Agricultural catchments often show elevated phosphorus concentrations during summer low flow. In a typical agricultural headwater we found out that geogenic phosphorus in groundwater was the major source of stream water phosphorus during these low-flow conditions and that stream sediments derived from farmland are unlikely to have increased stream phosphorus concentrations during low water. Agricultural land use was not the main cause of P loading in the stream during low-flow conditions.

We combine radon and stable isotopes in water to determine total submarine groundwater discharge (SGD) in the Ria Formosa and discriminate its component modes. We show that tidal action filters the entire water volume in the lagoon through local beaches 3.5 times a year, driving an estimated 350Ton nitrogen/year into the system. Conversely, fresh groundwater is discharged into the lagoon only occasionally, adding ~ 61 Ton nitrogen/year, but is capable of driving new production in the system.