Fifteen years in a global warming hotspot: Changes in subtidal mobile invertebrate communities

Temperate subtidal reefs are increasingly exposed to gradual warming and short periods of high temperatures (marine heatwaves; MHWs). These pressures can directly and indirectly affect the mobile invertebrate communities on these reefs.We investigate changes in mobile invertebrate communities from benthic surveys 15 yr apart (1999-2001 vs 2016-2019), spanning a 4° latitudinal gradient in Western Australia (30-34° S), expecting the biggest changes to the communities in the lower latitudes, where the cumulative effect of MHWs and warming is largest. The urchins Centrostephanus tenuispinus (warmtemperate affinity) and Phyllacanthus irregularis (cool temperate affinity) showed trends toward opposite responses over time; while P. irregularis densities declined (non-significantly at all sites), C. tenuispinus densities increased (significantly at one of 3 sites). The magnitude of the responses appeared to decrease with in creasing latitude, with C. tenuispinus recorded in significantly higher densities only at the lowest latitude location. Neither the densities, nor change in density over time of cool temperate gastropod Lunella torquatus and urchin Heliocidaris erythrogramma reflected a latitudinal gradient, suggesting other localized factors play a larger role in determining abundances of these species. However, size distributions of L. torquatus populations at the lower latitude locations were clearly impacted by the 2011 MHW, while those at higher latitudes remained relatively consistent. Overall, the biggest changes over time were seen at the warmedge of the temperate ecosystem. As temperatures continue to rise, the magnitude of these changes is not only expected to increase, but also to occur at higher latitudes.