Seasonal influence of surface and underground continental runoff over a reef system in a tropical marine protected area

Continental runoff is an important source of solutes and nutrients to coastal coral reefs. Nevertheless, they can also act as local stressors due to high coastal urbanization and the associated release of untreated effluents. Here we aimed to qualitatively evaluate the effects of seasonality over reefs from the Costa dos Corais Marine Protected Area (Brazil) and to identify the influence of rivers and submarine groundwater discharge (SGD) on the reefs' environmental status. Surface waters from the reef lagoon, local rivers and shoreline porewaters were sampled monthly during a year and abiotic parameters were determined. The results showed that the reef lagoon is oligotrophic throughout most of the year, with conditions that generally favor biological activities, such as calcification. Chlorophyll-a, with an average of 0.78 +/- 0.43 mg m- 3, was significantly correlated with salinity and radon in excess. This indicates the influence of rivers and SGD over the reefs, especially during the rainy season. These are important sources of nutrients, regulating primary production. However, signs of anthropogenic pressure were found in the local rivers. In addition, porewaters had a N:P ratio (27.68 +/- 24.45) which could potentially impact surface waters through SGD by increasing the local trophic status. In summary, the studied reefs still present good water quality conditions, although continental runoff is a significant transport vector of anthropogenic solutes to the area.

Automated summary

The study demonstrates that coral reefs in the Costa dos Corais Marine Protected Area in Brazil are affected by seasonality, with rivers and submarine groundwater discharge impacting the environmental status, especially during the rainy season. Although the water quality conditions of the reefs are good, signs of anthropogenic pressure were found in local rivers, and the N:P ratio of porewaters could potentially impact surface waters by increasing the local trophic status.