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%₀ for polychaeta; #x002D;17.3 to 13.9%₀ for brachyuran crabs; #x002D; 17.2 to #x002D; 15.3 for fish) and fall (#x002D; 16.2 to #x002D; 14.3%₀ for shrimps; #x002D; 14.0 to #x002D; 11.2%₀ for brachyuran crabs) were more enriched than that of the particulate organic materials of the surface water around sand banks (mainly phytoplankton) (#x002D; 20.5 ± 0.0%₀ in early summer and #x002D; 18.3 ± 0.1%₀ in fall). Organic materials attached to cobbles, representative of benthic microalgae, showed similar or more enriched signatures than consumers (#x002D; 14.7?±0.0%₀ in early summer and #x002D; 10.3#x002D; 0.0%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. © 2006, The Plankton Society of Japan, The Japanese Association of Benthology. All rights reserved.