Spatial Patterns of Reef Community Structure and Environmental Correlates in Biscayne National Park, Florida. Draft- 1

Weil, Ernesto; Miller, Margaret; Szmant, Alina

The coral reefs of Biscayne National Park, Florida, lie at the northern fringe of the Florida Reef Tract and of coral reef distribution in the western Atlantic. Environmental forcing of community structure is expected to be strong. Nine refal areas were quantitatively characterized within the Biscayne National Park and environmental factors such as temperature, sedimentation, and nutrient content in the sediment and water column were monitored to examine possible correlations with community structure patterns. Live cover, densities and diversity of scleractinian corals, octocorals, sponges, zoanthids and hydrocorals as well as cover of algae and other substrate types were quantified in 75-104, 1 m2 quadrats along 4-5 transects for each of nine shallow bank and patch reefs and hard bottom communities selected along four east-west transects distributed from north to south within the Park. Benthic cover was dominated in all sites by algal turf, crustose and macro algae. Sponges were the most diverse group of macro invertebrates (66 spp. in 41 genera) followed by scleractinian corals (52 spp. in 23 genera) and octocorals (43 spp. in 11 genera). Octocorals were the most abundant group on eight of the nine localities sampled. Despite closer proximity to hypothetically detrimental terrestrial and bay influences, near-shore patch reefs had higher live cover, densities and species richness of cnidarians and sponges, a greater abundance of large scleractinian corals and of octocorals, and reduced prevalence of bare substrate and algal turf compared to latitudinally-similar offshore localities. Inshore patch reefs were more spatially variable in mean percent cover of the different bio-groups, with increases for sponges, hydrocorals and crustose algae towards the south and decreases in cover of turf algae, scleractinians and octocorals. Octocoral densities were significantly higher in all but one reef compared to other cnidarians and sponges with significantly higher dens1t1es 1n the patch reefs compared to the offshore sites. Decrease cover of turf algae towards the south, away from Miami, was correlated with an increase in live cover, densities and diversity of cnidarians and sponges, cover of macro-algae and bare substrate in offshore sites. Juvenile coral densities were higher in southern patch reefs and were significantly correlated with adult densities for six common scleractinian species. Temperature regimes were more variable in the patch reefs, a pattern opposite to what would be expected to correspond to the greater coral cover in these localities. Sediment deposition rates, water column and sediment nutrient concentrations were temporally variable and provided no clear correlation with the observed spatial patterns of benthic community structure. This suggest that other factors such as historical effects of severe cold snaps, storms, disease outbreaks, coral recruitment and juvenile survival may be the dominant structuring forces limiting the abundances of large, reef-forming species in offshore sites in Biscayne National Park.