Cold-water corals in a changing ocean
Introduction
There are more coral species in waters over 50 m deep than there are on shallow, tropical coral reefs [1]. Set apart from tropical corals by the cooler waters they inhabit, cold-water corals are amongst the most significant ecosystem engineers on continental shelves, slopes, canyons, seamounts and ridge systems across the globe [2]. The habitats engineered by cold-water corals vary from ‘coral gardens’, frequently structured by bamboo or gorgonian corals, to the large deep-water framework reefs usually constructed by one or two of a small group of scleractinian corals (Oculina varicosa, Madrepora oculata, Lophelia pertusa, Solenosmilia variabilis, Goniocorella dumosa, Enallopsammia profunda and Bathelia candida). In all cases the coral skeletons provide complex three-dimensional structural habitat used by many other species as refuge or as a place from which to feed.
Research on cold-water corals grew rapidly from the mid-1990s onwards with international symposia beginning in 2000. This was first driven by technological advances coupled with a growing need to understand ecosystems on deep continental margin and slope settings as offshore hydrocarbon exploration and deep-water fisheries expanded into these areas. The huge strides made in acoustic seabed mapping using multibeam echosounders have revealed a previously unknown density of cold-water coral reefs and coral carbonate mounds [3, 4]. Both support extensive long-lived biogenic reefs that, through the dynamic processes of growth and (bio)erosion, can accumulate tens of metres above the surrounding seafloor, locally alter sedimentary patterns and provide niches for a diverse community [5]. In the NE Atlantic, cold-water coral reef frameworks structured by Lophelia pertusa developed during interglacials, but were absent during glacial climate periods indicating a very close coupling with global climate [6••]. The substantial structures produced by coral carbonate mounds are formed by sequences of interglacial coral reef framework overlain with glacial deposits (Figure 1). In other regions, cold-water coral research is growing rapidly with examples including studies of the solitary scleractinian Desmophyllum dianthus in Chilean fjords [7] and of colonial scleractinians in Mediterranean canyons [8].
In parallel with the work to map and characterise cold-water coral habitats has been a growing understanding of the environmental factors that control their distribution. Cold-water corals are frequently associated with certain temperatures and salinities, with evidence that L. pertusa in the NE Atlantic is found in a particular density layer (sigma-theta 27.35–27.65 kg m−3) perhaps because larvae disperse laterally along this horizon [9]. However, at local scales cold-water corals occurrence is closely associated to sites where fresh labile food material is transported rapidly from the surface [10], often in settings where seabed topography and water flow interact to promote enhanced food supply [11]. However, without a suitable hard surface for larval attachment it is unlikely that cold-water corals will be able to settle and flourish.
With the development of predictive habitat modelling approaches and greater availability of regional and global datasets of key environmental parameters (e.g. temperature, dissolved O2, and nutrients) work to develop predictive models of cold-water coral occurrence has begun. Following the first studies published in 2005 [12] using Ecological Niche Factor Analysis [13] a number of studies have been carried out both at regional and global scales. The most recent work uses the Maximum Entropy approach with examples including scleractinian corals on seamounts [14] and deep-water octocorals [15•]. Interestingly oceanic carbonate chemistry emerges as an important explanatory variable for both groups with the aragonitic scleractinian corals and calcitic octocorals related closely to aragonite and calcite saturation states respectively. As the progressive acidification of the oceans continues there is great concern that large areas of the global ocean will become undersaturated as the calcium carbonate saturation horizons shallow [16, 17].
Section snippets
CWC taxonomy and associated biodiversity
Cold-water corals are members of the Orders Scleractinia, Zoanthidea, the Subclass Octocorallia and the Family Stylasteridae (Table 1). Understanding their global distribution remains very limited by the lack of information from many deep-sea areas of the world. Where sufficient information exists it has only recently become possible to produce summaries of global species diversity. For example, the azooxanthellate Scleractinia show three high diversity centres (Figure 2), although recent
Threats
Although still largely unknown to the wider public, cold-water corals have been damaged by human activity and face an uncertain future in a global ocean forecast to rise in temperature and increase in acidity. From the 1970s onwards the trawl fishing industry has progressively moved to deeper waters as shallower stocks became depleted and vessel size, range and technology improved. As deep-water trawls were strengthened and rock-hopper gears developed to allow trawling on rougher grounds,
Conservation
It is now recognised that cold-water corals are threatened at the levels of species (e.g. coral collection for jewellery), habitat (e.g. trawl damage) and ecosystem (e.g. ocean acidification). Efforts to promote their conservation have focussed upon creating marine protected areas (MPAs) within which activities such as bottom fishing with towed nets and dredges are restricted. However, as with any protective measure these are only effective if enforced. The first MPA for a cold-water coral
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
JMR acknowledges support from Heriot-Watt University's Environment and Climate Change theme and the UK Natural Environment Research Council (grants NE/J021121/1 and NE/H017305/1). Many thanks to Les Watling (University of Hawaii) for supplying the images of Metallogorgia melanotrichos and Ophiocreas oedipus and to the Holland-1 remotely operated vehicle team and the captain and crew of RRS James Cook cruise 073 for assistance at sea.
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2021, Deep-Sea Research Part I: Oceanographic Research PapersCitation Excerpt :Because of their demography (Bramanti et al., 2019; Girard et al., 2019) and tridimensional structure, these animal forests are particularly vulnerable to mechanical injuries inflicted by anthropogenic pressures, such as direct fishing activities (bottom trawling, longlines and trammel nets) and their indirect consequences (Derelict Fishing Gears – DFGs, sediment resuspension and consequently silting), as well as the accumulation of marine litter (Puig et al., 2012; Clark et al., 2016; D'Onghia et al., 2017; Hinz, 2017; Giusti et al., 2019; Gori et al., 2019; Puig and Gili, 2019). Beside fishing activities, other aspects are further impacting MAFs assemblages worldwide, including climate-change driven events (i.e. warming and ocean acidification) (McCulloch et al., 2010; Roberts and Cairns 2014; Gori et al., 2017; Ragnarsson et al., 2017), chemical pollution (terrestrial nutrient loads, disposal of solid waste from mines and oil spills) (Montagna et al., 2006; Fabri et al., 2014; Otero et al., 2016), and seafloor drilling activities (Aguilar 2004; Larsson et al., 2013). The consequences of these pressures on MAFs have been extensively documented (Cau et al., 2017a; Gori et al., 2017; Ragnarsson et al., 2017; Galgani et al., 2018).