Nutrient fluxes in a tropical coastal lagoon

Abstract

In order to define the spatial and seasonal patterns of water and nutrient fluxes in a coastal lagoon, water residence times were estimated from salinities by using a simple salt balance model. Nutrient flows between different zones of the lagoon were measured during daily cycles in different seasons (dry: March to May; rainy: June to September; “nortes”: October to February). The shortest residence times (140–168 days) correspond to the rainy season. During this season, large inflows of nitrate-nitrogen (N0₃), soluble reactive phosphorus (SRP), and silicon (SRSi) take place associated with groundwater discharges. N0₃ and SRSi are trapped in the lagoon and ammonium and suspended particulate matter are released to the water column as water circulates through different zones of the lagoon for most of the year. During the dry season, moderate water residence times were estimated (175–201 days). Nitrogen and SRP are trapped in the inner zone of the lagoon and released in the outer zone, during this season. During the “nortes” season (water residence times between 240–286 days), SRP is released in the inner zone of the lagoon and a low net import from the sea takes place in the outer zone of the lagoon. In the annual budget, a net export of all the nutrients takes place from the lagoon to the sea. Mean sea level and freshwater discharges are the major factors regulating water turnover in the lagoon during all the year. Tidal and wind forces are also important in the outer zone of the lagoon during dry and “nortes” seasons.