Anthropogenic nutrient pollution of coral reefs in Southern Bahia, Brazil

Abstract

Spatial, temporal and anthropogenic controls on nutrient distribution were evaluated for nearshore and offshore reefs at Porto Seguro Bay, Southern Bahia. Water samples were analysed for total oxidised nitrogen (TON), soluble reactive phosphorus (SRP), reactive silica (DSi), and chlorophyll a (Chl). The results indicate that rainfall promotes a significant (F=19.873, p<0.001) increase in the load of nutrients to nearshore (average 12% increase) and offshore reefs (average 31% increase). Nutrient concentrations at the urbanized reef (TON=1.93-3.21µM; SRP=0.57-0.89µM; DSi=8.48-11.15µM) are on average 25% higher than at non-urban reefs and over 200% higher than the offshore reef. The urbanized reef also presented the highest rates of increase in nutrient input between dry and rainy seasons (TON=+0.36µM; SRP=+0.08µM; DSi=+0.70µM). Differences in nutrient concentration between nearshore and offshore reefs are more pronounced during dry season, when the bulk of land-based nutrient contribution is confined to the nearshore reefs. SRP values in the study area ranked among the highest in the world for coral reef areas and phytoplankton growth appears to be nitrogen-limited.