The cold‐water coral community as hotspot of carbon cycling on continental margins: A food‐web analysis from Rockall Bank (northeast Atlantic)

Abstract

We present a quantitative food-web analysis of the cold-water coral community, i.e., the assembly of living corals, dead coral branches and sediment beneath, associated with the reef-building Lophelia pertusa on the giant carbonate mounds at ,800-m depth at Rockall Bank. Carbon flows, 140 flows among 20 biotic and abiotic compartments, were reconstructed using linear inverse modeling by merging data on biomass, on-board respiration, d 15 N values, and literature constraints on assimilation and growth efficiencies. The carbon flux to the coral community was 75.1 mmol C m 22 d 21 and was partitioned among (phyto)detritus (81%) and zooplankton (19%). Carbon ingestion by the living coral was only 9% of the carbon ingestion by the whole community and was portioned among (phyto)detritus (72%) and zooplankton (28%). Carbon cycling in the community was dominated by suspension-and filter-feeding macrofauna associated with dead coral branches. Sediment traps mounted on a bottom lander trapped 0.77 mmol C m 22 d 21 (annual average), which is almost two orders of magnitude lower than total carbon ingestion (75.1) and respiration (57.3 mmol C m 22 d 21 ) by the coral community. This discrepancy is explained in two ways: the coral community intercepts organic matter that would otherwise not settle on the seafloor, and through their action as ecosystem engineers, the increased turbulence generated by the coral framework and organic-matter depletion in the boundary layer augment the influx to the coral community. A comparison of macrofaunal biomass and respiration data with soft sediments reveals that coral communities are hot spots of biomass and carbon cycling along continental margins.Oceans and seas cover almost 75% of Earth's surface and are underlain with predominantly soft sediments. In recent decades, however, increased use of underwater videos, video-guided sampling gear, and remotely operated vehicles (ROVs) has contributed to the discovery of other deep-sea ecosystems.

Cold-water coral ecosystem (Tisler Reef, Norwegian Shelf) may be a hotspot for carbon cycling

“…2b). This argument assumes that the high-biomass reef respires at a significantly greater rate than the surrounding benthic environment (van Oevelen et al 2009). If this was not the case, then oxygen uptake would not vary with a change in the local flow speed (residence time over the reef).…”

“…Well over 1300 species have been found living on L. pertusa reefs in the NE Atlantic (Roberts et al 2006). There is evidence to suggest that CWC ecosystems have elevated respiration rates compared with different benthic environments at similar depths with no coral present (van Oevelen et al 2009). Where CWCs are abundant, for example on the Norwegian Shelf (Mortensen et al 1995, Fosså et al 2002, they are likely to play an important role both in the maintenance of biodiversity and in the ecosystem services (e.g.…”

“…There have been very few studies regarding the role of CWC ecosystems in carbon and nutrient remineralisation. An ecosystem modeling approach for a location (≈800 m water depth) in the Rockall Trough (west of Ireland) has shown that CWC reefs may have an extremely high carbon uptake/respiration rate (van Oevelen et al 2009) . Only 9% of the total respiration was due to the coral Lophelia pertusa itself; much of the re spiration/uptake was ascribed to other fauna and bacteria, particularly those associated with the dead coral substrate that makes up a large part of the reef structures.…”

Benthic O2 uptake of two cold-water coral communities estimated with the non-invasive eddy correlation technique

“…echinoderms, crustaceans, sponges and fish (Roberts & Cairns 2014). The communities trap large amounts of organic material and are thus considered to have the capability of turning over considerable amounts of organic material (van Oevelen et al 2009, White et al 2012. This has emphasized the need to further investigate CWC metabolism and to better quantify CWC community contributions to local and regional carbon budgets (Roberts et al 2009).…”

“…Traditional methods are invasive and rely on ex situ incubations of reef coral fragments (Dodds et al 2007), in situ incubations of reef community sub-samples combined with complex food-web modeling (van Oevelen et al 2009), and in situ benthic chamber deployments (Khripounoff et al 2014). As per the current literature, the only non-invasive integrated estimates of CWC reef community respiration rates have been limited to open-water approaches that rely on benthic boundary layer (BBL) O 2 budget estimates from 2 or more coincident fixed-point O 2 measurements over the selected CWC reef area (White et al 2012).…”