Impact of burrowing crabs on C and N sources, control, and transformations in sediments and food webs of SW Atlantic estuaries

The intertidal burrowing crab Chasmagnathus granulatus is the dominant species in soft bare sediments and vegetated intertidal areas along the SW Atlantic estuaries (southern Brazil, 28° S, to northern Patagonia, 42° S). C. granulatus creates burrows that can reach densities of 60 burrows m–2, and its burrowing activities increase water and organic matter content of sediments. To evaluate the long-term effect of burrows on the origin and transformation of accumulated organic matter within sediments, we compared C and N stable isotope signatures of sediments, plants, and consumers within areas with and without crabs. 15N signatures of sediments and primary producers were enriched by 3 to 7‰ in areas with crabs. The enrichment was present in 4 different Argentine estuarine environments (Mar Chiquita coastal lagoon, 37° 46’ S, 57° 19’ W, Bahia Blanca, 38° 50’ S, 62° 07’ W, San Blas, 40° 33’ S, 62° 14’ W, San Antonio, 40° 48’ S, 64° 52’ W). Enrichment owing to crab activity appeared to overwhelm possible different N loads, anthropogenic influence, and other properties. Crab activity thus uncoupled the nitrogen dynamics in sediments from external controls. Enrichment of the heavier isotope of N could be the result of an increase in denitrification rates in areas with burrows. Crabs therefore forced faster transformation of available to unavailable nitrogen, making less inorganic nitrogen available to deeper waters. Food webs in areas with and without crabs were similar in shape, but less mobile benthic organisms (nematodes, fiddler crabs and the polychaete Laeonereis acuta) showed enriched N isotopic signatures. The benthic food web seemed separate from that of suspension feeders or water column consumers. Benthic microalgae were an important source for infauna, and marsh plants were particularly important for burrowing crabs.