As human populations have expanded in the coastal areas of tropical and subtropical oceans their influence on the environment that surrounds coral reefs has increased dramatically. Changes to the nutrient and sediment concentration of the waters surrounding corals have impacted the growth and calcification of a wide range of organisms. In the last two decades, these local impacts have been joined by global factors such as global warming and ocean acidification (see Chapters 9 and 10). Together, local and global factors have decreased the growth in calcification of reefs while at the same time probably increasing the rate of dissolution and/or bioerosion. These changes are complex and interactive, and have far-reaching consequences for both natural ecosystems and the human societies that depend on them.
The impact of coral bleaching, crown-of-thorns starfish (see Chapters 5 and 26) and a wide array of other factors has decreased the proportion of reefs covered by living coral, which normally maintains the carbonate reef substrates against infestation by bioeroders. The loss of corals has in turn provided an increase in supply of suitable substrata for bioerosion, so rates will increase across the reef after a bleaching event. These rates will either remain high or decline to prebleaching levels, depending on other factors such as water quality and supply of coral recruits. Clearly, if the growth and survival of coral reefs is to continue to decline under the rapid changes in global climate that are projected for this century, then there will be an increasing proportion of reefs that will be no longer growing and will be in net erosion. How fast a reef matrix can disappear is probably dependent on a number of factors. Some studies have suggested that accumulated calcium carbonate structures typical of many reefs can disappear quite quickly.
An additional problem has arisen from the buildup of carbon dioxide in the atmosphere as explained in Chapter 10. In this particular case, roughly 40% of the carbon dioxide that has entered the atmosphere has been absorbed by the ocean. In the ocean, carbon dioxide reacts with water to create a dilute acid called carbonic acid. This acid releases protons which, combined with carbonate, convert it to bicarbonate. The net effect is that the carbonate ion concentration has been declining and will decline further as carbon dioxide builds up in the atmosphere. Decreasing carbonate ion concentrations will decrease the ease with which calcification can occur and will increase the tendency for calcium carbonate crystals to dissolve. This has many people concerned about whether projected increases in carbon dioxide in the atmosphere will tip the balance of coral reefs away from the accumulation of calcium carbonate and towards the erosion of this important resource. The implications that reefs are eroding are highly significant and may involve a loss of the three-dimensional structure of reefs. This structure is important habitat for many thousands of species worldwide as well as being the 'front line' defence along coastlines throughout the world. The prospect of reef barriers disappearing as they erode in a warm and acidic sea may mean increased exposure of other ecosystems such as mangroves and seagrasses, which generally shelter behind the reef crests from the full force of ocean waves. These changes in wave energy, especially when combined with sea level rise, could have dire implications for the extensive human infrastructure that often lines tropical coastline, and which also shelters behind these crucial reef barriers.
In summary, the factors controlling rates and agents of bioerosion are complex and interrelated as are those controlling reef growth, and superimposed on these are location and regional factors. Anthropogenic impacts seriously modify rates and agents of bioerosion, and commonly these are cumulative. For example, a reef can recover from a single bleaching event but if such events become more regular, and if this is combined with a crown-of-thorns starfish plague, the reef has little chance to recover before it is hit by another bleaching event. In the meantime, rates of bioerosion are increasing and are causing physical loss of reef structure. When combined with yet other aspects such as the overfishing of key functional groups, then once healthy coral reefs can become rapidly algal dominated reefs.
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