Patterns in the distribution of juvenile corals and coral reef community structure in St. John, US Virgin Islands

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There is increasing awareness that coral reefs exhibit a high degree of spatio-temporal variability, yet the processes that create these patterns are poorly known. In this study, benthic community structure and scleractinian recruitment on shallow reefs (5 m depth) were quantified at 18 sites along 10 km of the coast of St. John, US Virgin Islands. The goal was to test 2 mechanisms that could create spatio-temporal variation-coral recruitment and early life-history events-by addressing 3 questions: (1) Is the percent cover of scleractinians correlated with the density of juvenile corals? (2) How is the density of juvenile corals affected by coral reef community structure? (3) What are the rates of mortality and growth of juvenile corals and how do they map onto the patterns of variation in juvenile density? Community structure was assessed as the percentage cover of the 4 major substratum components, and coral recruitment was estimated from the abundance of juvenile scleractinians. Temporal variation in the population of juvenile corals was examined over 4 yr (1994 to 1997) at 5 sites. The same 5 sites were used to assess the growth and survivorship of juvenile corals over 1 yr. Overall, the results demonstrate that there is a high degree of spatio-temporal variability in the shallow reefs of St. John. The community structure varied significantly among sites, and the density and taxonomic composition of juvenile corals varied significantly among sites and years. However, the density of juvenile corals was not correlated with the percentage cover of scleractinians, and the overall community structure did not explain a significant portion of the variation in the density of juvenile corals. Similar results were obtained when the juvenile corals were separated by reproductive mode (brooders vs spawners). The variation in density of juvenile corals was unlikely to have resulted from differential growth, as growth rates did not vary among sites. Additionally, mortality was not correlated with density of juvenile corals, although it did vary among sites. Together, these results suggest that the community structure of coral reefs is related only loosely to the contemporaneous distribution of juvenile corals and early life-history events (i.e., survivorship and growth of juvenile corals). Studies on larger spatial (> 10 km) and temporal (> 4 yr) scales probably are necessary to quantify the relationships between coral recruitment and coral community structure.