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.