IMPACTS OF DENSE CRAB POPULATIONS ON CARBON EXCHANGES ACROSS THE SURFACE OF A SALT-MARSH

Laboratory mesocosm experiments were conducted to quantify the effects of 2 crab species, Sesarma catenata and Cleistostoma edwardsii, on ca rbon exchanges across the surface of a south temperate salt marsh. The grapsoid crab S. catenata was most abundant in the vegetated marsh fl ats, and the ocypodoid C. edwardsii in the unvegetated tidal creek. Re plicate mesocosms of the marsh flats and tidal creek were incubated wi th and without crabs of the species dominant in that region. Both spec ies enhanced the losses of carbon relative to uptake by the marsh, but they did so via different mechanisms. In the tidal creek, C. edwardsi i increased the net fluxes of total organic carbon (TOC) by 60 % (or 4 48 mg C m-2 d-1), but had no significant impact on community net produ ction (NP). (Bioturbation accounted for 95 %of the enhanced TOC fluxes , and excretion only 5 %). In the marsh flats, S. catenata had no meas urable impact on the fluxes of TOC, but decreased community NP by 1132 mg C m-2 d-1. (Ninety-two percent of this reduction was caused throug h reduction in epibenthic NP through crab grazing of microalgae. Crab respiration accounted for only 8 % of the reduction). The crab effects were sufficient to determine whether, for a tide of a particular tide elevation, the marsh functioned aS d carbon source or sink. When crab s were absent, the marsh functioned as a carbon sink at all except the very highest tides that, in turn, accounted for only 3 % of all tides inundating the marsh. When crabs were present, the marsh functioned a s a carbon source at 87 %, and all except the very lowest tides.