Abstract
The trophodynamics of macrobenthic communities on two pocket sandy beaches (Eastern Cape, South Africa), situated between two estuaries including a mangrove estuary, was investigated by means of stable light isotope (δ 13C and δ 15N) analyses. Up to three consumer trophic levels were identified, with benthic carnivores acting as important higher consumers in these communities. Mangrove material and terrigenous inputs were not driving the sandy beach food webs. Instead, marine allochthonous inputs (carrion, macroalgae), possibly phytoplankton, sediment organic matter, and resident macroinfauna were the dominant food sources. The macroinfauna displayed generalist/omnivorous feeding strategies, but within the limits of predominantly marine food sources. There was evidence that carnivores actively preyed on resident beach fauna. Omnivory and intraguild feeding might also be important biological processes in these communities. Seasonal and spatial variability in stable isotope composition of the fauna was observed, but with few patterns evident. It was hypothesised that these isotopic and biochemical changes were in response to increased food availability during summer. Although mangrove material appeared not to play an important direct role in the nutrition of these sandy beach communities, it was suggested that a high retention time of particles in the bay could enhance bacterial decay of particulate mangrove material, which could then act as fine, bacterial-enriched particulate food to the macrobenthos. This remains to be tested. The implications of these findings in terms of macrobenthic stability and beach conservation are discussed.