Identifying the Source of Nutrient Contamination in a Lagoon System

Abstract

Nutrient concentrations within watercourses are often associated with the input of sewage or the runoff of fertilizers. Due to population increases, there has been a dramatic rise in the amount of fertilizer applied to land, as well as in the further development of sewage treatment plants (STPs), both of which can lead to significant discharges with associated eutrophication risks in coastal waters. The implementation of the European Union Water Framework Directive (EU-WFD) should improve the management and quality of European water bodies. The Ria Formosa Lagoon, Portugal, is one such water body monitored under the WFD where two inter-calibration sites have been developed, that of the Ancão Basin, which has a status of “high/good,” and that of the Ramalhete Channel, with a status of “good/moderate.” Spatial nutrient concentrations (NH4 +, NO2 –, NO3 –, PO4 3–) and lipids were measured in two areas that were thought to contribute nutrients into the Ancão Basin; a river that flows through several golf courses and the Ramalhete Channel, which receives discharge from both Faro Airport and a STP. Nutrient analyses showed that waters from the Ramalhete Channel had substantial concentrations, the highest of which was ammonium, which exceeded 180 μ M near the sewage discharge site; however, concentrations diminished towards the sea, reaching 2.7 μ M suggesting the utilization of NH4 + by seagrasses (Zostera noltii). The Atlantic Ocean was also a source of nitrate, contributing 1.8 μ M, whereas sources of phosphate originated in the Ancão Basin catchment and included a commercial horticulture site, several golf courses, and the STP. Principal component analysis (PCA) and cross plots of the 5β-coprostanol/cholesterol and 5β-coprostanol/(5β-coprostanol + 5α-cholestanol) ratios identified areas of fecal contamination, highlighting several sites previously unknown to contain sewage matter. In general, nitrogen sources were associated with agriculture and phosphate originated from the golf complex and sewage discharges. These effects were partly mitigated, however, by eutrophic ponds between the river and the lagoon.