Food Sources And Trophic Level Of Macrobenthics In Cave By Means Of Stable Carbon And Nitrogen Isotopes

The cave ecosystem is a special subterranean heterotrophic ecosystems.The cave macrobenthos are the main biological taxa in cave aquatic ecosystem,and in the middle of the cave aquatic food web.They directly participate in the material cycle and energy flow in the cave aquatic ecosystem,and play an important role in maintaining the structure and function of the cave aquatic ecosystem.At present,there are few studies on the food source,trophic level and trophic niche of consumers in cave stream food web in China.In the present study,we investigated and collected the macrobenthos in Mawan Cave,Xianming Cave and Xiaoshui Cave in Zhengan County,Guizhou Province in June 2022.The carbon(δ¹³C)and nitrogen(δ¹⁵N)isotope characteristics of cave macrobenthos and their organic carbon sources were analyzed.Bayesian mixing model（SIAR）are used to analysis the main food sources of macrobenthos in the light zone and dark zone of caves.The trophic level and trophic niche indexes of cave macrobenthos were calculated,and the differences of trophic niche of macrobenthos in light zone and dark zone were analyzed.This study aims to provide theoretical basis for revealing the energy contribution of different organic carbon sources to macrobenthos and understanding the material cycle and energy flow of cave ecosystem.It can also provide the basic data for understanding the coexistence pattern and biological evolution among consumers in low productivity environment as well as provides scientific reference for the conservation of aquatic biodiversity and the balance of cave ecosystem.The main research results are as follows.（1）A total of 1203 cave macrobenthos were obtained in three caves,with 51 taxon belonging to 4 phyla,6 classes,15 orders and 37 families.The species composition was mainly aquatic insects,and the Ephemera sp.,Baetis sp.,and Gammarus sp.are common dominant species in the three caves.There was no significant difference in the average number of species and individuals of cave macrobenthos between the light zone,weak light zone,and dark zone,but it overall followed the order of light zone>weak light zone>dark.The functional feeding groups of cave macrobenthos are dominated by predators（PR）and gather-collectors（GC）in species composition,and the relative abundance of cave macrobenthos is dominated by gather-collectors（GC）.（2）Theδ¹³C andδ¹⁵N analyses of macrobenthos and their potential organic carbon sources indicate that theδ¹³C values of cave organic carbon sources ranged from-37.60±0.03‰to-23.76±0.23‰,and theδ¹⁵N values that of cave organic carbon sources ranged from-2.83±0.53‰to 9.16±0.90‰.Theδ¹³C andδ¹⁵N values of different organic carbon sources in the same light zone are significantly difference,theδ¹³C values of the same organic carbon source in the same light zone of different caves also have significant differences,but theδ¹⁵N values of most organic carbon sources in the same light zone of different caves have no significant difference.Theδ¹³C andδ¹⁵N values between different macrobenthos show large differences among species regardless of whether the light zone or dark zone,and the range ofδ¹³C andδ¹⁵N of macrobenthos in the light zone are generally higher than those in the dark zone.Theδ¹³C values of the same species and most functional feeding groups are significant difference in the same light zone of different caves,while theδ¹⁵N values are relatively stable.Theδ¹⁵N values of different macrobenthos functional feeding groups in the same light zone are also significant difference,and theδ¹⁵N values of predators are significantly higher than those of non-predators.In contrast,theδ¹³C values among functional feeding groups of macrobenthos did not show obvious variation regularity.（3）Analysis of the main food sources of cave macrobenthos based on Bayesian mixed model（SIAR）.The results show that coarse organic particulate matter（COPM）or sediment organic matter（SOM）had the highest contribution rate to 87.87%of non-predators in light zone,and the contribution of photosynthetic primary productivity such as moss and periphyton to non-predators in light zone is relatively low.Similarly,the coarse organic matter（CPOM）or sediment organic matter（SOM）had the highest contribution rate to 95.83%of non-predators in the dark zone.The coarse organic matter（CPOM）and sediment organic matter（SOM）are the main food source of non-predatory macrobenthos in caves.Both non-animal organic carbon sources and non-predator functional feeding groups have higher contribution to predators in light zone and dark zone,the cave predators show high omnivore.Moreover,the average contribution of non-animal organic carbon sources to most predators is higher than that of non-predatory animals,and coarse organic particulate matter（CPOM）or fine organic particulate matter（FPOM）had the highest food contribution to most predators.（4）The results of trophic level analysis showed that the trophic level of the consumers rang from 2.01 to 4.45,most of the same species have relatively stable trophic level in the same light zone in different caves or in different light zone in the same cave,and some organisms have different trophic level in different caves.The trophic level of most cave non-predatory macrobenthos is between 2 and 3,and the trophic level of some non-predatory macrobenthos is close to or higher than 3.The 53.85%of predators in light zone and 75.00%of predators in dark zone also had trophic levels between 2 and 3,the trophic level of predators and non-predators partially overlap.（5）The results of trophic niche analysis showed that the core ecological niche space（SEAc）occupied by light zone and dark zone macrobenthos generally overlap,but the CR,NR,SEAc values of macrobenthos and of most functional feeding groups in the light zone are higher than those in the dark zone.The predators occupy the highest trophic niches in both light zone and dark zone,and the trophic niche of predators and non-predators in the two light zone are partially overlapped.Moreover,the niche overlap of different functional feeding groups of macrobenthos in the light zone is generally higher than that of the dark zone.