Coral Ecosystems and Their Services

Corals are the most studied organisms in relation to impacts of ocean acidification. Should these impacts occur in the natural environment they will have a large impact on the ecosystems they support. Corals are, therefore, a useful example of how ecosystems and their services may be impacted in the future.

4.4.1. Tropical Coral Reef Ecosystems

Tropical corals have adapted, over millions of years, to live in warm, sunlit waters highly saturated in aragonite. They are among the most diverse marine ecosystems, supporting about a quarter of all marine biodiversity. They are very important in local shore protection, important to tourism, and supply a critical level of subsistence protein as well as providing an income source in the developing world through fishing and tourism [7]. Unabated CO2 emissions will result in suboptimal aragonite saturation states for coral growth by 2070 such that many reefs could be threatened resulting in reduced coral cover [6,32,55 57]. At this time erosion will outpace calcification so that reef structures will not be able to withstand the waves nor rebuild sufficiently after a storm.

Indeed, coral reefs in the waters off Panama and Galapagos, which live in a naturally more acidic and high CO2 environment, suffer some of the highest erosion rates measured. They contain extremely low percentages of interske-letal pore cement to hold them in place compared to the coral reefs off the Bahamas that live in waters with less CO2 and higher pH [58]. These reefs may be a vision into the future of reefs worldwide, since the Panama and Galapagos environments replicate the expected increased in acidity and CO2.

4.4.2. Cold-Water Corals

Scleractinian cold-water corals, often referred to as deep-water or deep-sea corals, are long-lived (hundreds of years old), are found around 200 1000+ m depth throughout the worlds' oceans and can form large (100 km2) reef frameworks that persist for millennia. They are biodiversity hotspots and play an important role as a refuge, feeding ground and nursery for deep-sea organisms, including commercial fish [23,59,60]. However, they may be the most vulnerable marine ecosystems to ocean acidification [23,61]. Future projections of global aragonite saturation state indicate that 70% of cold-water corals are likely to experience undersaturation this century through the shoaling of the ASH and in some instances this could be as early as 2020 [6,23,61]. It would seem unlikely that scleractinian cold-water corals would be able to calcify under these conditions; it would be more likely that aragonitic structures would experience dissolution in these corrosive waters. As yet there have been no experiments on their reaction at high CO2 but if they respond in the same way as their warm-water cousins their calcification rates may decrease well before aragonite under-saturation occurs.

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