Differential responses in recovery, growth and survival between intertidal and subtidal corals after acute thermal stress

Abstract Sea temperature increases may compromise ecological restoration as a tool for recovering degraded coral reefs. A potential solution may lay within using corals with naturally higher thermal resilience, such as intertidal corals. This study aimed at comparing thermal resilience, growth and survival between intertidal and subtidal corals in a reciprocal transplant experiment. Sixty coral nurseries were installed in a shallow coral reef area in Kenya: half were placed in the intertidal zone and half in the subtidal zone. At both zones, intertidal and subtidal Pocillopora cf. damicornis coral fragments were cultured in equal proportions, resulting in 15 replicate nurseries for four treatments. After an initial culture phase of 1 month in situ, six nurseries per treatment were thermally stressed ex situ by exposing corals for 5 days to a temperature of 32 °C (3 °C above summer maximum), after which they were returned in situ to recover. Fragment brightness was measured as the response variable to thermal stress. Intertidal and subtidal corals increased brightness (i.e., bleached) at a similar rate, but during recovery intertidal corals returned quicker to their original brightness in both culture environments. Coral growth was highest for intertidal corals in the intertidal zone during cooler months and was highest for subtidal corals in the subtidal zone during peak temperatures. Intertidal corals transplanted to the subtidal zone registered the lowest survival. Thus, intertidal corals display higher thermal resilience through quicker recovery, but potential trade-offs require further investigation before these corals can be used as a climate-proof broodstock for reef restoration.