Regional-scale variability in the response of benthic macroinvertebrate assemblages to a marine heatwave

Extreme climatic events are predicted to increase in severity as a consequence of anthropogenic climate change. In marine ecosystems, the importance of marine heatwaves (MHWs) - discrete periods of anomalously high sea temperatures - is gaining recognition. In 2011, the highest-magnitude MHW ever recorded impacted the west coast of Australia (southeast Indian Ocean). The MHW was associated with widespread mortality of habitat-forming species, including corals and kelps, and structural changes in assemblages of macroalgae and fish. However, the responses of benthic macroinvertebrate assemblages have not yet been fully documented. Here, we resurveyed 2 subtidal habitat types (reef 'flats' and 'slopes') at 4 locations (spanning > 800 km of coastline and > 6 degrees of latitude) during the period 1999-2015 to examine the effects of the 2011 MHW on herbivorous macroinvertebrates (i. e. sea urchins, gastropod molluscs). Responses to the MHW varied with latitude; at our warmest study location, abundances were severely depleted, whereas no effects were detected at the coolest location. Across the entire study region, subtle but significant shifts in assemblage structure were observed due to decreased abundances of more southerly-distributed species (i. e. 'cool' affinity) and increased abundances of several more northerly-distributed species (i. e. 'warm' affinity). The 2011 MHW has had profound effects on the marine biota off the west coast of Australia, across multiple trophic levels and taxonomic groups. Here, as in many other regions, contemporary warming events are superimposed onto gradual warming trends, increasing the likelihood of abrupt changes in ecosystem structure and functioning.