Coral recruitment and juvenile mortality as structuring factors for reef benthic communities in Biscayne National Park, USA

Coral communities of Biscayne National Park (BNP) on offshore linear bank-b arrier reefs are depauperate of reef corals and have little topographic rel ief, while those on lagoonal patch reefs have greater coral cover and speci es richness despite presumably more stressful environmental regimes closer to shore. We hypothesized that differences in rates of coral recruitment an d/or of coral survivorship were responsible for these differences in commun ity structure. These processes were investigated by measuring: (1) juvenile and adult coral densities, and (2) size-frequency distributions of smaller coral size classes, at three pairs of bank- and patch-reefs distributed al ong the north-south range of coral reefs within the Park. In addition, smal l quadrats (0.25 m(2)) were censused for colonies <2 cm in size on three re efs tone offshore and one patch reef in the central park, and one intermedi ate reef at the southern end), and re-surveyed after 1 year. Density and si ze frequency data confirmed that large coral colonies were virtually absent from the offshore reefs, but showed that juvenile corals were common and h ad similar densities to those of adjacent bank and patch reefs. Large coral colonies were more common on inshore patch reefs, suggesting lower survivo rship (higher mortality) of small and intermediate sized colonies on the of fshore reefs. The more limited small-quadrat data showed similar survivorsh ip rates and initial and final juvenile densities at all three sites, but a higher influx of new recruits to the patch reef site during the single ann ual study period. We consider the size-frequency data to be a better indica tor of juvenile coral dynamics, since it is a more time-integrated measurem ent and was replicated at more sites. We conclude that lack of recruitment does not appear to explain the impoverished coral communities on offshore b ank reefs in BNP. Instead, higher juvenile coral mortality appears to be a dominant factor structuring these communities.