be an inevitable further decline in fisheries (McGowan et al., 2003; Field et al., 2006).

Of even greater concern, because of the seemingly inevitable effects on all calcareous marine organisms, is ocean acidification due to the increased solution of carbon dioxide that forms carbonic acid in seawater (Feely et al., 2004). Measurements have already demonstrated a drop of 0.1 pH units in the oceans (Caldeira and Wickett, 2005), and laboratory and mesocosm experiments demonstrate that calcareous planktonic coccolithophores, pteropods, and foraminifera exhibit decreased calcification and growth under even mildly acidic conditions (Riebesell et al., 2000; Riebesell, 2004). The biogeochemical implications are staggering. These organisms are among the greatest producers of biogenic sediments in the ocean, are vital to particle aggregation and the production of marine snow that enhances the vertical flux of biogenic material, and are major components of the cycling of carbon and the CO₂ storage capacity of the ocean (Riebesell et al., 2000; Riebesell, 2004).

Coral reefs are the most diverse marine ecosystems and among the most threatened (Knowlton, 2001; Hughes et al., 2003). Just 15 years ago, many coral reef scientists still referred to coral reefs as pristine (Jackson, 1997), yet today many scientists believe that the cumulative forces of overfishing, pollution, and climate change are so great that coral reefs may virtually disappear within a few decades (Hoegh-Guldberg et al., 2007; Knowlton and Jackson, 2008).

Corals are dying out around the world and are being replaced by fleshy macroalgae or algal turfs that may carpet the entire reef surface (Hughes, 1994; Pandolfi et al., 2005; Newman et al., 2006). In the Caribbean, live coral cover has fallen from an average of ≈55% in 1977 to 5% in 2001 (Table 1.1), whereas macroalgal cover has risen from an average of ≈5% to 40% (Gardner et al., 2003; Paredes, 2007). The demise of formerly ubiquitous and abundant elkhorn and staghorn corals (Acropora palmata and Acropora cervicornis) is particularly striking; these corals were the major rock formers on shallow Caribbean reefs for at least a million years (Jackson, 1992; Aronson et al., 2004; Pandolfi and Jackson, 2006) but are now officially listed as endangered species. The story is a little better in the Indo-West Pacific where live coral cover still averages ≈22%, which is about one-half of that in 1980 (Bruno and Selig, 2007). However, even the Great Barrier Reef, which is arguably the best-protected coral reef system