Study On The Food Web Structure And Energy Flow In River Ecosystems Based On Biomarker Analysis

The river ecosystems are rich in biodiversity and provide many important ecological services to human society,while increasing anthropogenic disturbance are causing river ecosystems to suffer from multiple environmental stresses.Quantifying food web structure and energy flow is a fundamental ecological research for understanding ecosystem structure and function of river ecosystems,which can provide important insights into how river ecosystems respond to anthropogenic disturbance.However,there are differences in the effects of environmental factors on riverine ecosystems at different spatial scales,and the understanding of how riverine food web structure and energy flow respond to anthropogenic disturbance at different scales is still unclear.This study sampled and analyzed basal resources,consumers and environmental variables from Lhasa River on the Qinghai-Tibet Plateau,three rivers（Xuerongzangbu,Jinshui River and Jurong River）on the Three Gradient Terrain of China and streams that manipulated light and nutrient in the upper Jinshui River to investigate multi-scale anthropogenic disturbance on primary producers,carbon sources,trophic structure and energy flow in river food webs by using the stable isotope techniques and fatty acid analysis.The main results were summarized as follows:（1）Dietary analysis showed that allochthonous resources contributed more than autochthonous resources to macroinvertebrate primary consumers regardless of the disturbance intensity in the adjacent land areas in the Lhasa River.Anthropogenic activities increased theδ¹⁵N values in epilithic algae and isotopic variation in basal resources and fish but reduced the trophic length and redundancy（i.e.,fewer species or taxon at each trophic level）in food webs.Additionally,the total nitrogen concentration in waters was the most important environmental variable affecting trophic diversity and redundancy.Therefore,the reduction of nitrogen inputs into streams is critical for sustainable river management and biodiversity conservation in the streams on the Tibetan Plateau.（2）The macroinvertebrates in the stream food webs of Xuerongzangbu,Jinshui River and Jurong River relied mainly on leaf litters,leaf litters and epilithic algae,and seston and FBOM,respectively.The trophic diversity and redundancy of stream food webs in the Xuerongzangbu were lower than those of Jinshui River and Jurong River,indicating that the ecosystem is more vulnerable.Meanwhile,there was a significant positive correlation betweenδ¹⁵N values of consumers and nitrogen input related to anthropogenic activities in the watershed,which can be used as a potential indicator of nitrogen pollution in waters.In addition,riparian canopy increased the trophic diversity,and moderate increases in the nutrient content increased the trophic redundancy in stream food webs.（3）Nutrient addition increased the epilithic algae and diatom density,colonial life-forms,planktonic diatoms,and high-profile diatoms but decreased the low-profile diatoms.The shaded canopy increased the low-profile diatoms but decreased the adnate life-forms,planktonic diatoms,and high-profile diatoms.Epilithic metabolism,including gross primary production（GPP）,net daily metabolism（NDM）,and P/R（Production/Respiration）increased with nutrient addition,while respiration（R）only increased when the canopy was open with nutrient addition.Epilithic GPP had a significant positive correlation with diatom density（p<0.01,R²=0.94）.The results indicated that changes in epilithic algae structure due to changes in riparian canopy coverage and nutrients could affect epilithic algal community function.（4）The results showed that nutrient addition increased theδ¹⁵N ratios of periphyton and macroinvertebrates,indicating theδ¹⁵N enrichment of benthos could reflect the nitrogen inputs to streams.Dietary analysis showed that the relative contribution of autochthonous resource（i.e.,periphyton）to the diets of stream macroinvertebrates increased and that of allochthonous resource（i.e.,leaf litter）in their diets decreased at the treatment of canopy open and nutrients addition.The trophic diversity also increased with the elevated light intensity and nutrients,while the trophic redundancy decreased in stream food webs,which indicated a decreased capability in the resistance of ecosystems to environmental stress.The results suggest the urgent need for protection and restoration of riparian vegetation and the reduction of nutrient inputs to streams due to the sensitive responses of the food web to human activities especially light and nutrient alteration in stream ecosystems.（5）Nutrient enrichment increased the carbon sources from autochthonous resources（i.e.,algae especially diatoms）indicated by fatty acid（FA）biomarkers within biofilms and grazers.The nutritional quality indicated by eicosapentaenoic acid（EPA）content of biofilms was increased with nutrient enrichment and more so with the combined effect of canopy shading,consequently leading to an increase in the nutritional quality,density,and biomass of grazers.In particular,the trophic linkages between biofilms and grazers were mainly influenced by EPA concentration in the biofilms,and strengthened with the combined effects of canopy shading and additional nutrients.The results suggest that human activities associated with changes in riparian canopy and nutrient inputs in the uplands,can alter carbon sources and polyunsaturated fatty acid（PUFA）composition of biofilms and subsequently decrease food quality by lowering PUFA transfer in their respective stream food webs.This study reveals the mechanisms of river food web structure and energy flow in response to multi-scale anthropogenic disturbance,and provides a theoretical basis for studying the dynamics of river ecosystems and the conservation and restoration of river ecosystem health in the context of global change.