A study has been conducted in a salt marsh, located in the coast of the Mar Menor lagoon (SE Spain), in which three watercourses overflow. Water samples were regularly collected over a one year period from two transects established through the salt marsh towards the lagoon. All the samples were analysed for electrical conductivity, pH, sulphides, chlorides, nitrate, ammonium and dissolved phosphorus. The quality of the water flowing from the watercourses to the salt marsh showed a seasonalpattern, with higher contents of nitrates (>200 mg NO−3 L−1) in periods of maximum agricultural activities in the nearby areas, as well as a higher content of ammonium (>30 mg NH+4 L−1) and phosphorus (>10 mg PO3−4 − P L−1) when the human population increased in the zone as a result of tourism. The general spatial pattern of nutrient retention in the salt marsh, indicated by a reduction in the nutrient concentrations in the water closer to the lagoon, was modulated depending on the season. In the driest months, the marsh was 100% effective in reducing nutrient concentrations, but in the rainy periods the effectiveness was reduced. This reduction was more evident for phosphorus in autumn, whose concentration increased in the lower part of the salt marsh closer to the lagoon until it reached that of the inflow. This could mainly be explained by the accumulation of water in some sites near the lagoon, which can act as sinks of pollutants. Our data support the existence of polluted water in the surface watercourses of the area, and the associated risk can include soil salinization and the eutrophication of aquatic systems. Based on our data, the Mar Menor coastal marshes have an important role as filters to reduce pollution in this lagoon