PARENT SESSION
OOS 4: Organisms as Ecosystem Engineers: Conceptual Progress, Limits and Challenges.
Organized by: JP Wright and CG Jones
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room E 146.

Ecosystem-engineering by burrowing crabs in a salt marsh and consequences for carbon export.

Gutiérrez, Jorge^{*,1, 2}, Jones, Clive¹, Groffman, Peter¹, Findlay, Stuart¹, Iribarne, Oscar², Ribeiro, Pablo ², Bruschetti, Martín², ¹ Institute of Ecosystem Studies, Millbrook, New York, USA² Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata, Mar del Plata, Buenos Aires, Argentina

ABSTRACT- Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the export characteristics of tidal marshes. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers if their habitat modification significantly affects detritus retention. Salt marshes located from Southern Brazil to Northern Patagonia (Argentina) support high densities (up to 60 ind m^-2) of burrowing crabs Chasmagnathus granulata. Studies conducted in Mar Chiquita coastal lagoon (37° S, Argentina) show that both the burrowing activity of crabs and the physical presence of their burrows have important consequences for the retention/export of detritus and sedimentary organic carbon within the marsh. Burrows function as sediment and detritus traps, collecting an important proportion (>30 %) of the macrodetritus produced in the marsh. Although crabs excavate macrodetritus from their burrows during burrow maintenance, most (90 %) detritus is retained for several months in sediment mounds that result from crab excavation. Furthermore, mound production by crabs leads to the burial of detrital fragments that would otherwise remain as litter on the marsh surface (50% of the litter standing stock is buried under mounds in a month). While the excavation of sedimentary organic carbon by crabs exceeds its deposition into their burrows, sediments excavated by crabs have a lower content of readily (10 d) labile carbon relative to these collected by their burrows. Collectively, these findings indicate that ecosystem engineering by burrowing crabs (1) increase retention and in situ processing of macrodetritus within the marsh by increasing burial rates, and (2) increase the probability of export of sedimentary organic carbon by increasing total carbon availability, and decreasing the proportion of readily labile carbon in surficial marsh sediments. Incorporating the activities of ecosystem engineers in current models of marsh export emerges as important for a more adequate understanding of the function of marshes in the fluxes of nutrients and materials in estuarine ecosystems.

Key words: salt marsh, burrowing crabs, ecosystem engineering, carbon export