Crumbling Reefs and Cold-Water Coral Habitat Loss in a Future Ocean: Evidence of “Coralporosis” as an Indicator of Habitat Integrity

Abstract

Ocean acidification is a threat to the net growth of tropical and deep-sea coral reefs, due to gradual changes in the balance between reef growth and loss processes. Here we go beyond identification of coral dissolution induced by ocean acidification and identify a mechanism that will lead to a loss of habitat in cold-water coral reef habitats on an ecosystem-scale. To quantify this, we present in situ and year-long laboratory evidence detailing the type of habitat shift that can be expected (in situ evidence), the mechanisms underlying this (in situ and laboratory evidence), and the timescale within which the process begins (laboratory evidence). Through application of engineering principals, we detail how increased porosity in structurally critical sections of coral framework will lead to crumbling of load-bearing material, and a potential collapse and loss of complexity of the larger habitat. Importantly, in situ evidence highlights that cold-water corals can survive beneath the aragonite saturation horizon, but in a fundamentally different way to what is currently considered a biogenic cold-water coral reef, with a loss of the majority of reef habitat. The shift from a habitat with high 3-dimensional complexity provided by both live and dead coral framework, to a habitat restricted primarily to live coral colonies with lower 3-dimensional complexity represents the main threat to cold-water coral reefs of the future and the biodiversity they support. Ocean acidification can cause ecosystem-scale habitat loss for the majority of cold-water coral reefs.

Using the Goldilocks Principle to model coral ecosystem engineering

“…Importantly, once a coral dies, this framework persists until it is bio-eroded, settled by other organisms or infilled with sediment, and the ratio of live : dead coral is an important determinant of biodiversity found on reefs. Due to the diverse communities they support, and the threats they face such as bleaching [ 8 ] and ocean acidification [ 9 , 10 ], considerable attention is now given to understanding the future state of coral reef assemblages. A key issue to understand is how stony corals grow in different conditions in the present day, as that will have a major bearing on the diversity of these ecosystems in the future.…”

Hidden structural heterogeneity enhances marine hotspots’ biodiversity

“…The habitat suitability for calcifying organisms in the deep sea will get severely decreased; [ 80% reduction is predicted for scleractinians and octocorals (Morato et al 2020). Research on the impacts of acidification on calcified organisms pinpoint to more fragile skeletons with greater porosity (Byrne and Fitzer 2019;Hennige et al 2020) and altered anchoring ability of bivalves (O'Donnell et al 2013). Furthermore, ocean warming has been shown to destabilize the microbial symbionts in corals, sponges, and bivalves increasing their susceptibility to parasites (Baker et al 2018).…”

“…Models predict large-scale shifts in water-mass characteristics by 2100 (e.g. pH reduction by 0.37 units; Puerta et al 2020) which in turn will have negative impacts on the distribution (Morato et al 2020) and skeletal integrity of habitat-forming cold-water corals (Hennige et al 2020). These are expected to damage the role of these corals in supporting high biodiversity (Henry and Roberts 2017) and their overall health and environmental status (Kazanidis et al 2020;Morato et al 2020).…”

Benthic foraminiferal faunas associated with cold‐water coral environments in the North Atlantic realm

“…On-going ocean acidification leads to the dissolution of the coral's aragonitic skeleton (Davidson et al, 2018;Hennige et al, 2015), which weakens individual coral branches and in the long term the entire reef (Hennige et al, 2020 and references therein). Hennige et al (2020) showed that this loss of dead coral will result in a shift from high 3-dimensional complexity provided by both living and dead coral, to a habitat restricted primarily to live coral colonies with lower 3-dimensional complexity. This phenomenon, given the importance of dead coral as a substratum for epibenthic foraminifera (Figures 7 through 12), may over time reduce benthic foraminiferal macrohabitat and microhabitat availability and ultimately lead to diversity loss within benthic foraminiferal communities.…”