Effects of sediment disturbance on the structure of benthic fauna in a subtropical tidal creek of southeastern Brazil

Abstract

An experimental analysis of benthic response to sediment disturbance by the blue crab Callinectes danae was carried out in an unvegetated flat adjacent to a salt marsh of the tidal Perequ& Creek (Paranagua Bay, SE Brazil). In order to mimic the natural pits dug by C. danae, we made randomly arranged holes, which were subsequently sampled together with undisturbed control sites on 6 different sampling dates, up to 18 d after disturbance. Analyses of variance were performed to determine if significant differences in macrobenthic population densities and abiotic parameters existed at different sampling times, and between control and disturbed areas. Apart from water content, physical variables did not differ significantly between disturbed and control areas, suggesting that adaptive strategies of the organisms themselves were the maln structuring factor of the recolonization process. We provide evidence that active immigration, together with passlve deposition, is a major colonization vector for the fauna of local low-energy tidal flats. Recolonization rates of the local fauna paralleled the mobility degrees of individual species or taxa. The numerically dominant surface crawler Heleobia australis and the burrower Lumbrineris tetraura, which are the more mobile species of local macrofauna, also exhibited the more rapid recolonization. There was no evidence that the catchment action of pits actually concentrated food resources, since sediment organic content was essentially similar between disturbed and control sites. Evidence for passive deposition was shown for the sedentariate species Laeonereis acuta, Capitella capitata and an unidentified nematode, whose recolonization rates were moderate and followed the sediment infill of pits. The only species to exhibit evidence of exploiting conditions created by pit simulation was the suspension-feeding bivalve Anomalocardia brasiliana that exhibited fast recolonization rates and higher abundance at disturbed sites. The infaunal predator Sigambra grubji was the only species present at significantly lower densities in pits throughout the experiment, probably due to the removal of its putative food source. Disturbances made by blue crabs, though short-lived, are abundant and frequent enough actually to maintain high densities of species that are otherwise scarce in similar tidal habitats. Such biogenic disturbances can be considered as a mechanism that allows for the persistence and numerical dominance of opportunistic species in tidal flats of local tidal creeks.