Dissolved Nutrients from Submarine Groundwater in Flic en Flac Lagoon, Mauritius

Abstract

The aim of this study was to investigate dissolved nutrients in
a submarine groundwater discharge (SGD) in Flic en Flac lagoon on the west coast of the volcanic island of Mauritius. The SGD enters Flic en Flac lagoon through a thin blanket of unconsolidated sediment through a fracture system and is concentrated along the irregular surface of the buried rock. The results show that the major inputs of dissolved nitrate and phosphate in Flic en Flac lagoon arise from agricultural and domestic sources during periods of heavy rain. Dissolved nitrate (150-470 µg/l) and phosphate (10-115 µg/l) concentrations in the SGD, lagoon and offshore in the Flic en Flac region fell within the range of seawater values in the Pacific region. Rainfall caused dissolved nitrate and phosphate transfer
through groundwater conduits in the coralline basement of the lagoon, delivering a significant discharge during the cyclone season in summer. The lagoon is subjected to diffuse SGD flows which may contribute to its high dissolved nutrient values. This will lead to a reduction in the lagoon’s assimilative capacity, exacerbating the problem of excessive nutrient input and associated eutrophication in the lagoon. The submarine groundwater discharge thus merits consideration in Mauritian coastal zone management and similar tropical volcanic lagoonal systems.