Abstract
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 delta C-13 and delta S-34 values for Z. capricorni was wide (> 5 parts per thousand), which highlights the importance of accounting for spatial and temporal variability in primary producer isotopic signatures. The range of delta N-15 values for Z. capricorni was comparatively small (< 1 parts per thousand). Mixing models based on delta C-13 and delta N-15 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 S-34-depleted sulfur to consumers.