The objective of assessing algae is to quantify the abundance (as percent cover) of several key functional groups (crustose corallines and macroalgae, note turf algae are no longer included), and to relate these abundances with herbivory and coral condition on the same reefs. Percent cover estimates for algae are rapid, non-destructive methods to assess community dominance. Macroalgae are distinguished as fleshy or calcareous and the average canopy height of each type is also measured. Macroalgal indices (a better assessor of macroalgal dominance than cover) are easily approximated as the product of cover X height.
Herbivorous fishes and sea urchins significantly affect the distribution, abundance and species richness of algae and over the past two decades, there has been a noticeable change on many reefs, especially around the Wider Caribbean, from coral-dominated communities to those dominated by macroalgae (e.g., Done 1992, Hughes 1994, Hallock et al. 1993, Dustan and Halas 1987, Lewis 1986, McClanahan 1996, Steneck and Detheir 1994). The causes for these community shifts have been attributed, in part, to a loss of these key herbivores, particularly those whose grazing and scraping activities would otherwise restrain the growth of non-encrusting algae (Woodley 1979, Hay 1984, Hughes 1994, Roberts 1995). Reefs in decline often have high fleshy (noncalcified) macroalgal biomass, sometimes accompanied by high amounts of Halimeda. (Hughes 1994, Steneck 1994). Reefs with a low macro:crustose coralline ratio are interpreted to be “healthier” than those with a high ratio (Steneck 1994).
In addition to assessing coral condition and algal abundance, the numbers of Diadema are counted. Sea urchins, particularly Diadema, affect coral reef structure and composition including algal composition and abundance, competition with other grazers, particularly certain fishes, and erosion of coral skeletons (Steneck 1994, Roberts 1995, McClanahan 1996). The dramatic decline of Diadema that occurred after the 1983 Diadema die-off (Lessios 1988), lead to the dominance of some reefs by filamentous and fleshy algae, increased coral mortality and decrease in coral recruitment (Hughes 1994). The presence of Diadema as an important factor in determining the condition of Caribbean coral reefs has been emphasized by many researchers (e.g., Hughes 1994, Roberts 1995, Jackson 1997). Recent reports have suggested the abundance of Diadema has been increasing in localized areas. Although there is information available at the local scale, a region-wide assessment of its abundance has not been conducted throughout the western Atlantic and its current status is not known in detail.
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