Carbon movement and assimilation by invertebrates in estuarine habitats at a scale of metres

Theories of large-scale (kilometres) movement of carbon within and from estuaries are often not supported by empirical data, and this provided the basis for a smaller-scale (i.e. <100 m) analysis of carbon movement and assimilation between adjacent habitats. We tested 3 models that potentially explained the movement and assimilation of carbon by resident animals in estuarine habitats at different spatial scales: coarse (>30 m), intermediate (2 to 30 m) and fine (<2 m). The carbon stable isotope signatures of 2 crab and 2 gastropod species were measured at different positions in saltmarsh and mangrove habitats (centre, intermediate and edge in the saltmarsh, intermediate and centre positions in adjacent mangroves) at 5 sites. The δ13C signatures of crabs collected from the saltmarsh (–15.6 ± 0.2‰) were significantly more enriched than those of crabs from the mangrove habitat (–22.1 ± 0.3‰), but did not differ between positions within each habitat. The δ13C signatures of crabs in the saltmarsh were similar to those of the dominant macrophyte, the salt couch grass Sporobolus virginicus (–14.9 ± 0.1‰). The δ13C signatures of crabs in the mangrove habitat were enriched relative to those of the mangroves (–27.6 ± 0.2‰), but were similar to those of the microphytobenthos in that habitat (–24.6 ± 0.7‰). The crabs thus fitted the fine-scale model of assimilation of carbon produced in their immediate vicinity, although the signatures for crabs in the mangrove habitat were also consistent with a food source comprising a mixture of mangroves and a more enriched source, possibly the salt couch grass S. virginicus. Gastropods were found only in the saltmarsh habitat. Their δ13C signatures did not differ among central and intermediate positions (–15.3 ± 0.2‰) but were lower at edge positions (–17.0 ± 0.1‰). The δ13C signatures of gastropods indicated assimilation of carbon from sources 2 to 15 m away, at the lower end of the intermediate scale. The extent of carbon movement and assimilation varies among estuaries, and our results show that in some situations it occurs at scales much smaller than previously realised.