Food Web Analysis Of Salt Marshes Of The Yangtze River Estuary By Using Stable Isotopes

Salt marsh is a special ecosystem that holds high primary production,speices biodiversity,complicate trophic structure and carbon flow pathway of food web,and important ecological function.Previous studies have indicated that marsh primary production not only supports the food web within the salt marsh,but also provides trophic support for estuarine secondary production.However,there is no consistent paradigm that indicates which primary producer is the energy base of salt marsh food web,and the extent to which salt marsh supports the estuarine fishery production.In the Yangtze River estuary,there is a large area of salt marshes.At present,the roles that salt marshes play in supporting the secondary production in the estuary are not well understood.In addition,exotic C4 plant Spartina alterniflora is rapidly invading the salt marshes in the Yangtze River estuary,and displacing the native C3 vascular plants Phragmites australis and Scirpus mariqueter.It is no clear whether the C4 vascular plant invasion has serious impact on food web structure and trophic function of the salt marshes of the Yangtze River estuary.In this study,we determined the stable carbon and nitrogen isotope ratio(δ¹³C andδ¹⁵N)of primary producers, dominant macroinvertebrates and fishes in two salt marshes(Dongtan and Jiuduansha) of the Yangtze River estuary,China.Based on stable isotope technique,the study analyzed the food source of dominant macroinvertebrates and fishes,traced the carbon flow pathway of salt marsh food web,estimated the trophic level of consumers and built up a simplified salt marsh food web model of the Yangtze River estuary.The main results are shown as follows.(1)The exotic plant S.alterniflora,a C4 plant,had a significantly more enrichedδ¹³C value(-12.64±0.04%o)than other primary producers(P＜0.05).The two native C₃ vascular plants P.australis(-27.12±0.19%0)and S.mariqueter(-27.73±0.35‰)were the most depleted inδ¹³C values.The microalgae(benthic microalgae -20.02±0.07‰and Phytoplankton -22.80±0.28%o)had intermediateδ¹³C values between those of C₃ vascular plants and C4 vascular plant. (2)Both marsh vascular plant and microalgae were the bases of the food web supporting these nektons.For C.haematocheilus and S.ommaturus,vascular plants constitute a larger fraction of their carbon source than microalgae.The invasive C4 plant S.alterniJIora contributed more than 50%of total organic carbon of these two fishes and was more important than the native C3 plants being utilized as food sources.(3)The individuals of five macroinvertebrates(Assiminea latericea,Cerithidea sinensis,Sesarma denaani,Helice tientsinensis and Sesarma haematocheir)from the invasive plant monocultures had more enrichedδ¹³C values than those from the native plant monocultures.The meanδ¹³C values of macroinvertebrates from the invasive plant monocultures varied between those of benthic microalgae and S. alterniflora,while their meanδ¹³C values in the native plant monocultures resembled with stable isotope signatures of microalgae(benthic microalgae and phytoplankton).Therefore,the rapid spread of S.alterniflora in the salt marshes of the Yangtze River estuary changed the food source composition of macroinvertebrates and the percentage of organic carbon from plant detritus increased.Moreover,its invasion maybe changed the energy base of salt marsh food web,namely from the algae to the mixing organic carbon of microalgae and Spartina detritus in the food web base.(4)The stable nitrogen isotope ratios indicated a total of three trophic levels in the salt marsh food webs of the Yangtze River estuary.Eighteen macroinvertebrates and four fishes represented the primary consumers(TL2).Their trophic levels ranged from 2.0 to 2.7,and the mean TL was 2.4.Secondary consumers(TL3) included 2 macroinvertebrates and seven fishes.The trophic levels of these consumers varied between 3.0 and 3.9,and the mean TL was 3.4.The impact of body size on the trophic levels of nektons was also examined.The trophic levels of some fishes including C.haematocheir,L.macultus,Eleutheronema tetradactylum and Cynoglossus gracilis showed an increase as fishes grew. (5)Benthic microalgae and Spartina detritus were the main food source of deposit feeders(crabs and snails)in the vegetated zone of salt marshes.Three bivalves (Potamocorbula ustulata,Sinonovacula constricta,Corbicula fluminea)and two nektonic crabs(E.leptognathus and Orithyis sinica)from tidal creek and mudflat had similarδ¹³C values with microalgae(benthic microalgae and phytoplankton), which demonstrated that these consumers mainly assimilated the organic carbon source from microalgae.However,several consumers including bivalve species (Moerella iridescens)and crabs(Macrophthalmus dilatatus and Philyra pisum) had relative enrichedδ¹³C values and assimilated the mixing organic carbon from S.alterniflora and microalgae.The result demonstrated that the extent to which different nekton species depend on salt marshes showed a large variation.(6)A simplified food web model of the salt marshes of the Yangtze River estuary was built up based on the stable isotope analysis.All the consumers in the study were divided into three trophic groups such as detritivorous/algae feeders,omnivores and carnivores.Among 31 species,17 species represented detritivorous/algae feeders.Both omnivores and carnivores included 7 species,respectively.Two main carbon flow pathway in the simplified food web were found.(1)Detrital food chain;Spartina detritus / benthic microalgae→macrobenthos / detritivores nekton(polychaetes,oligochaetes,crabs and C.haematocheila and E.carinicauda)→Camivorous fishes(Synechogobius ommaturus and Lateolabrax macultus).(2) Grazing food chain;phytoplankton / benthic microalgae→bivalves/zooplankton→zooplanktivorous / carnivorous fishes.The detrital food chain was the main energy pathway in the salt marsh food webs of the Yangtze River estuary.S. alterniflora and microalgae were the energy base of the salt marsh food web of the Yangtze River estuary.