auratus)is one of the dominant fishes in all kinds of freshwater bodies in China.This fish could be dominated in both macrophyte-and phytoplankton-dominated lakes.As a small-sized omnivorous benthivorous fish,the food sources of crucian carp are include zooplankton,phytoplankton,zoobenthos and submerged macrophytes.So far,the effects of crucian carp on the shallow lake ecosystem and the potential mechanisms have not been fully studied.In the practice of lake restoration,the control strategies and key parameters of omnivorous fish such as crucian carp are still unclear.In this paper,we have conducted a series of outdoor mesocosm experiments to explore the impacts and mechanisms of crucian carp on shallow lake ecosystems in subtropical regions.Our study could provide a basis for lake management and restoration.The main contents and results are shown below.In restoration of eutrophic shallow lakes in warm regions,efforts were usually made to remove all fishes.Because of the strong reproductive ability of crucian carp,zero-age(0+)crucian carp(2-4 g)tends to be dominated in the later stage of restoration.To date,the effects of small crucian carp remains to be studied.Meanwhile,it is not clear whether it needs to be controlled in practice of lake restoration.We designed a single-factor outdoor mesocosm experiment to explore the size effects of crucian carp on water turbidity,nutrients and phytoplankton biomass.Three treatments were set up,included the crucian-free control group,small carp group and large carp group.Our results showed that under similar biomass,small crucian carp may have stronger negative effect than that of the large crucian carp.Thus,more attention should be paid in the practice of lake restoration.Based on the above chapter,we further explored the density effects of the small-sized crucian carp.Three treatments were named the crucian-free control group,the low-density group and the high-density group.Results showed that the water turbidity and light attenuation coefficient of the low-density crucian carp group were significantly higher than those of the control and high-density groups.Our study suggested that in shallow lake ecosystems,the effects of the crucian carp were not density-depended,and a density threshold may existed.Besides the disturbance effect,crucian carp may also prey on zooplankton,and therby affect algal growth and nutrient cycle via top-down control.For shallow lakes,the relative importance of the effects of "disturbance" and "predation" on ecosystem remains to be studied.In this chapter,we have compared the effects of crucian carp on water turbidity,nutrients and plankton biomass under two conditions(whether it can touch the sediment)by adding a net on the surface of the sediment.Our study shows that only when crucian carp increase water turbidity and nutrient levels only when they could contact sediments.Therefore,in shallow lakes lacking submerged vegetation,the "bottom-up effect" induced by crucian carp may be the main mechanism of its negative impacts on the shallow lake ecosystem.External loading control is the prerequisite for ecological restoration of shallow lakes.In subtropical and tropical regions,disturbance of crucian carp on sediments may promote the internal release of P.In this case,external N inputs may promote the growth of phytoplankton and thereby boost eutrophication.We set up a two-factor(N input and carp disturbance)mesocosm experiment to explore the impact of N inputs on lake ecosystems under the condition of crucian disturbance.Our study suggested that lake restoration needs both N and P control in warm regions where lakes often dominated by crucian carp.