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

ALEJANDRO BORTOLUS; OSCAR IRIBARNE,

MARINE ECOLOGY PROGRESS SERIES

INTER-RESEARCH

Año: 1999 vol. 178 p. 79 - 88

0171-8630

ABSTRACT: In this work we evaluate the effect of the burrowing crab Chasmagnathus granulata on the soil quality, and on the cordgrass Spartina densiflora in a SW Atlantic coastal lagoon (Argentina, 37º 32´ S; 57º 19´ W). Soil hardness and percolation rates were higher outside the crab inhabited area than inside, and soil profiles showed two different strata in areas without crabs. Crabs were associated to low soil water content, a higher variability in pH, and a homogeneous distribution of organic mater. Meiofaunal abundance was lower inside the area inhabited by crabs than outside. The water content of aboveground structures of S. densiflora was lower inside the crab populated areas. Leaf survival was lower inside the area inhabited by crabs than outside. In areas with new shoots (after burning by a brush fire) crabs depleted the aboveground plant structures. A caging experiment showed that crabs decreased leaf survival due to herbivory. During the experiment substrata was covered by sediment removed by crabs, and the area showed lower soil hardness and percolation rate than controls. Crabs decreased water content and organic matter on upper sediment layers. Meiofaunal abundance (arachnids and insects) was higher in control plots (without crabs) than in treatment plots. A field selection experiment showed that crabs decreased survival of young stems, but did not affect old stems. All this evidence suggests that C. granulata affects the cordgrass production by herbivory on new shoots, and can change the physical characteristics of the environment which may indirectly affect S. densiflora production. Thus, the general belief that in Spartina marshes only a small proportion of primary production is consumed in situ may not apply to the SW Atlantic Spartina marshes inhabited by crabs.