Effects of the SW Atlantic burrowing crab Chasmagnathus granulata on a Spartina salt marsh

In this work we evaluated the effect of the burrowing crab Chasmagnathus gr anulata on the soil quality and on the cordgrass Spartina densiflora in a S W Atlantic coastal lagoon (Argentina, 37 degrees 32' S, 57 degrees 19' W). Soil hardness and percolation rates were higher outside the crab inhabited area than inside, and soil profiles showed 2 different strata in areas with out crabs. Crabs were associated with low soil water content, a higher vari ability in pH, and a homogeneous distribution of organic matter. Meiofaunal abundance was lower inside the area inhabited by crabs than outside. The w ater content of aboveground structures of S. densiflora was lower inside th e crab populated areas. Leaf survival was lower inside the area inhabited b y crabs than outside. In areas with new shoots (after burning by a brush fi re) crabs depleted the aboveground plant structures. A caging experiment sh owed that crabs decreased leaf survival by herbivory. During the experiment , substrata was covered by sediment displaced by crabs, and the area showed less soil hardness and lower percolation rates than controls. Crabs decrea sed water content and organic matter on upper sediment layers. Meiofaunal a bundance (arachnids and insects) was higher in control plots (without crabs ) than in treatment plots. A field selection experiment showed that crabs d ecreased survival of young stems, but did not affect old stems. All this ev idence suggests that C, granulata affects cordgrass production by herbivory on new shoots, and can change the physical characteristics of the environm ent, which may also indirectly affect S. densiflora production. Thus, the g eneral belief that in Spartina marshes only a small proportion of primary p roduction is consumed in situ may not apply to SW Atlantic Spartina marshes inhabited by crabs.