Extreme spatial variability in sessile assemblage development in subtidal habitats off southwest Australia (southeast Indian Ocean)

Abstract

The development of marine benthic communities is strongly influenced by patterns of settlement, recruitment and survival, which may vary across multiple spatial scales in concordance with the scale-dependent processes that drive them. The temperate subtidal reefs off southwest Australia support highly diverse assemblages of macroalgae and sessile invertebrates, yet little is known about spatial variability in the structure of developing assemblages compared with established assemblages. Here, settlement panel arrays were deployed adjacent to subtidal rocky reefs, in 13–15m depth, at 3 locations spanning 400km of temperate coastline in Western Australia. Panel assemblages were allowed to develop for ~14months before they were harvested. Variability in ecological pattern was analyzed at 4 spatial scales, spanning centimeters to 100s of kilometers. The structure of sessile assemblages was vastly different between the 3 locations, in that one location (Geographe Bay) supported an impoverished assemblage comprising a single macrofaunal species whereas assemblages at the other two locations (Jurien Bay and Marmion Lagoon) supported fairly rich assemblages of macroalgae and sessile invertebrates. Multivariate assemblage structure, total richness and total cover varied significantly between the locations, although variability at the smallest spatial scale (centimeters) was consistently pronounced. Variability patterns for key taxa were less consistent across spatial scales. While explanations for the extreme between-location variability remain unclear, there was some evidence to suggest that herbivory by demersal fish may inhibit assemblage development at Geographe Bay, although local hydrodynamic factors (i.e. relatively lower water movement and influence of the dominant regional-scale oceanic current) could also be important.