Ocean Acidification, Deep Sea Corals and US Fisheries

Debate surrounds the influence carbon dioxide has on global warming and the future climate of the Earth. Despite documented increases in atmospheric CO2, and understood sources of these emissions, naysayers argue that there is no clear evidence that increasing greenhouse gas emissions are altering our climate. However, there is clear and unambiguous scientific evidence that documents how rising atmospheric CO2 is leading to increasingly acidic seawater. There is no debate as to what is causing ocean acidification. As a result, ‘the other CO2 problem’ leads us to one solution – limit CO2 emissions and mitigate future atmospheric CO2 levels.

Seawater chemistry is rapidly changing as atmospheric CO2 levels rise. The ocean absorbs the excess CO2 through air-sea gas exchange as the partial pressure of CO2 in the atmosphere equilibrates with it.  It is estimated that the oceans have become 30% more acidic since the beginning of the industrial revolution (Feely, 2004).  The concentration of CO2 in the atmosphere is now approximately 385 parts per million (350 ppm is a target level suggested to be the safe upper limit) and is likely to increase at 0.5% per year throughout the 21st century, a rate 100-times faster than has occurred in the past 650,000 years (Meehl et al. 2007). This increase will likely lead to a pH drop in the oceans of 0.3-0.4 units by the end of the century (Feely et al. 2008), a 150% increase in acidity (since the industrial revolution).

This increasing acidity is a significant problem for ocean ecosystems. Marine animals that use calcium carbonate to make shells, skeletons or tests such as pteropods, coccolithophores, corals, oysters, clams, and sea urchins (to name a few) are likely to have increasing difficulty building and maintaining their carbonate structures (Guinotte and Fabry 2008). As acidity increases, conditions become more corrosive to calcified organisms; water with low pH becomes depleted of calcium carbonate ions and is referred to as “undersaturated” with respect to the two major forms of calcium carbonate used by organisms, calcite and aragonite. This will result in negative impacts on growth, metabolism and survival, and ultimately, reef growth may cease and/or reverse. At projected future pH levels, reef-building corals will erode faster than they can build up—leading to a net loss of coral reefs and the species they support.

As a result of increasing CO2, commercial fisheries are now confronted with unknown future impacts from acidification. First, fish may experience direct physiological changes that impact metabolism, growth and reproduction. Second, the food web that supports them may be altered as their prey (e.g., pteropods), which require calcium carbonate structures, decline. Finally, key habitats such as coral forests and reefs will be affected. This mix of ecological impacts is enough cause for alarm, but the combination of changes in ocean chemistry and pressure from destructive fishing methods such as bottom trawling, is a recipe for disaster.

Cold-water coral communities in Alaskan waters are highly diverse (141 species to date) and far more abundant than most other high latitude areas, with an unusually high number of endemic species (Heifetz et al. 2009). National Marine Fisheries Service biologist, Robert Stone (pers. comm.) estimates that up to 50% of corals species and 30% of sponge species found in the Aleutian Islands are endemic. The most important structure-forming taxa in Alaskan coral ecosystems are gorgonians (>60 species) and hydrocorals (primarily stylasterids) (>25 species) (Stone and Shotwell 2007), also known as lace corals. Large Paragorgia or bubblegum corals (gorgonians) may be 2-3 m in height and some of the erect Stylaster species may be over 1 m in diameter.

The structures formed by these coral colonies provide important habitats for ocean life. Biologically diverse deep coral ecosystems are found throughout the world’s oceans in areas where the combination of rocky or hard-bottom substrate is exposed to rich ocean currents. In these areas, productivity is high and the corals are important nurseries and spawning sites for a number of commercially important fish species.  Cold, deep waters are lower in pH than waters bathing shallow reefs, and future projections indicate that 70% of cold-water corals could experience corrosive conditions by the end of this century (Guinotte et al. 2006).

Coral communities, whether in the shallow or deep sea, are among the most biologically diverse marine environments, and understanding the origins of this diversity is an important conservation objective. A study on Stylasterid corals (Lindner et al. 2008) shows that this important group of tropical shallow water marine animals originated and diversified in the deep sea and subsequently invaded shallow waters. It is also possible that deep-sea black corals, gorgonians and stony corals have also contributed to shallow water communities (Lindner et al. 2008). This has very important implications for deep-sea coral communities of the North Pacific, the likely evolutionary origin of these corals.

Adding to this richness, sponges are commonly found in association with deep-water corals.  The sponges in Alaska are diverse and abundant and a structurally important component of these ecosystems. From the few collections that have been made so far, 126 species have been identified, mostly demosponges (Stone, pers. comm.), which are a different taxa from the hexactinellids or ‘glass sponges’ usually found on deep coral reefs. Coral reefs in Alaska are therefore unusual in many ways, and the complex structures formed by the rich assortment of coral and sponge colonies provide habitat for myriad invertebrate and fish communities, including commercially valuable fish species.  Many pharmaceutically promising compounds have come from sponges, and the diverse Alaskan species are completely untapped.

Better understanding of the threats, future changes and mitigation options to these cold-water coral communities is an important conservation objective. This project will provide a better understanding of the locations of these communities, current protections for these areas, and how and where increasing acidification is likely to impact these communities. We will use this analysis to educate decision makers and interested parties, and we fully expect that this information will add impetus to take action to improve management measures and reduce CO2 emissions.

References

Guinotte, J., J. Orr, S. Cairns, A. Freiwald, L. Morgan, and R. George (2006) Will human-induced changes in seawater chemistry alter the distribution of deep-sea scleractinian corals? Frontiers in Ecology and the Environment 4(3): 141-146

Guinotte, J.M. and V.J. Fabry (2008) Ocean acidification and its potential effects on marine ecosystems. In The Year in Ecology and Conservation Biology 2008. R.S. Ostfeld & W.H. Schlesinger, Eds. Annals of the New York Academy of Sciences.

Heifetz, J., B.L. Wing, R.P. Stone, P.W. Malecha and D.L. Courtney (2005) Corals of the Aleutian Islands. Fisheries Oceanography 14 (Suppl. 1): 131–138.

Morgan, L.E., Tsao, C.F. and J. Guinotte (2006) Status of deep-sea corals in US waters, with recommendations for their conservation and management. Marine Conservation Biology Institute, Bellevue, WA. 64 pp.

Stone, R.P. (2006) Coral habitat in the Aleutian Islands of Alaska: depth distribution, fine-scale species associations, and fisheries interactions. Coral Reefs 25(2): 229-238.

Turley, C.M., Roberts, J.M. and J.M Guinotte (2007) Corals in deep-water: will the unseen hand of ocean acidification destroy cold-water ecosystems? Coral Reefs 26: 445-448

Watling, L. and E.A. Norse (1998) Disturbance of the seabed by mobile fishing gear: A comparison with forest clear-cutting Conservation Biology 12(6)1180-1197.
Morgan, L.E. and R. Chuenpagdee (2003) Shifting Gears: Addressing the collateral impacts of fishing methods in US waters. PEW Science Series, Island Press, Washington DC. 42 pp.