MEPS 220:103-117 (2001)  -  doi:10.3354/meps220103

ABSTRACT: Carbon and sulfur stable isotope ratios, as well as fatty acid composition of tissues, of dominant consumer species were determined and compared to those of potential food sources in an isolated community of Zostera marina in a shallow, semi-enclosed inlet of the Sea of Japan. Of the 6 dominant species of invertebrates, 4 species were enriched in ¹³C, compared to all sampled carbon sources alternative to Z. marina. Among them, the grazing gastropods Littorina squalida and Homalopoma sangarense exhibited the most enriched δ¹³C values. On the dual δ¹³C versus δ³⁴S plot, these mollusks occupy an intermediate position between Z. marina and epiphytes, suggesting nearly equal proportions of organic carbon from both nutritional sources. In lipids of H. sangarense there was a high content of the 18:1(n-7) acid characteristic of aerobic bacteria; however, another grazer (L. squalida) showed the lowest content of bacterial fatty acids among all consumers. Other highly ¹³C-enriched consumers were the surface-deposit-feeding mollusks, the gastropod Batillaria cumingii and the bivalve Macoma incongrua; however, their δ³⁴S values were markedly lower than those of any of the primary producers sampled, including Z. marina. Although the high δ¹³C values of grazers and surface-deposit feeders are suggestive of a great contribution of Z. marina organic carbon, no substantial concentrations of seagrass marker fatty acids were detected. Significant interspecific variations of both the sulfur isotope ratios and the fatty acid composition of these consumers suggest that there are a variety of pathways by which seagrass organic matter reaches invertebrates at lower trophic levels of the community food web. Dominant filter feeders, the bivalves Ruditapes philippinarum and Pillucina pisidium, had carbon drastically different in isotopic composition from Z. marina organic matter. Body tissues of P. pisidium and especially its gills, which bear symbiotic bacteria, were dramatically depleted both in ¹³C and ³⁴S compared to all sources of photosynthetically fixed carbon in the bay. Very low δ¹³C and δ³⁴S values of P. pisidium body together with a high content of 18:1(n-7) acid suggest a leading role of sulfur-oxidizing symbiotic bacteria in the nutrition of this species. R. philippinarum was only slightly ¹³C-enriched, compared to POM, and was the only consumer which had the high concentration of fatty acids characteristic of plankton, particularly 22:6(n-3). At the same time, it was much more ³⁴S-depleted than would be expected, assuming negligible contribution of Z. marina detritus to its food. This mollusk showed a high content of branched fatty acids, especially the iso17:0 and anteiso17:0 acids characteristic of bacteria from sediment; this suggests that R. philippinarum assimilated notable amounts of bacteria from resuspended sediment. Furthermore, the contribution of ³⁴S-depleted bacteria, which inhabit reduced sediment, to R. philippinarum nutrition was high enough to result in the observed depletion of ³⁴S in mollusks. Further progress in food web studies of seagrass ecosystems using a complex of multiple stable isotope and fatty acid analyses would appear possible on the basis of analysis of separate components of the seagrass epiphytic community and micro- and meiobenthic organisms, inhabiting surface sediments.

KEY WORDS: Eelgrass community · Mollusks · Stable carbon isotope · Stable sulfur isotope · Fatty acids · Food web