Groundtruthing assessments of lab-based coral thermal tolerance with large-area imaging

Identifying upper-thermal thresholds in corals is paramount to uncovering species and reefs with high thermal resistance and resilience. Recognition of the need to improve ex situ experiments (e.g., standardization of heat-stress experiment methodology) and a shift toward more robust and repeatable in situ monitoring (e.g., large-area imaging) can serve to improve thermal tolerance assessments. Yet rarely are the results derived from lab- and field-based approaches compared, leaving the congruence of findings unclear. Here, we selected three reefs in southern Taiwan with different thermal histories and compared the performance of the common scleractinian coral Pocillopora acuta using 1) an acute heat-stress assay experiment and 2) large-area imaging before and after a marine heatwave. Chlorophyll a, algal symbiont density, and protein concentration in the acute heat-stress assay declined sharply in high temperature treatments, with only subtle differences among sites observed. Dose-response curves of photochemical efficiency (ED50) in the acute heat-stress assay revealed differences among corals across reef sites that generally matched with the bleaching trends observed in situ. Site-specific coral–Symbiodiniaceae associations mirrored lab-based ED50 patterns as well as field-based bleaching observations (i.e., higher thermal tolerance when hosting Durusdinium spp. than Cladocopium spp.). However, site-specific bleaching trends observed in the lab experiment were opposite of that observed in the field, perhaps as a result of differential nubbin vs. colony response strategies, or as a consequence of reef conditions (e.g., community composition, structural complexity). A collective effort to compare lab- and field-based thermal responses can help build confidence in thermal tolerance categorization in corals.