Environmental drivers and indicators of change in habitat and fish assemblages within a climate change hotspot

Abstract

Climate-driven environmental change is predicted to accelerate over the coming decades. Anticipating the ecological consequences requires a fundamental understanding of how environmental variables influence key habitats and species. Southeastern Australia is a marine climate change hotspot, characterised by rapidly increasing sea surface temperatures, strengthening currents and increasing storm frequency. To understand ecosystem responses to environmental drivers in this region, and to anticipate future climate-driven change, we examined the spatial distribution and environmental relationships of key marine habitats, fish assemblages, and the ecologically important urchin Centrostephanus rodgersii along ∼1000 km of reef off southeastern Australia using towed video transects. We also identified potential biological indicators of climate-driven changes to rocky reef ecosystems, based on ecological importance, ease of measurement, and sensitivity to one or more climate-affected environmental drivers. Significant latitudinal gradients were identified for biota, with habitats, fish assemblages and urchin densities all varying latitudinally with sea surface temperature and current strength. Wave energy and distance offshore were identified as drivers of biota at local-scales (100–1000 m). These results indicate that climate-driven changes to various environmental factors will alter the composition and distribution of marine biota, at a variety of scales. The average thermal niche for fish assemblages varied significantly with changing mean sea surface temperature and may be a sentinel indicator of climate-driven changes. Coverage of the kelp Ecklonia radiata, densities of the urchin C. rodgersii, and coral cover were also confirmed as potential valuable biological indicators of climate-driven changes. These indicators, and the widespread baseline data assembled in this study, will facilitate detection of future changes to rocky reef ecosystems in southeastern Australia.