Effects Of Eutrophication And Benthivorous Fish Activity On Macrophytes Under Climate Warming Envelope

Shallow lakes theory claims that there are two alternative state conditions,which are a clear water state dominated by submerged macrophytes,and a turbid water state driven by phytoplankton.Climate warming,eutrophication and benthivorous fish activities will lead to a decline of macrophyte communities,and then reinforce processes involved in forward switching between ecosystem stable states.Previous studies had focused on effect of single factor and synergistic effects of climate warming and eutrophication on macrophytes.However,maternal environment effects of warming and eutrophication on the growth characteristics of macrophytes' offspring are still unknow.Meanwhile,the effect of climate warming and benthic fish disturbance on aquatic plants is also relatively scarce.Here,we set up an outdoor mesocosm experiment in order to discuss maternal environment effect of warming and eutrophication on the emergence of Potamogeton crispus,and reveal the response of different life-forms aquatic macrophytes to warming and benthivorous fish activity.The main results are as follows:(1)Effects of warming and eutrophication on the growth characteristics of Potamogeton crispus showed that maternal warming and eutrophication significantly affected the early life stages of curled pondweed,P.crispus,a submerged macrophyte.The emerged number of P.crispus shoots were significantly higher in T and TP treatments than the control.The final number of P.crispus shoots in T treatments were notably higher than in other treatments.The results indicated that warming promoted the emergence of P.crispus shoots,whereas phosphorus addition did not affect the emerged number of P.crispus shoots significantly.Besides,elevated temperature led to a decrease of total nitrogen concentrations and an increase of carbon: nitrogen ratios in plant tissues,which reflected an increased nutrient use efficiency of P.crispus,and relatively low demand for nutrients(e.g.carbon,nitrogen)involved in physiological and biochemical processes.(2)A subsequent germination experiment in the same ambient condition showed that maternal warming significantly promoted the emergence of propagule of P.crispus.Propagules produced in warmed conditions had higher emerged shoot number and germination rate than those under ambient conditions.Meanwhile,maternal warming had a significant positive effect on the early growth of offspring of P.crispus,which could benefit the early establishment of its population.However,due to the influence of maternal environment eutrophication,the number of emerged shoot and germination rate of propagule of P.crispus in phosphorus addition treatments were relatively low even at same ambient temperature.P.crispus grew in warmed and eutrophic waters may adapt to the changing environment by altering the ecological stoichiometry of plant tissue and their propagules,thereby affecting the emergence of propagule and early growth of offspring seedlings.(3)Effects of warming and benthivorous fish activity on aquatic macrophyte communities indicated that warming did not significantly influence the abundance of phytoplankton,but significantly increased the attached algae abundance.When synergistic effects of climate warming and benthivorous fish activity were considered,we found that benthivorous fish activity was significantly increased as indicated by water turbidity,phytoplankton and attached algae biomass were also significantly higher than other treatments.Warming advanced the emergence of P.crispus and promoted the growth of P.crispus significantly.Bioturbation caused by benthivorous fish and combined effects of warming and benthivorous fish increased water turbidity,phytoplankton and attached algae biomass,which finally lead to P.crispus decline.In addition,warming had no effect on the coverage of floating-leaved macrophyte,T.bispinosa.While the bioturbation caused by benthivorous fish would enhance the release of nitrogen and phosphorus from sediments,which promoted the growth of T.bispinosa in summer.Our results suggested that there were different responses to warming and benthivorous fish bioturabation of free-floating and submerged macrophytes.