Nutrification and its effects on coral reefs from Southern Bahia, Brazil
OZÉAS DA SILVA COSTA JR.

Coral reefs are undergoing increasing deterioration worldwide, one of the major causes of coral reef demise being nutrification. At early stages, nutrification causes phytoplankton blooms. Continued nutrient load may lead to changes in the community structure, decrease in reef calcification and benthic dominance by algae, deposit and suspension feeders.

To assess nutrification levels on Brazilian reefs, measurements of the distribution patterns of nutrients and chlorophyll concentrations were conducted in three coastal reefs with distinct nutrient inputs along the south coast of Bahia State. Seawater and porewater samples were analysed for phosphorus, nitrogen and silica. Benthic surveys were performed at all sites to investigate the relationships between community composition and nutrient and chlorophyll concentrations. In order to account for seasonal variation, sampling was undertaken in dry and rainy seasons.

Results of both seawater and porewater nutrient measurements revealed the occurrence of consistent spatial and temporal patterns. An inshore-offshore gradient reflects the existence of land-based point sources, with significant amount of nutrients being delivered by human activities on the coast (untreated sewage and groundwater seepage). Another spatial gradient is related to distance from a localized source of pollution (an urban settlement without sewerage treatment). Seasonal variations suggest that submarine groundwater discharge (SGD) is the primary source of nutrients to nearshore reefs during the rainy season. The data also suggests that the SGD effect is not restricted to nearshore reefs.

Benthic community assessment revealed that turf algae are the dominant group on all studied reefs and that zoanthids are the organisms most adapted to take advantage of nutrient increase in coastal areas. Highly spatial variability in coral cover ultimately reflects the patchy distribution of stony corals over the reefs.

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