Analysis On Carbon Sources And Their Utilization Rate In Fish Stocking And Existing Vegetation Areas Of Lake Wuliangsuhai Using Stable Isotope Technique

The over-growth of the submerged plants caused by the eutrophication had accelerated the swamping process and thus threatened the various ecological functions and values in the Lake Wuliangsuhai,including the maintainence of fish species diversity.Stocking fishes can play a role in delaying the swamping process through top-down control by feeding on and transforming the overgrown vegetation,speeding up the material circulation and energy flow.But the pathway and utilization rate are not clear.In this study,carbon and nitrogen stable isotope technique was used to compare and analyze the transformation pathway and utilization rate of carbon sources in the two areas（fish stocking area and existing vegetation area）of Lake Wuliangsuhai,so as to explore the influence of fish stocking on material circulation and energy flow in the Lake Wuliangsuhai ecosystem,and provide reference for the swamping control.There were significant differences in the δ¹³C and δ¹⁵N values of five potential food sources（phytoplankton,particulate organic matter,debris,water grass and reeds）in the vegetation area（Kruskal-Wallis analysis,P<0.01）.The mean values ofδ¹³C ranged from-32.67‰to-17.39‰,and the mean values ofδ¹⁵N ranged from 7.22‰to 15.66‰.The mean values of δ¹³C and δ¹⁵N from consumers in the vegetation area varied from-32.50‰to-23.54‰and 11.06‰to 17.80‰,respectively.There were also significant differences in the δ¹³C and δ¹⁵N values of the four potential food sources（phytoplankton,particulate organic matter,debris and reeds）in the fish stocking area（Kruskal-Wallis analysis,P<0.001）.Due to the less influence of exogenous nutrient input,compared with the vegetation area,theδ¹³C of the four potential food sources in the fish stocking area were more enriched,and the mean values ranged from-30.12‰to-16.90‰.The δ¹⁵N level was lower with the means ranged from 7.11‰to 12.43‰.The mean values of δ¹³C from consumers in the fish stocking area ranged from-29.58‰ to-20.28‰,while the consumer’s δ¹⁵N value was low,and the means ranged from 9.11‰to 13.74‰.The δ¹³C values of all analyzed components were more abundant in the fish stocking area,while the δ¹⁵N values were higher in the existing vegetation area.The trophic level was calculated using the δ¹⁵N of debris as a baseline,and the food chain of the vegetation area（TLmax=4.11）was longer than that of the fish stocking area（TLmax=3.24） because there were two top predators,Channa argus and Silurus lanzhouensis in the vegetation area.The total trophic level（2.30⁴.11） of fish in the vegetation area was higher than that in the fish stocking area（1.60³.24）.The increased interspecific competition for food caused by stocked aquatic animals in the fish stocking area resulted in the decline in trophic levels of the consumers because they were obligated to feed more on the food sources with relatively lower trophic level.The five species common in the two areas,Acheilognathus macropterus,Pseudorasbora parva,Carassius auratus,Abbottina rivularis,and Hypseleotris swinhonis had lower trophic levels（1.69,2.50,2.20,2.68 and 2.95,respectively） in the fish stocking area than those（3.21,3.37,3.44,3.42,and 3.66,respectively）in vegetation area.The composition and utilization rate of carbon sources in the existing vegetation area were significantly different from the fish stocking area.The calculation based on SIAR model showed that the contributions of five types of carbon sources for aquatic animals in the existing vegetation area were quite different.Phytoplankton was the most important source with the mean contribution rate of 29.9%,followed by POM with the mean contribution rate of 24.3%,the sum of the two is 54.2%.The mean contribution rates of reed,water grass（Potamogeton pectinatus,Myriophyllum spicatum）and debris were 17.1%,15.6%and 13.1%respectively,and the contribution rate of water grass and debris was low,and the sum of the two was less than 30%.The four carbon sources in the fish stocking areas were all important to consumers.Notably,POM was the most important one with an average contribution rate of 27.7%,followed by reed,debris,and phytoplankton with the mean contribution rates of26.1%,23.5%,and 22.7%,respectively.Compared with the vegetation area,the contribution rate of reed and debris increased significantly,and the sum of them increased from 30.2%to nearly 50%.In order to systematically explain the process of energy flow and the contribution of different types of food source in the two areas,the food web models of the vegetation and fish stocking areas were constructed separately.Phytoplankton and POM together constituted the energy basis of food webs in the vegetation area,which were the most important organic carbon sources for consumers in this area.Considering the approach of energy flow,there were two main pathways of carbon flow in the vegetation area:（1）the Pastoral food chain:Phytoplankton/POM→zooplankton/Chironomus sp→crustaceans（Macrobrachium nipponense,Gammarus sp）/small omnivorous fish（Carassius auratus,Abbottina rivularis,Pseudorasbora parva,Triplophysa scleroptera,Acheilognathus macropterus）→large carnivorous fish（Channa argus,Silurus lanzhouensis）;and（2）detrital food chain:detrital/C₃plants→Radix auricularia/crustacean/small omnivorous fish→large carnivorous fish.The pastoral food chain was the main energy flow pathway of food web in the vegetation area.The four carbon sources together constituted the energy basis of the food web in the fish stocking area.Among them,POM was the most important organic carbon source for consumers.However,there were two main carbon flow pathways in the fish Stocking area:（1）Pastoral food chain:Phytoplankton/POM→zooplankton/Macrobrachium nipponense/Chironomus sp→Hypophthalmichthys molitrix,Aristichthys nobilis/small omnivorous fish（Carassius auratus,Abbottina rivularis,Pseudorasbora parva,Hemiculter leucisculus,Triplophysa orientalis,Hypseleotris swinhonis）,and（2）detrital food chain:detrital/Reed→herbivorous fish（Ctenopharynodon idellus,Acheilognathus macropterus,Cyprinus carpio）/Eriocheir sinensis/Macrobrachium nipponense/Hypophthalmichthys molitrix,Aristichthys nobilis.The detrital food chain was the main pathway of food web in the fish stocking area.In summary,stocking of fish and Chinese mitten crab and other aquatic animals had promoted the material circulation and energy flow in the ecosystem,and played a positive role in controlling the swamping process of the Lake Wuliangsuhai.However,it is necessary to comprehensively study the influence of stocking aquatic animals such as fish and Chinese mitten crab on the ecosystem in the Lake Wuliangsuhai from various aspects,so as to providing references for promoting the health of this lake ecosystem while controlling the swamping process.