Effects of seasonality, length of immersion, locality and predation on an intertidal fouling assemblage in the Northern Gulf of Mexico

Abstract

Recruitment of the intertidal fouling assemblage was studied by placing out tiles at four sites along the Louisiana Gulf of Mexico coast in fall, spring and summer, for immersion periods of one, two and three months. The dominant species were the barnacle Balanus eburneus (Gould), which recruited in all three seasons, but at highest densities in the spring; the oyster Crassostrea virginica (Gmelin), which recruited only in fall, and the bryozoan Membranipora savartii, which recruited in spring and fall. Thus, the species that recruited varied with the season, as in earlier studies of colonization of free space in intertidal fouling assemblages. However, unlike previous studies, barnacle abundances or oyster and bryozoan percentage cover tended to decrease with length of immersion. To determine if these post-settlement decreases were caused by predation, we enclosed tiles in mesh cages to exclude predators, and replicated experiments at both coastal and estuarine sites to determine site-specific differences in recruitment. Barnacles recruited at higher rates at coastal sites and oysters at estuarine sites, as predicted by the literature. For barnacles, densities after one month of colonization were lower on open or partially caged tiles than on caged tiles, supporting the hypothesis that predators, like blue and mud crabs, or southern oyster drills, may regulate barnacle abundance. Caging did not increase the percentage cover of either oysters or bryozoans. Oyster abundance may be limited by larval supply, as oyster recruitment never reached above 5% cover. Small predators, like flatworms, which were not excluded by cages, may also be important for the latter two prey.