Food Web Analyses in Aki-Nada Using Stable Isotopes:Ecological Significance of Sand Banks in Coastal Waters

摘要

There are many sand banks in Seto Inland Sea,making patchy shallow zones less than 10 m deep.In this study,we tried to evaluate ecological significance of sand banks at the ecosystem level in Hojo area,the southern part of Aki Nada,Shikoku Island,Japan.We investigated the food-web structure using stable isotopes in early summer,mid summer and fall.Carbon isotopic signatures of several consumers in early summer(ca-17 to-15 per thousand for polychaetes;-17 to 14 per thousand for brachyuran crabs;-17 to-15 for fish)and fall(-16 to-14 per thousand for shrimps;-14 to-11 per thousand for brachyuran crabs)were more enriched than that of the particulate organic materials of the surface water around sand banks(mainly phytoplankton)(-20.5 ± 0.0 per thousand in early summer and-18.3 ±0.1 per thousand in fall).Organic materials attached to cobbles,a benthic-microalgae substitute,showed similar or more enriched signatures than consumers(-14.7 ± 0.0 per thousand in early summer and-10.3 ± 0.0 per thousand in fall).These results suggest that the ecosystem of Hojo area has the food web structure that benthic microalgae contribute greatly to the secondary or higher production.As sand banks often have a larger amount of benthic microalgae than other areas,sand banks have a role in fueling the production of coastal waters.