ABSTRACT
The role of nitrate-contaminated groundwater discharge in the appearance of algal blooms in the Ria Formosa lagoon, south Portugal, is assessed by setting up a regional water balance for its drainage basin and determining annual N loads on groundwater. Of the estimated 80.8 million m3 (hm3) of natural recharge, 34% is consumed for irrigation, mainly in the western sector near Faro, largely reducing coastal groundwater discharge (CGD) in that area. In the east, since 2001 irrigation is performed with surface water, which has lead to an average annual increase in CGD by 24 hm3, 3.5 hm3 of which is artificial recharge. It is not known what part of CGD enters the lagoon, but estimates point towards 40-50%. Geophysical surveys are carried out to study groundwater seepage along geological faults. Total N load on the drainage basin of the lagoon is calculated in the order of 570 ton/yr, but due to restricted outflow near Faro and deep circulation, 300 ton/yr is estimated to enter the lagoon. Groundwater discharge may be important for triggering winter algae blooms on the mudflats, with little access to ocean water, as well as for the summer algae, which seemingly depend on the Ria Formosa as a nutrient source.
