of dissolved organic matter into the surrounding water that destabilizes microbial communities on corals and promotes coral disease (Nugues et al., 2004; Kline et al., 2006; JE Smith et al., 2006; Hoegh-Guldberg et al., 2007), or by smothering the crustose coralline algae that are necessary cues for coral larvae to recruit (Carpenter and Edmunds, 2006). Increased abundance of fish or sea urchins is associated with a decline in macroalgae (Newman et al., 2006; Mumby et al., 2007) and increased coral recruitment (Carpenter and Edmunds, 2006). Coral cover has not increased, however, presumably because of the slow growth and long generation times of corals compared with fish and macroalgae (Jackson, 1991; Pandolfi et al., 2005; Newman et al., 2006).

Rising temperatures and falling pH are as ominous for the future of corals and coral reefs (Knowlton, 2001; Hughes et al., 2003; Kleypas et al., 2006; Hoegh-Guldberg et al., 2007) as for calcareous plankton (Riebesell, 2004). Warming has caused mass mortality of corals by coral bleaching that has increased in frequency and intensity over the past two to three decades. Reduction of pH reduces coral growth rates and skeletal density, and may eventually stop calcification entirely, so that corals lose their skeletons and resemble small colonial sea anemones (Fine and Tchernov, 2007). Regardless of whether or not the corals can survive under such circumstances, reef formation would be severely reduced or halted if acidification proceeded at current rates.

Climate change exacerbates local stress due to overfishing and decline in water quality (Pandolfi et al., 2005; Newton et al., 2007; Knowlton and Jackson, 2008), but the reverse is also true to the extent that the unpopulated, unfished, and unpolluted atolls of the central Pacific still possess ≈50% coral cover while other reefs in the Pacific have less than half that amount (Bruno and Selig, 2007; McClanahan et al., 2007; Knowlton and Jackson, 2008). This is the only good news I know of for coral reefs, and there is a pressing need to study these reefs to determine why corals have so far persisted in such abundance and the degree to which coral community composition is shifting toward more physiologically resilient species or to those with shorter generation times and faster growth. Ultimately, however, it is difficult to imagine how corals will be able to survive or reefs persist if the rise in CO₂ continues unabated.

The overall status of the four major categories of ocean ecosystems and the principal drivers of their degradation are summarized in Table 1.3.