Seawater fogging reduces mortality and bleaching in two coral species during a heatwave and subsequent recovery

The escalating frequency and severity of marine heatwaves are driving factors behind mass coral bleaching, necessitating both rapid emission reductions and the development of scalable intervention tools. Shading coral to reduce irradiance stress is a promising approach, yet most studies focus on static structures, and few include recovery dynamics. Here, we tested the efficacy of a laboratory-scale seawater fogging system as a shading intervention during a simulated thermal/light stress and recovery experiment. An orthogonal design with two temperature levels (control (26.5 °C) vs heat-stressed ((MMM) at the collection site, 29.1 + 3.7 °C) and two light treatments (fogged, 33.3% shading ± 6.46 SD for 6 h daily vs non-fogged) was used to test the response of Acropora hyacinthus and Pocillopora damicornis over 13 days of thermal stress (3 °C-weeks) followed by a 24-day recovery period. Variable shading from seawater fog reduced mortality risk in heat-stressed A. hyacinthus by 55%. Only two mortalities occurred in P. damicornis, both in the heat-stressed treatment without fog. Fog lowered per cent whiteness and improved Fv/Fm in both species, starting at 0.84 °C-weeks, with the effects peaking near 3 °C-weeks. Fogging during the recovery period did not inhibit coral recovery but provided some additional benefits, including increased Fv/Fm and lower catalase activity. Seawater fog can not only be used to delay/reduce bleaching while thermal stress is high but can also enhance recovery and post-stress repair processes of corals. In order to advance seawater fogging as a practical reef intervention tool, engineering advances should be accompanied by in-situ trials that capture complex coral-environment interactions and assess ecosystem-wide responses.