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

Ratios of C-13/C-12 and N-15/N-14 were measured in dissolved inorganic carbon (DIG), 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-13 with average parts per thousand(13)C values (-9.5+/-0.5 parts per thousand) being typical of limnetic waters. Benthic detrital particles (delta(13)C -24.1+/-0.3 parts per thousand) originated mainly from marginal vegetation (delta(13)C -25.7+/-0.3 parts per thousand), but their slightly elevated carbon ratio suggests additional input from C-13-rich sources - possibly C-4 plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma orbiculare 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 (delta(13)C -32.5+/-0.3 parts per thousand) foraged by filter-feeding on fine suspended particulate organic matter (delta(13)C -31.2+/-0.5 parts per thousand). Amphipods (delta(13)C -28.0+/-0.6 parts per thousand) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (delta(13)C -23.8+/-1.5 parts per thousand) 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.