| Food sources of filter-feeding bivalve in seaweed and filter-feeding bivalve polyculture ecosystem were determined and quantified using stable carbon and nitrogen isotopes and lipid biomarkers. The contribution of seaweed organic materials to bivalves was also evaluated. Results were listed as follows:1. The ecological functions of seaweed and filter-feeding bivalve and their couple in seaweed-based ecosystems were reviewed. Seaweed has a firm top-down control on the nutrient level of the water; meanwhile, the majority of its primary production is released as particulate and dissolved organic materials, which can supply important food sources to filter feeding bivalves.2. The application of stable isotope ratios and lipid biomarker in marine food web analysis was reviewed. Detailed analyzing procedures of two methods were established.3. The detritus release process of kelp Laminaria japonica and its trophic importance to co-cultured scallop Chlamys farreri were examined for duration of 6 weeks in a laboratory polyculture system using fatty acid biomarkers and stable isotope analysis. There was a loss of 27% of wet tissue weight in the fresh kelp after the 6 week experiment. The C:N ratio of decomposing kelp blade and detritus were significantly lower than fresh tissue. Lipid biomarkers indicated that, bacteria were mostly responsible for the biodegradation of kelp tissue. Diatom and protozoan/flagellate were also found in kelp detritus. The stable isotopic results suggested that kelp detritus is trophically important to suspension feeding bivalves in the laboratory polyculture system.4. Food sources of intertidal oyster Crassostrea gigas, mussel Mytilus galloprovincialis and subtidal cultured scallop C. farreri in Jiaozhou Bay, (Qingdao, China) were determined by fatty acid and stable isotope analysis. Cultured scallop feeds mainly on diatoms together with some amount of bacteria and terrestrial materials. Flagellates are part of intertidal bivalves'planktonic food source besides diatoms, and Ulva pertusa (Chlorophyta) seaweed bed also supplied important food sources to intertidal bivalves. Stable isotope analysis showed that phytoplankton contributed 86.2-89.0% to intertidal bivalves'carbon budget; macroalga U. pertusa origin source had a contribution of 8.7-11.0%.5. Food sources of cultured scallop C. farreri were investigated in a seaweed and filter-feeding bivalve polyculture area in Sanggou Bay (Rongcheng, China). The trophic importance of kelp-derived particulate organic materials in the polyculture system was also evaluated. From February to August, detritus carbon in the water ranged from 75.52 to 265.19μg l-1, which occupied 25.6% to 73.8% of total particulate organic carbon (POC). Both of the carbon amount and proportion changed along with the growth and harvest process of the kelp. Cultured scallop obtained 14.1% to 42.8% of its tissue carbon from kelp detritus, and the percentage varied seasonally with the detritus carbon proportion in the water, and a close correlation was found between them (F=0.992, P=0.004). In May, the detritus amount in the kelp cultivation area outside of the bay was significantly higher than the bivalve cultivation area in the bay, and scallops cultured in these two sites also had different proportions of detritus diet. Evaluating result showed that, of all the 6967 tons (wet weight) of scallop production each year, kelp contributed about 57.1 tons of carbon (219.6 tons in dry mass) as a whole. Lipid biomarkers indicated that, the percent of diatom declined in scallop's food composition, whereas that of bacteria increased from February to August. Scallops cultured in Sanggou Bay showed a higher EPA/DHA ratio than Jiaozhou Bay, suggesting that there were much more food source rich in DHA in Sanggou Bay. |
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