Marine Heatwave Hotspots in Coral Reef Environments: Physical Drivers, Ecophysiological Outcomes, and Impact Upon Structural Complexity

Abstract

A changing climate is driving increasingly common and prolonged marine heatwaves (MHWs) and these extreme events have now been widely documented to severely impact marine ecosystems globally. However, MHWs have rarely been considered when examining temperature-induced degradation of coral reef ecosystems. Here we consider extreme, localized thermal anomalies, nested within broader increases in sea surface temperature, which fulfill the definitive criteria for MHWs. These acute and intense events, referred to here as MHW hotspots, are not always well represented in the current framework used to describe coral bleaching, but do have distinct ecological outcomes, including widespread bleaching, and rapid mass mortality of putatively thermally tolerant coral species. The physical drivers of these localized hotspots are discussed here, and in doing so we present a comprehensive theoretical framework that links the biological responses of the coral photo-endosymbiotic organism to extreme thermal stress and ecological changes on reefs as a consequence of MHW hotspots. We describe how the rapid onset of high temperatures drives immediate heat-stress induced cellular damage, overwhelming mechanisms that would otherwise mitigate the impact of gradually accumulated thermal stress. The warm environment, and increased light penetration of the coral skeleton due to the loss of coral tissues, coupled with coral tissue decay support rapid microbial growth in the skeletal microenvironment, resulting in the widely unrecognized consequence of rapid decay, and degeneration of the coral skeletons. This accelerated degeneration of coral skeletons on a reef scale hinder the recovery of coral populations and increase the likelihood of phase shifts toward algal dominance. We suggest that MHW hotspots, through driving rapid heatinduced mortality, compromise reefs' structural frameworks to the detriment of long term recovery. We propose that MHW hotspots be considered as a distinct class of thermal stress events in coral reefs, and that the current framework used to describe coral bleaching and mass mortality be expanded to include these. We urge further research into how coral mortality affects bioerosion by coral endoliths.

Will coral reef sponges be winners in the Anthropocene?

“…This is caused by the increase in solar insolation during the summer months, and results in a strong density gradient at the bottom of the MLD that prevents the mixing of nutrient‐rich waters within the shallow euphotic zone creating a zone of nutrient limitation above the MLD and light limitation below the MLD for phyto‐ and icoplankton (Figure b). Regional changes in the physical oceanography of tropical oceans can also contribute to the increasing occurrence of local perturbations such as marine heatwave hotspots which have similar oceanographic features; increased heat absorbed in shallow waters, increased stratification, and a decrease in the MLD (Fordyce, Ainsworth, Heron, & Leggat, ). The depths observed for the maximum MLD overlap with the maximum depths of the euphotic zone (i.e., 1% of downwelling irradiance [ E d ]) observed on many coral reefs from 58 to 102 m (Lesser, Slattery, et al, ).…”

“…Illustration showing (a) the physical oceanography of a contemporary coral reef from shallow (<30 m) to mesophotic (30–150 m) depths down to the mixed layer depth (~100 m), and (b) the same coral reef in 2100 with the changes in physical forcing, productivity, and changes in reef community structure (see text for detailed description). Redrawn from Fordyce et al () with permission from the authors…”

Detecting Marine Heatwaves With Sub-Optimal Data

“…This metric is the integral of the temperature anomalies of a MHW, and so has units of • Cdays, and represents the sum of temperature anomalies over the duration of the MHW. Cumulative intensity is comparable to the degree heating day metrics used in coral reef studies (Fordyce et al, 2019).…”

Bleaching resistant corals retain heat tolerance following acclimatization to environmentally distinct reefs

“…This dysbiosis has a myriad of negative consequences, ranging from declines in coral growth and reproduction to extensive coral mortality (Ward et al 2000;Baird and Marshall 2002;Baker et al 2008;Hughes, Kerry, et al 2018). These bleaching-associated outcomes affect the function of the entire reef ecosystem, as coral biomineralization is necessary to build and maintain the physical framework that is required to support the immense biodiversity typical of a healthy coral reef (Fordyce et al 2019;Leggat et al 2019;.…”