The Fate of Cold-Water Corals in a Changing World: A Geological Perspective

Abstract

As "ecosystem engineers," framework-forming scleractinian cold-water corals (CWC) build reefs that are unique biodiversity hotspots in the deep sea. Studies using common biological techniques such as correlating the spatial occurrence of the most common CWC species with modeled environmental conditions have revealed the ecological requirements and tolerances of these species. However, limited field observations and poorly understood geographical distribution patterns of the CWC restrict the application of existing knowledge toward assessing their fate (e.g., local extinction, newly established populations) under ongoing global change. Hence, the risk to cross ecological tipping points causing the demise (or establishment) of entire CWC reefs remains unclear. A major challenge is to identify the key environmental parameters (or stressors) having the potential to control CWC vitality by providing such tipping points. This is largely hampered by the overall lack of present-day observations of such tipping point crossings. However, evidence for such events is frequently preserved in geological records revealing that entire CWC ecosystems vanished or returned at specific moments in the past. Here, a geological approach is presented that by correlating geological CWC records with paleoceanographic data describing past environmental changes allows to identify a set of key environmental drivers that directly or indirectly control CWC vitality. Thus, by combining such a geological approach with common biological techniques (see above) to describe the ecological tolerance of the most important reefbuilding CWC has a great potential to better assess their future spatial distribution in times of accelerating global change and to improve the sustainable management of the important deep-sea ecosystems formed by CWC.

Influence of Water Masses on the Biodiversity and Biogeography of Deep-Sea Benthic Ecosystems in the North Atlantic

“…Off Nova Scotia (Canada) and the Mid-Atlantic Bight (United States), CWCs occur (Buhl-Mortensen et al, 2015;Csa Ocean Sciences et al, 2017) mainly in areas influenced by warm southwestward meanders of the Gulf Stream (Mienis et al, 2014). Similarly to the NE Atlantic, L. pertusa re-established at ∼7 kya in the NW Atlantic, in the Cape Lookout area (offshore North Carolina, United States), after a glacial period (Hebbeln et al, 2019). Ocean surface productivity and deep-sea temperatures reached interglacial levels several thousand years earlier, but did not facilitate L. pertusa to return to this area.…”

“…Only the dramatic increase of the hydrodynamics ∼7 kya was likely to have enabled the return of this CWC (Matos et al, 2015). The intrusion of the Gulf Stream into the upper slope and the consequent increase of food supply is assumed to have promoted the re-establishment of L. pertusa (Matos et al, 2015;Hebbeln et al, 2019).…”

“…The occurrence of CWCs reefs and gardens (Mortensen et al, 2001) and sponge grounds (Murillo et al, 2012;Beazley et al, 2015;Howell et al, 2016) have been tightly related to water mass properties and circulation (e.g., Davies et al, 2009;Mohn et al, 2014;Kenchington et al, 2017;Lacharité and Metaxas, 2018) as well as the forcing of climate conditions such as glacial periods, the presence and strength of fronts, etc. (e.g., Frank et al, 2011;Montero-Serrano et al, 2011;Hebbeln et al, 2019). However, climate models forecast that anthropogenic climate change will modify circulation patterns and fluxes, energy flow and water mass properties of the Atlantic Ocean by 2100 (Mora et al, 2013;Rahmstorf et al, 2015;Sweetman et al, 2017;Osman et al, 2019).…”

“…However, many knowledge gaps remain in the biological processes and oceanographic dynamics in the deepsea. Critical information for predicting tipping points, adaptation or resilience are still unknown for most of the deep-sea benthic organisms at species, population, and community-level processes (Hebbeln et al, 2019). An improved understanding of these drivers would help enable us to more clearly identify and describe the various impacts of climate change.…”

“…Re-organizations in the North Atlantic water mass structure have been occurring since the Atlantic's genesis 200 mya, influencing the biodiversity, distribution and connectivity of deep-sea communities and species. For instance, the re-organizations in water mass structure related to AMOC and Sub-Polar Gyre dynamics during the early Holocene triggered an unprecedented post-glacial re-expansion of the cold-water coral (CWC) Lophelia pertusa over 7,500 km in ∼400 years (Frank et al, 2011;Henry et al, 2014;Wienberg and Witschack, 2017;Hebbeln et al, 2019). Thus, changes in the North Atlantic deep-water circulation are of particular interest in order to understand deep-sea species distribution, community composition, ecosystem functioning and dynamics.…”