Sand banks contribute to the production of coastal waters by making a habitat for benthic microalgae in the sublittoral zone: food web analyses in Aki-Nada using stable isotopes

Abstract

There are many sand banks in Seto Inland Sea, making patchy shallow zones less than 10 m deep. Due to the shallow environment, the surface sediment on the sand banks in the Hojo area, in the southern part of Aki Nada, Shikoku Island, Japan, often has a larger amount of benthic microalgae than other areas. We hypothesized that benthic microalgae contributed to the secondary production of coastal waters around sand bank areas, and investigated the food-web structure of the Hojo area using stable isotopes in early summer, mid summer and fall. Mean carbon isotopic signatures of several consumers in early summer (−16.9 to −15.1‰ for polychaeta; −17.3 to 13.9‰ for brachyuran crabs; −17.2 to −15.3 for fish) and fall (−16.2 to −14.3‰ for shrimps; −14.0 to −11.2‰ 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‰ in early summer and −18.3±0.1‰ in fall). Organic materials attached to cobbles, representative of benthic microalgae, showed similar or more enriched signatures than consumers (−14.7±0.0‰ in early summer and −10.3±0.0‰ in fall). These results suggest that benthic microalgae on the sand bank contribute greatly to the secondary or higher production of coastal waters in Hojo area.