BIOLOGICAL-GEOLOGICAL INTERACTIONS - STORM EFFECTS ON MACROALGAL COMMUNITIES MEDIATED BY SEDIMENT CHARACTERISTICS AND DISTRIBUTION

The North Carolina mid-continental shelf is characterized by a complex sequence of rock substrates, or hardbottoms, which support rich commu nities of invertebrates and benthic macroalgae. Three hardbottom sites in southern Onslow Bay have been studied over 3 years, including stud ies of sedimentology and dynamics of the surficial sand bodies and the benthic community structure. Between 1991 and 1993 at two of the site s, there were large differences in sediment characteristics and depth at several of the zones sampled. These differences coincided with cons iderable variability in the abundance and species composition of the m acroalgal community. In sampling zones where there was consistently lo w sediment coverage, algal biomass was uniformly high. There was only sparse, if any, macroalgal growth where gravelly sand or fine sand cov ered the hard substrate. Based on hindcasting of bottom currents gener ated by the March 1993 'storm of the century', it appears that this ep isodic event cleared fine sand from upper flat hardbottoms, resulting in considerable colonization and growth of macroalgae between the summ ers of 1992 and 1993. Macroalgae continued to be excluded from areas o f gravelly sand, even after storm passage. Furthermore, the storm did not have any significant effects on scarps and ramp areas where sedime nt cover was already low. Therefore, distribution and characteristics of sediments influenced where the storm was to have biological consequ ences. Based on the wide areal extent of the upper flat hardbottoms in Onslow Bay, the newly formed macroalgal meadows represent a significa nt increase in benthic biomass and habitat complexity on a regional sc are with potentially important ecological consequences. Copyright (C) 1996 Elsevier Science Ltd