Bleaching in sponges on temperate mesophotic reefs observed following marine heatwave events

•Large-scale benthic monitoring across a region experiencing rapid warming.•Impacts on deeper mesophotic reefs not well studied.•Time-series of bleaching in sponges in imagery analysed with spatial models.•Bleaching found to increase in surveys following marine heatwaves.•Example species a potential indicator of climate change on deeper reef systems. Climate change driven extreme events such as marine heatwaves (MHWs) can have dramatic impacts on ecosystems, with thermal stress often resulting in localised die-offs and visible signs of impacts such as bleaching of organisms. Such impacts are reported widely in shallower ecosystems but are less studied on deeper mesophotic ecosystems (MEs) where collecting data is more expensive. However, these deeper ecosystems are often biodiverse and play important ecological roles, and so understanding climate change impacts at these depths is important. Here we use benthic imagery collected as part of a large-scale monitoring program to explore bleaching in a cup sponge ‘morphospecies’ (i.e. morphologically distinct organisms readily identified in imagery) in MEs across eastern Tasmania, a region experiencing rapid ocean warming. We find an increased incidence of bleaching in surveys following MHWs, but currently no evidence for mass mortality following bleaching. Our results suggest that this cup sponge morphospecies may be useful for tracking climate change impacts on MEs in the region. Future efforts should be directed towards a better understanding of the physiological limits of this morphospecies across its range and timing surveys to more closely follow MHW events. Sponges form an important and dominant component of temperate MEs and monitoring the impacts of climate change on sponges across these ecosystems should therefore be an ongoing priority.