Research On The Food Web Structure In The Costal Areas Of Gouqi Island

There tends to be higher marine primary productivity in the coastal areas, which usually are good habitat of sea creatures like fish, shrimp, crabs etc. The material circulation and energy flow within the ecosystem have a combined impact on the production of the organics, the circulation of nutrient substance and the energy flow. At present, domestic understanding about the coastal area ecology, especially about the trophic interrelation among most organisms is limited, so it’s hard to evaluate the effect of the sustainable use, ecology protection and restoration behavior. Therefore, the research on the web food structure in the coastal areas has become the key to the theory and application research of reef protection, rebuilding or exploitation. To put forward scientific decision basis of protection and management of the coastal areas, this research first analyzed the species composition and temporal and spatial variation of main biological community in the web food structure in the coastal areas of Gouqi Island. Meanwhile, the research used stable isotope technology to measure the carbon and nitrogen stable isotope characteristics of the dominant and common species among primary producers and consumers of all levels in the food web. Combined with the analysis of the stomach content of the main fish, food composition and food source were confirmed. Besides, it analyzed the contribution of various food sources to the food web, estimated the trophic levels of consumers, defined nutrition levels and built a simplified model of the food web here. The main results are as follows.First, the sea areas for this research--- Gouqi Island, it is located in north Zhoushan Islands, Shengsi county, Zhejiang Province, which belongs to the Special National Marine Reserve of Ma’ an Archipelago. Ninety percent of the Archipelago is made up of reefs, and the depth of the coastal water is mostly from 0 to 30 meters. Silty clay mud and clay mud form the main seabed. The intertidal zone and subtidal zone of the reefs are abundant in benthic algae resources, which form a certain area of algae with its dominant species of macroalgae including Sargassum horueri and Ulva pertusa etc. In the coastal areas of Gouqi Island, the main dominant species of phytoplankton were Pseudonitzschia pungens(Y=8.66), Prorocentrum dentatum(Y=5.07), Ceratium fusus(Y=2.98), Coscinodiscus asteromphalus(Y=2.33), Coscinodiscus argus(Y=1.23), Coscinodiscus jonesianus(Y=1.21), Gonyaulax spinifera(Y=0.82), Gymnodinium spp.(Y=0.58), Skeletonema costatum(Y=0.37), Paralia sulcata(Y=0.32), Noctiluca scintillans(Y=0.27) and so on.Diatoms are dominant species both in number and species number. The Copepoda was the main zooplankton group, whose dominant species are mainly Calanus sinicus and Euchaetidae etc. Apart from Copepoda, the relatively important zooplankton group also included Krill, jellyfish, arrow worms and the larvae of shrimps and crabs, etc. Calanus sinicus was the absolute dominant species in spring and winter, whose dominance was also significant in summer and autumn. In summer, the larvae of Euchaetidae and Doliolum denticulatum became dominant species as well. In autumn, Sagitta bedoti was largely dominant, and the following were Diphyes chamissonis, Calanus sinicus and the larvae of Euchaetidae. In the nekton community of the coastal areas of Gouqi Island, Sebastiscus marmoratus, Nibea albiflora, Pseudosciaena polyactis, Agrammus agrammus, Thryssa kammalensis, Platycephalus, Paralichthys olicaceus and Astroconger myriaster were main dominant species. Meanwhile, Charybdis japonica, Temnopleurus hardwickii, Cantharus cecillei, Hyastenus diacanthus, Turbo Cornulus etc. were main dominant species of macroinvertebrates.Second, within the ecosystem of macroalgal bed in the coastal areas of Gouqi Island, small invertebrates such as zooplankton, amphipod, polychaetes and echinodermata were important parts of the food web, which bore the responsibility of transfering the energy of the primary producers to advanced consumers. In autumn and winter the δ13C value of primary producers was between-21.7‰ and-13.8‰, and the average value was-17.8‰. Among them, the δ13C value(-13.8‰±1.0‰) of benthic microalgae was the maximum. The following was the value of Sargassum horueri(-15.4‰±0.7‰) and Ulva pertuca(-16.2‰±0.9‰). The δ13C value of SOM was the minimum. Besides, the δ15N of primary producers was between 2.3‰ and 11.6‰, and the average value was 5.7‰, among which the maximum δ15N value of epiphyte was 11.6‰±0.8 ‰, and the δ15N value of other primary producers was less than 6.7‰. In spring and summer, the δ13C value of primary producers was between-21.2‰ and-14.2 ‰, and the average value was-17.6‰. The δ13C value of Ulva pertuca(-14.2‰±0.1‰) was the maximum, followed by the value of Sargassum vachellianum(-15.7‰±1.6‰) and Undaria pinnatifida(-16.5‰±0.3 ‰), and the δ13C value of SOM was the minimum. The δ15N value of primary producers in spring and summer was between 4.0 ‰ and 7.3‰ and the average value 5.8 ‰, among which the δ15N value of SOM was the maximum and that of the phytoplankton was the minimum. The stable isotope composition of primary producers had different varitions between the period of autumn and winter and that of spring and summer. Among them, the δ13C value of SOM, phytoplankton, POM and Sargassum horueri had no significant seasonal difference(P>0.05), while the δ13C value of epiphyte, Ulva pertuca and benthic microalgae had significant seasonal difference(P<0.05). Among all the primary producers, only the δ15N value of epiphyte had very significant seasonal difference, while the δ15N value of all the others’ had no significant seasonal difference. Meanwhile, among all the primary consumers, only the δ13C and δ15N value of copepoda had significant seasonal difference(P<0.05), while the δ13C、δ15N value of all the others’ had no significant seasonal difference(P>0.05).Based on the multi-source linear mixed model, the research found phytoplankton was the main carbon source of Spongia and zooplankton such as Copepoda, Euphausia pacifica and Acetes chinensis, Caprellidae and Gammaridae larvae etc. mainly fed on epiphyte, and small benthic invertebrates, such as Gammaridae, Anthocidaris crassispina and Turbo cornutus etc., their main nutrition source were macroalgae and benthic microalgae.Third, Sebastiscus marmoratus, Agrammus agrammus, Nibea albiflora, Pseudosciaena polyactis and Astroconger myriaster were five common economic fishes of Zhoushan Islands, which played a very important part in local fishery. Based on the analysis results of stomach content of the fishes, this research used Carbon and nitrogen stable isotope technology to analyze the feeding habit of the five fishes like Sebastiscus marmoratus and evaluated the energy support to the fishes from food organism such as Zooplankton, amphipoda, shrimp, crabs and small fishes. The δ13C value and δ15N value and body length of Sebastiscus marmoratus had an insignificant seasonal difference(P>0.05), and only the winter C/N value had a significant difference. At different development stages, the δ13C value and δ15N value of Sebastiscus marmoratus were significantly different. The δ15N value had an increasing tendency along with the increase of the body length.The results of stomach content analysis showed that the five main economic fishes like Sebastiscus marmoratus fed on Amphipoda, fish, crabs, Shrimps, Acete chinensis, Polychaetes, mollusk, zooplankton etc. All these fishes fed on benthic nekton. The stable isotope analysis of these fishes and their food organism in autumn & winter as well as in spring & summer found that the δ13C value of these fishes differed insignificantly in different seasons and only the δ15N value of Pseudosciaena polyactis differed significantly(P<0.05), the δ15N value of the other four species had no significant difference. Among the food organism, zooplankton’s δ13C and δ15N value both had a significant seasonal difference(P<0.05), so was the δ15N value of the shrimp(P<0.05), all the others’ δ13C and δ15N value had no significant difference. The δ13C value of Sebastiscus marmoratus etc. was much closer to the area with a higher δ13C value of the food organism such as Caprellidae, Gammaridae etc., which, as the research showed, was the main food source while the contribution of zooplankton was relatively less. When analyzing the nutrition contribution of food organism with Iso Source Software, the research found that Sebastiscus marmoratus, Agrammus agrammus, Nibea albiflora, Pseudosciaena polyactis and Astroconger myriaster were benthos feeders rather than zooplankton feeders. In autumn and winter, the food organism of Sebastiscus marmoratus— Gammaridae, its potential average contribution value was the maximum(50.3%), and its range was 32%~74%. The following was that of Caprellidae, which was 19.7%. The nutritional contribution of Caprellidae to Agrammus agrammus was the maximum, which average value was 38.9%. Next was the contribution of Gammaridae and it was 18%. The maximum contribution of Acete chinensis to Nibea albiflora was 54%. Among the food organism of Pseudosciaena polyactis, zooplankton and Gammaridae contributed relatively more. Fish, shrimps and crabs’ potential contribution rate to Astroconger myriaster was high, and their contributions were all above 15%. The reason why zooplankton’s contribution rate was the highest was that they had eaten more fish larvae. In spring and summer, in Sebastiscus marmoratus’ food organism, the potential maximum average contribution value of Caprellidae was 49.2%, which ranged from 4% to 80%. Next was the average contribution value of Gammaridae, and it was 29.2%. Caprellidae’s nutritional contribution to Agrammus agrammus was as high as 72.7% while the contribution of other food organism was relatively small. Fish, shrimps and crabs’ contribution to Nibea albiflora was relatively high. The maximum nutritional contribution of zooplankton to Pseudosciaena polyactis was 55.1%. Except that the contribution of zooplankton to Astroconger myriaster was below 10%, the contributions of Gammaridae, fish, shrimps and crabs were all above 15%.Fourth, the δ13C value of potential food sources-the phytoplankton, POM, SOM and macroalgae ranged from-21.4‰ to-15.2‰, and the δ13C value of zooplankton, macroinvertebrate and fish ranged from-23.8‰ to-8.7‰. The average C/N ratio of the primary producers was 8.6, and 3.8 of the consumers, the difference of which was quite obvious(P<0.001). Based on the δ15N value, the consumers could be divided into four trophic levels named Filter-feeding zooplankton, detritivorous/alage small-sized benthic animals, omnivorous animals, and fierce carnivorous fish.Based on the method of stable isotope and stomach content analysis, the food web had four trophic levels, and the consumers’ trophic level was between 2.0 and 4.74. Among them, the average trophic level of twenty five species of primary consumers was 2.26. They were Calanus sinicus, Temnopleurus hardwickii, Anthocidaris crassispina and Euphausia pacifica etc. The average trophic level of fourteen species of secondary consumers like Exopalaemon annandalei was 2.97. The average trophic level of omnivorous fish such as Sebastiscus marmoratus etc. and macroinvertebrates was 3.47, and that of the Astroconger myriaster, Miichthys miiuy, Nibea albiflora, Mugil cephalus and Sparus macrocephlus was 4.04.Cluster analysis divided the biome of the food web into two groups(fig.1), one of which mainly fed on phytoplankton, SOM and POM. This group included suspension feeders and grazers/detritus feeders, such as zooplankton, amphipoda and meiofauna etc. The other group included Carnivorous fish and some omnivorous large benthic animals. The difference between them is very significant(ANOSIM test,R=0.835,P<0.01).The carbon flow in the food web mainly went in three ways. Firstly, phytoplankton and POM were filter-fed by zooplankton, and the zooplankton was eaten by the predatory fish such as Sardinella fimbriata, Iso flosmaris, decapterus maruadsi etc. Afterwards, these small-sized forage fish were finally eaten by Nibea albiflora and Sparus macrocephlus. Secondly, macroalgae or its attached microalgae were eaten by Gammaridae and Caprellidae etc. and then these eaters became food of omnivorous fish including Sebastiscus marmoratus, Hexagrammos otakii, Agrammus agrammus and so on. Thirdly, organic detritus was eaten by small shrimps and crabs, and in turn they finally became food of benthic fish like Astroconger myriaster. In summary, the ecosystem in the coastal areas of Gouqi Island is an ecosystem of high primary productivity, in which macroalgae, phytoplankton and organic detritus were main energy source, and the route of the energy flow was relatively short.