Application of isotope mixing models to discriminate dietary sources over small-scale patches in saltmarsh

Intertidal grazing crabs play an important role in estuarine ecosystems, transforming carbon fixed by autotrophs into forms available to a wide range of consumers. Whether the autotrophic carbon is derived primarily from intertidal vegetation or microalgae is an important question to be resolved, as the modification of estuaries alters the balance between these potential food sources, and restoration efforts are best guided by an understanding of the primary drivers of ecosystem energy flow. We utilised the mosaic of C-3 and C-4 vegetated patches in a temperate saltmarsh to clarify the relative contributions of potential sources of carbon and nitrogen to the diet of 2 species of grapsid crabs: Paragrapsus laevis and Helograpsis haswellianus. The 2 vegetation communities occupied the same position in relation to tidal elevation. We analysed stable isotopes of carbon (delta C-13) and nitrogen (delta N-15) to discriminate 3 potential sources of dietary carbon using an IsoSource mixing model: microphytobenthos (MPB); fine benthic organic matter (FBOM); and fresh plant material. We found enrichment of delta C-13 and depletion of delta N-15 in crabs sampled from patches of the C-4 grass Sporobolus virginicus, consistent with the use of C-4 derived carbon compared to those sampled in the C-3 chenopod Sarcocornia quinqueflora. However, microphytobenthos was similarly depleted within large patches of S. virginicus, implying uptake of dissolved inorganic carbon originating from plant respiration. Multiple-source mixing (IsoSource) models indicated a primary role for MPB and FBOM in crab diets, with locally derived plant material making little contribution to crab diet. The result contrasts with those of studies from subtropical and tropical systems.