Compositional responses to global warming
The response of the Honduras sites showing a clear shift in benthic composition related to global warming conforms to the expectation of Caribbean coral reef vulnerability under global climate change. Such change is likely driven by the limited resilience of Caribbean coral reefs to disturbance (Roff and Mumby 2012), along with reduced functional redundancy of key taxa (McWilliam et al. 2018; Siqueira et al. 2019). Surprisingly, a reduction in sponge coverage under global warming (Fig 3) contradicts the expectations of how sponges will cope in warmer waters (Bell and Smith 2004; Powell et al. 2014; Pawlik et al. 2016; Bell et al. 2018). However, a few of these studies on sponges were undertaken at the WNP sites (Bell and Smith 2004; Powell et al. 2014), perhaps indicating the processes driving sponge coverage are different across regions, as our results also indicate that sponge coverage will increase at the Indonesia site under warmer temperatures. However, within the Caribbean an expectation of increased sponge coverage driven by a negative feedback-loop of dissolved organic carbon linked to coral reduction under global warming, exacerbating conditions which favour sponges on coral reefs has been reported (Pawlik et al. 2016). Yet, proliferation of sponges may actually be constrained in the long term owing to cascading trophic processes (Lesser and Slattery 2020). Therefore, the processes determining how sponge coverage will respond to global warming remains contested and requires further investigation. Furthermore, a reduction in soft coral coverage under global warming at the Honduras site contradicts the expectation of these taxa under global climate change (Inoue et al. 2013). This process, however, is primarily related to ocean acidification (OA), as the reduced calcification under OA is assumed to allow soft corals to outcompete calcifying corals for reef space. Yet, if temperature negatively impacts soft coral coverage globally, as identified at the Honduras sites in this study, this benthic group may also see declines as ocean temperatures continue to rise.