Biogeographical patterns of rocky shore community structure in south-east Australia: effects of oceanographic conditions and heat stress
Aim Environmental variability can be a major driver of large-scale patterns of distribution and abundance and, within the marine environment, benthic community structure is usually thought to reflect several oceanographic processes and coastal morphological features. The aim of this study was to quantify spatial and temporal variability in large-scale rocky intertidal community structure along the south-east coast of Australia and ask whether these are associated with variation in sea-surface temperatures, chlorophyll-a concentration, heat stress and coastal geomorphology. Location Rocky intertidal shores of south-east Australia spanning > 1750 km, 13° of latitude (26°24′23″ S to 39°07′47″ S) and known biogeographical barriers. Methods From 2009 to 2011 one-thousand and twenty photoquadrats located within the midshore region of 17 rocky shores (20 quadrats site−1 year−1) were used to estimate spatial and temporal variability in the percentage cover of dominant benthic invertebrates and macroalgae. We used satellite data to generate annual estimates of nearshore sea-surface temperatures (SST) and chlorophyll-a concentrations ([Chl-a]) and a meteorological model to estimate heat stress during summer low-tides. Results Non-metric multidimensional scaling and cluster analysis revealed a strong and consistent biogeographical break in rocky intertidal community structure between southern and eastern sites. Nearshore oceanographic conditions and summer heat stress did not appear to influence this discontinuity as abundances of functional groups (filter-feeders, grazers and macroalgae) generally did not vary in response to geographical patterns of SST, [Chl-a] or mean daily maximum air temperatures during low-tide. At the species level, however, the distribution and abundance of several species varied with nearshore SST and [Chl-a] including the barnacle Tesseropora rosea and limpets within the genus Patelloida, respectively. Main conclusions We found no evidence of strong bottom-up effects on rocky intertidal functional group abundance. This was somewhat expected because bottom-up effects in regions characterized by persistent downwelling are generally weak or non-existent. Instead, broad-scale patterns of community composition were best explained by coastal geomorphology. This study provides an important and necessary comparison for the role of broad-scale environmental variability in structuring marine benthic communities within downwelling rather the more commonly reported upwelling systems.