Trophic Structure Of The Major Species In The Jiaozhou Bay Considered Through Stable-isotope Analysis

Jiaozhou Bay is an important spawning and feeding ground for manycommercially important marine species. However, its trophic function remains poorlyunderstood. In recent years, stable isotope (e.g., carbon and nitrogen) analysis hasbecome a powerful tool for studying food webs in rivers, flood plains, salt marshes,lakes and marine ecosystems. In this study, stable isotope analyses were employed toexplore the trophic spectrum of food web in Jiaozhou Bay, based upon the datacollected from two surveys in spring and autumn of2011. The species includedplankton (including phytoplankton, zooplanktons of four sizes),29invertebrates(including decapoda, brachyuran, cephalopod, bivalve, gastropod and polychaete) and34fishes. All these species accounted for95%of the total biomass of the catches, andcovered all the dominant invertebrates and fish species in the Jiaozhou Bay ecosystem.Carbon and nitrogen stable isotope ratios (δ13C and δ15N) of these species weremeasured by isotope ratio mass spectrometer (IRMS, Isoprime; GV, Manchester, UK).The results showed that δ13C values for these species in Jiaozhou Bay ranged from-25.63‰to-17.16‰, with the highest13C-enriched values exhibited by Portunustrituberculatus and the lowest one by300~500μm zooplankton. The δ15N valuesranged from4.15‰to14.11‰, with the highest15N-enriched values exhibited byOdontamblyopus rubicundus and the lowest by phytoplankton. The average value ofδ13C and δ15N was-19.42±1.80‰and11.98±1.77‰, respectively. A hierarchicalcluster analysis was performed on Normalisd Euclidean distances of δ13C and δ15Nvalues. Base on the cluster analysis, the major species of food web in Jiaozhou Baywere classified into four trophic groups: primary producers (phytoplankton), primaryconsumers (zooplankton), secondary consumers (Philine Kinglippini and two fishes-Ammodytes personatus and Syngnathus acus), top predators (other invertebrates andfishes).Based on the1515N), trophic levels for these species were estimated. Almost all these species in this study belonged to trophiclevels between1.0and4.0, with the lowest trophic level of phytoplankton (1.10) andthe highest of O. rubicundus (4.03). Trophic levels estimated from nitrogen stableisotope ratios (TLN) were compared with those estimated by stomach content analysis(TLD) from the literatures. In all37species whose TLDwere available, the differencebetween TLNand TLDwere less than0.5trophic levels in29species. So the nitrogenstable isotope analysis was an effective method to study the trophic position oforganisms in the aquatic ecosystem. The continuous trophic spectrum for food web inJiaozhou Bay was established according to the trophic levels of these species. Trophiclevels of most species (59of63) were between3.0and4.0, which indicated that thefood web of Jiaozhou Bay consisted mainly of lower and mid-level carnivorousspecies. We recommend the use of both stable isotope and stomach content analysesto better understand the food web characteristics of aquatic ecosystems.Stable isotopes are increasingly being used as tracers in environmental studies.One application is to use isotopic ratios to quantitatively determine the proportionalcontribution of several autotrophic sources to consumers. A Visual Basic program(IsoSource) was created by Phillips and Gregg to perform these calculations foruser-specified data. This paper, using the IsoSource program, attempted to identify theultimate autotrophic sources for demersal fishes at Jiaozhou Bay, and mainly focusedon their δ13C values. A total of33demersal fish species in Jiaozhou Bay wereanalyzed, belonging to7orders,20families and29genera. The studied demersalfishes had δ13C values between-22.49‰and-17.26‰, with the average of-19.15±0.99‰, and Tridentiger barbatus was the most13C-enriched fish, while A.personatus was the most13C-depleted species. The δ13C values of the studied organicsources ranged from-21.00‰to12.61‰，with SOM (sediment organic matter) beingthe most13C-enriched sources (-12.61±0.51)‰，POM (particulate organic matter,including phytoplankton) being the most13C depleted source (-21.00±0.51)‰，andMPB (Microphytobenthos) having δ13C values in the middle. Using δ13C values asreferences，we then estimated the contributions of three organic sources as foodsources for these33demersal fishes living in Jiaozhou Bay. The results showed that POM was the most important ultimate autotrophic sources for demersal fish species;MPB, second only to POM, was also indispensable autotrophic sources for somefishes; SOM was a contributor to only a few fishes. These demersal fishes could bedivided into four groups according to their ultimate trophic sources. The fishes amongthe first group derived almost all ultimate trophic sources from POM. The first groupincluded A. personatus, Lophius litulon, Pholis fangi, Tridentiger trigonocephalusand Larimichthys polyactis. The δ13C value of A. personatus was higher than that ofPOM, so all A. personatus’s trophic sources come from POM. IsoSource modelingsuggested that the contributions of POM as food sources for L. litulon, P. fangi, T.trigonocephalus and L. polyactis were84%,77%~87%,75%~89%and71%~89%，respectively. The fishes among the second group assimilated most trophic sourcesfrom POM and some trophic sources from MPB. More than half demersal fishes (18species) were included in this group, Sebastes schlegelii, Apogon lineatus, Sauridaelongate, whose δ13C values were between-20.0‰and-18.5‰. The demersal fishesin the third group gained most trophic sources from MPB and some from POM. Total8species e.g. Kareius bicoloratus, Paratrypauchen microcephalus et al. were amongthe third group. Cynoglossus joyneri and T. barbatus in the fourth group had themixed organic carbon source of MPB, POM and SOM.