Ninety-six percent of surveyed shallow-water Dry Tortugas reef corals died during the severe winter of [1976][1977]. Data from skeletal stains indicate that death occurred during the mid-January intrusion of 140C water onto the reef. In deeper water, community parameters such as percent cover, species number, and relative abundance showed no significant change. However, an analysis of competitive interactions at the growing edges of adjacent colonies reveals a 70% reduction in space competition during this environmental disturbance. These results can explain high variability in the growth rate of Floridian reefs and demonstrate the importance of obtaining long-term spatial information to interpret successional dynamics of complex communities.Successional changes in complex natural communities are among the most difficult ecological phenomena to measure due to the often subtle influence of controlling mechanisms, the posited great length of time between observable changes, and the large sample size required to generalize with statistical confidence. In this study we report changes in coral reefcommunity structure that occurred during the climatically severe winter of 1976-1977 (1-3).Corals require space for plankton capture by their tentacles and adequate surface area for photosynthetic light capture by their intracellular symbiotic algae (4-6). They compete for space both intra-and interspecifically by overtopping one another and by extracoelenteric destruction, whereby the mesenterial digestive filaments ofa dominant species extend onto the living tissues ofan adjacent subdominant and destroy it (7,8). Abilities in these competitive mechanisms are species-specific and, in. areas of high densities on the reef, have been shown to affect coral abundance and distribution patterns (7)(8)(9)(10)(11)(12)(13)(14)(15) The Dry Tortugas coral reef community was drastically altered during the winter of 1976-1977. Ninety-six percent ofthe living coral cover in the survey depths of <2 m died (Fig. 1). As seen in the sample frames, the shallow water A. cervicornis suffered tissue death only. The skeletons were fully intact, indicating that death was not induced by physical damage due to storm or wave stress. At the time of sampling, the skeletons supported a thick mixed assemblage of filamentous and fleshy epiphytic algae. Table 1 shows contrasting coral community changes between shallow and deep water stations. Both stations experienced a reduction in percent cover, but only significantly so in shallow water, where a significant reduction in species number and species diversity also occurred. These parameters did not change (P < 0.01) in deep water. However, at both depths, the amount of living coral in close proximity (1-5 cm from each other) decreased significantly.Acquisition of substrate occurs both by growth involving indirect exploitative means (shading) and direct interference competition (extracoelenteric destruction) as well as by settlement or physical displacement after stochastic events. Because the eff...