Origin and trophic importance of detritus-evidence from stable isotopes in the benthos of a small, temperate estuary.

Ratios of C/C and N/N were measured in dissolved inorganic carbon (DIC), marginal vegetation, benthic macrodetritus (diameter > 1 mm) and selected invertebrate consumers in the Gamtoos estuary, South Africa to: (1) trace the provenance of benthic detrital deposits, and (2) determine the extent to which three abundant species of macroinvertebrates utilise this resource. DIC was strongly depleted in C with average δC values (-9.5±0.5‰) being typical of limnetic waters. Benthic detrital particles (δC-24.1±0.3‰) originated mainly from marginal vegetation (δC-25.7±0.3‰), but their slightly elevated carbon ratio suggests additional input from C-rich sources-possibly C plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma robiculare together account for 96% of total benthic biomass in the upper regions of this estuary. Marked differences in trophic niches were evident among these three consumer species. Ghost shrimp (δC -32.5±0.3‰) foraged by filter-feeding on fine suspended particulate organic matter (δC-31.2±0.5‰). Amphipods (δC-28.0±0.6‰) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (δC-23.8±1.5‰) appeared to utilise benthic detrital particles to any significant extent. In the benthic consumer community of the upper Gamtoos estuary, suspension feeders make up 98% of biomass and thus clearly dominate over deposit feeders. This can be traced to the low contribution of higher plants (c. 13%) to overall carbon production, and detritus originating from macrophytes is consequently relatively unimportant in supporting invertebrate secondary production in this particular system.