On the paradox of thriving cold‐water coral reefs in the food‐limited deep sea

Abstract

The deep sea is amongst the most food‐limited habitats on Earth, as only a small fraction (<4%) of the surface primary production is exported below 200 m water depth. Here, cold‐water coral (CWC) reefs form oases of life: their biodiversity compares with tropical coral reefs, their biomass and metabolic activity exceed other deep‐sea ecosystems by far. We critically assess the paradox of thriving CWC reefs in the food‐limited deep sea, by reviewing the literature and open‐access data on CWC habitats. This review shows firstly that CWCs typically occur in areas where the food supply is not constantly low, but undergoes pronounced temporal variation. High currents, downwelling and/or vertically migrating zooplankton temporally boost the export of surface organic matter to the seabed, creating ‘feast’ conditions, interspersed with ‘famine’ periods during the non‐productive season. Secondly, CWCs, particularly the most common reef‐builder Desmophyllum pertusum (formerly known as Lophelia pertusa), are well adapted to these fluctuations in food availability. Laboratory and in situ measurements revealed their dietary flexibility, tissue reserves, and temporal variation in growth and energy allocation. Thirdly, the high structural and functional diversity of CWC reefs increases resource retention: acting as giant filters and sustaining complex food webs with diverse recycling pathways, the reefs optimise resource gains over losses. Anthropogenic pressures, including climate change and ocean acidification, threaten this fragile equilibrium through decreased resource supply, increased energy costs, and dissolution of the calcium‐carbonate reef framework. Based on this review, we suggest additional criteria to judge the health of CWC reefs and their chance to persist in the future.

Regional Variations in Live Proportions of Southwest Pacific Cold-Water CoralSolenosmilia variabilisReefs

“…variabilis (Maier et al, 2023). The corals in this study are also genetically different between the two regions (Zeng et al, 2017), which could explain some of the observed differences in live:reef ratios.…”

“…Additionally, subtropical surface waters and subantarctic waters from the Southern Ocean meet near the Graveyard Seamount Complex, causing higher primary productivity than is found at the Louisville Seamount Chain (Tracey et al, 2023). Higher than average surface productivity provides greater levels of food to CWC, driving reef distribution for several species, including S. variabilis (Maier et al, 2023). The corals in this study are also genetically different between the two regions (Zeng et al, 2017), which could explain some of the observed differences in live:reef ratios.…”

Tiger reefs: Self‐organized regular patterns in deep‐sea cold‐water coral reefs

“…Several explanations already exist for the conundrum of highly productive reefs under seemingly food-limited conditions (Cathalot et al, 2015;van Oevelen et al, 2009). For example, cold-water corals typically occur at locations of increased export production (da Costa Portilho-Ramos et al, 2022;Maier et al, 2023;. Self-organization in cold-water coral reefs can be an additional explanation for how cold-water coral reefs can maintain a high productivity with many associated fauna in the deep sea, or simply an emergent characteristic of cold-water coral ecosystems.…”

Patterns in the deep sea

“…Several explanations already exist for the conundrum of highly productive reefs under seemingly food-limited conditions (van Oevelen et al, 2009;Cathalot et al, 2015). For example, cold-water corals typically occur at locations of increased export production (Wienberg and Titschack, 2017;da Costa Portilho-Ramos et al, 2022;Maier et al, 2023). Selforganization in cold-water coral reefs can be an additional explanation for how coldwater coral reefs can maintain a high productivity with many associated fauna in the deep sea, or simply an emergent characteristic of cold-water coral ecosystems.…”

“…By forming regular patterns in space, an organism can extend the area from which it extracts resources, allowing it to thrive in resource limited circumstances (Rietkerk et al, 2004;Rietkerk and van de Koppel, 2008;Bastiaansen et al, 2020). Spatial self-organization should thus be added to the list of explanations for the conundrum (Maier et al 2023) of highly productive cold-water coral reefs in the resource limited deep-sea (chapter 2 and 4).…”