Macroinvertebrate diet in intertidal seagrass and sandflat communities: a study using C, N, and S stable isotopes

Most seagrass community food-web studies using stable isotopes have been carried out in subtidal habitats during one sampling event. We used C, N, and S stable isotopes to characterise the diet of the dominant macroinvertebrates found in intertidal Zostera capricorni and sandflat communities of southern New Zealand in late summer and winter. The range of δ¹³C and δ³⁴S values for Z. capricorni was wide (>5‰), which highlights the importance of accounting for spatial and temporal variability in primary producer isotopic signatures. The range of δ¹⁵N values for Z. capricorni was comparatively small (13C and δ¹⁵N signatures suggested that Z. capricorni was a potentially important contributor (24-99%) to the diet of most consumers sampled at the seagrass site, whereas microphytobenthos dominated the diet of the same consumers at the sandflat site. The main exception was the bivalve Austrovenus stutchburyi, which had a diet consisting mostly (up to 85%) of Ulva and Polysiphonia spp. at both sites. S isotopes proved to be of limited use owing to the difficulty of sampling microphytobenthos and to potential nondietary sources of ³⁴S-depleted sulfur to consumers. Mixing models also suggested that Z. capricorni contributed more to the diet of deposit feeders in August (late winter) than in March (late summer).