Overview of Threats to Deep-Sea Corals
Structure-forming deep-sea corals are generally slow-growing and fragile, making them and their associated communities vulnerable to human-induced impacts, particularly physical disturbance. With the exception of a few areas (e.g., the Oculina Banks), the extent of habitat degradation as a result of anthropogenic impacts is largely unknown. Activities that can directly impact deep-sea coral communities include fishing using bottom-tending fishing gear, deep-sea coral harvesting, fossil fuel and mineral exploration and extraction, and submarine cable/pipeline deployment. Invasive species, climate change and ocean acidification represent additional serious threats.
Disturbances to deep-sea coral ecosystems from bottom-tending fishing gear, especially bottom trawl gear, have been well documented where they have been studied in U.S. waters and in other regions around the world. Bottom trawling is widespread and considered the major threat to deep-sea corals in most U.S. regions where such fishing is allowed and overlaps with areas where deep-sea corals are present. The area of seafloor contacted by bottom trawls is relatively large, the force against the seafloor from the trawl gear is substantial, and the spatial distribution of bottom trawling is extensive. Although not as destructive as bottom trawls and dredges, other types of fishing gear can also have detrimental effects on deep-sea corals. Bottom-set gillnets, bottom-set longlines, pots and traps all impact the seafloor. Vertical hook and line fishing, used in both recreational and commercial fishing, has the potential for some damage to fragile corals by the weights used, but such damage is minimal compared to other bottom-tending gear. [a,b]
Exploration for and production of oil and gas resources can impact deep-sea coral communities in a variety of ways. Potential threats include the physical impact of drilling, placement of structures on the seafloor (e.g., platforms, anchors, pipelines, or cables), discharges from rock-cutting during the drilling process, and intentional or accidental well discharges or release of drilling fluids. Smothering and death of corals by drilling muds and cuttings was observed on Lophelia pertusa colonies living on an oil platform in the North Sea close to drilling discharge points. [c] The use of anchors, pipelines, and cables for oil exploration/extraction can be destructive to sensitive benthic habitats as well. While deployment of oil and gas pipelines can cause localized physical damage to deep-sea corals. The use of anchors, pipelines, and cables for oil exploration/extraction can be destructive to sensitive benthic habitats as well.
The potential impacts of climate change on deep-sea coral ecosystems are unknown, however, the associated threat of ocean acidification is a significant concern. Since pre-industrial times, over half of the additional CO2 attributed to human activities released in the atmosphere has been absorbed by the ocean. Once in the ocean, CO2 lowers the saturation of the minerals (principally calcium carbonate) used to form skeletal structures in many major groups of marine organisms, including corals. The saturation level of calcium carbonate decreases with depth, and therefore the effect of ocean acidification on deep-sea corals could be significant. Projected increases in ocean acidity could result in severe ecological changes for deep-sea corals, and may influence the marine food chain from carbonate-based phytoplankton up to higher trophic levels. [d]
Other activities that can adversely impact deep-sea corals and the communities that depend on them include coral harvesting; marine debris; and submarine cable/pipeline deployment; and invasive species.