Does transported seagrass provide an important trophic link in unvegetated, nearshore areas?

Abstract

The contribution of detritus from seagrass and other primary producers to faunal production in unvegetated nearshore areas was examined primarily using stable isotopes. Fish, macroinvertebrates, meiofauna and primary producers (seagrasses, macroalgae, seston and benthic microalgae) were sampled from sites in south-western Australia. All samples were analysed for δ 13C and δ 15N values and fish gut contents were determined. δ 13C values for seagrasses in the region were high compared to other macrophytes, ranging from 49.9 to −8.2‰ compared to −19.8 to −12.6‰ for macroalgae. The δ 15N values ranged between 4.0 and 7.7‰ for the red, brown and green algae, and between 3.2 and 5.9‰ for seagrasses. Seston and benthic microalgae samples had a mean δ 13C value of −12.8 and −14.0‰, respectively, and their δ 15N values were comparable to the macroalgae. All invertebrate fauna had mean δ 13C values considerably lower than seagrasses. However, individual samples harpacticoid copepods and polychaetes had a value as high as −11.7‰. δ 15N values for consumers were higher than those of the primary producers, except for copepods and amphipods. The δ 13C values for fish had a relatively small range, between −16.6 and −13.1‰, and the δ 15N values of fish were elevated compared to the invertebrates and primary producers, ranging mostly between 10.0 and 12.6‰. Mixing model analysis based on δ 13C values indicated that seagrass ranked low as a likely carbon source for all invertebrates other than harpacticoid copepods at a single site and some samples of polychaetes. The δ 13C values for fish were similar to those of a combination of harpacticoid and calanoid copepods, amphipods and polychaetes. The consumption of harpacticoid copepods by some fish species indicates that Amphibolis and Posidonia species in south-western Australia can contribute to the food web of unvegetated nearshore areas as detritus, but brown algae is likely to make a greater contribution. At least for the time of year that was sampled, the flow of detrital seagrass material into the foodweb may be mediated by specific detrivores, in this case harpactacoid copepods, rather than by all detritivores.