Trophic heterogeneity in salt marshes of the northern Gulf of Mexico

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Predator-prey interactions in salt-marsh ecosystems are driven primarily by interactions among tidal dynamics, local physiography, and macrophyte density. Distinct seaward-to-landward predation gradients have been described for the continuous and expansive meadows of Spartina alterniflora on the Atlantic coast of the United States. In the northern Gulf of Mexico, Juncus roemer- ianus dominates and Spartina is generally limited to fringing bands or disjunct patches in low-lying areas. It is not known whether predation regimes vary consistently among subhabitats in these dis- junct marshes. We investigated whether small-scale variations in topography and habitat structure produce persistently distinguishable predation regimes among marsh patches in coastal Alabama. Each of our study sites was representative of the principal subhabitats found within continuous Spartina meadows: vegetation bordering open water, tidal creeks within the marsh, and interior sub- habitat. We assessed a strong trophic interaction in these subhabitats: predation pressure exerted by the blue crab Callinectes sapidus on the periwinkle Littoraria irrorata. The distribution of sublethal repairs in Littoraria shells demonstrated that attacks by Callinectes were randomly inflicted, irre- spective of attack frequency. The occurrence of sublethal shell repair was positively related to the frequency of predatory attacks on experimentally tethered snails, the abundance of crabs, and the degree of antipredatory architectural defense in the snail shells. Among-site differences in shell repair and architectural defense were related to topography and habitat structure, and these differ- ences persisted over multiple seasons and years. Physical variations thus maintain strong, persistent predation regimes among subhabitats in disjunct Spartina marshes.