Partial mortality of massive reef-building corals: An index of patch reef condition, Florida Reef Tract

Patch reefs are by far the most numerous reef type of the Florida Reef Tract. As they occur along the inner shelf, they are the nearest reefs to potential anthropogenic impacts from the islands of the Florida Keys by runoff of pollutants, fishing, boating and diving. Massive corals of four genera (Montastraea spp., Diploria spp., Siderastrea siderea and Colpophyllia natans) are the major reef builders of the patch reefs distributed discontinuously along some 200 km between the latitudes of Miami and Key West. To assess the condition of this coral community, we estimated and measured percent dead tissue of 3232 colonies by species and size on 22 shallow patch reef sites, Most colonies have diameters between 10-60 cm; those that are larger, to 300 cm, usually occur in clusters, which provide sheltered cavities for fishes and lobster. The size distributions of all but one of the nine species examined are skewed toward the smaller sizes. The smaller number of larger colonies is believed to be the result of the cumulative effects of disease, bioerosion, and removal by storm waves. For colonies <1 m in diameter, we made visual estimates of percent dead tissue by thirds (< 1/3, 1/3-2/3, >2/3) of each colony. Large variations are the rule at most sites and between sites. However, of all colonies combined 74% have less than 1/3 dead tissue, with only a small standard deviation, and cluster analysis revealed no major spatial variations. Overall, species of Diploria have less dead tissue than most other species and M. annularis generally has more than other species. The amount of dead tissue on clusters of colonies >1 m. was measured by the line point intercept method of Loya (1978) along radial transect lines. It shows the same large variations within and between reefs as found for the smaller specimens. However, the amount of dead tissue on 60% of the 65 clusters measured is between 40-70%, much larger than on smaller colonies. The amount of dead tissue in the clusters in the Middle and Lower Keys is significantly higher than for clusters off the Upper Keys (Key Largo-Elliot Key) according to the Kruskal-Wallis analysis of variance. Given the overall marginal conditions for reef corals in the Florida Reef Tract-periodic low or high temperatures; periodic high turbidity, locally unfavorable water quality-we believe that the condition of massive corals does not show clear evidence of direct anthropogenic impacts. Our results do indicate the negative impacts of storms and of locally unfavorable water quality either in the nearshore zone or from the effluent of shallow, lagoonal Florida Bay.