MEPS 290:97-107 (2005)  -  doi:10.3354/meps290097

ABSTRACT: During daytime low tides, assemblages living in sediments below boulders differ from those living in sediments not overlaid by boulders. It is, however, not known whether these differences are consistent under different tidal or diel conditions. In this study, the composition of assemblages living in sediments associated with boulder fields (below and adjacent to boulders) under different diel (day and night) and tidal (high and low) conditions, was investigated. Significant differences between assemblages living in sediments below boulders and those living in sediments without boulders persisted during different combinations of daylight and state of tide. Species responsible for these differences did, however, change in relation to the level of the tide, but not to time of day. The presence of boulders in sediments allows the coexistence of species usually found on rocky substrata with those usually found in sediments. Mechanisms to explain intertidal movements of invertebrates in either hard or soft-bottom benthic assemblages have been very well described in the literature. The patterns of spatial distribution, in relation to diel and tidal fluctuations, of individual taxa found in this study, however, cannot easily be explained by these mechanisms. Consequently, the mechanisms of migrational movements of benthic assemblages associated with boulder fields need development of new conceptual models. A model considering the structural complexity of the boulder fields and the interactions between hard- and soft-bottom habitats is proposed herein.

KEY WORDS: Tidal movements · Diel movements · Intertidal boulder fields · Macrobenthos · Intertidal migration