Nutrification impacts on coral reefs from northern Bahia, Brazil

Abstract

Coral reefs extend for 20 km along the north coast of the state of Bahia, Brazil. Over the last 15 years, this region has experienced an acceleration of generally unplanned urbanisation, with the irregular and indiscriminate use of septic tanks in urban centres contaminating the groundwater. This infiltration of nutrients and pathogens is facilitated by both the soil permeability and an accented hydraulic head, which eventually leads to the percolation of nutrient-rich groundwater seaward to the reefs. The groundwater nutrient concentrations (nitrate, nitrite, ammonia, phosphate and silicate) from Guarajuba beach (a highly urbanised area) are over 10 times higher than groundwater from Papa Gente beach, an area of low human occupation. The pH values of the groundwater samples also indicate the predominance of reducing conditions in Guarajuba, due to the high availability of organic matter and consequent bacterial activity. Additionally, faecal coliform data indicate domestic wastewater as the source of groundwater contamination. High densities of macroalgae and heterotrophic organisms on the impacted reefs, as well as higher concentrations of nutrients, evoke the effects of eutrophication on this coral reef ecosystem. These data suggest that the high availability of nutrients is affecting the trophic structure in the study area, especially in Guarajuba, with the increased turf and macroalgae growth reducing light penetration to the coral colonies, competing with them for space and inhibiting the settlement of new coral larvae.