Urban wastewater treatment is one of the most important challenges in villages of southern Spain. This is especially outstanding in arid and semiarid regions in which wastewater are discharged to temporary streams or wetlands. The treatment plant of the Fuente de Piedra village discharges its wastewater, passing previously through four semi-natural wetlands, into Fuente de Piedra Lake, a RAMSAR wetland. In summer 2016, a very dry year, water affluent to Fuente de Piedra was limited to wastewater plant effluents without dilution. In order to study the natural assimilation capacity of the wetland system, four key points were sampled. Physico-chemical and biological indicators were analyzed (temperature, pH, conductivity, total phosphorous, total nitrogen, bacteria, phytoplankton and zooplankton). The results show very high chlorophyll a concentration (>500 mg/l) at the water inlet, which decreased to concentration lower than <20 mg/l before discharging into the RAMSAR wetland. Zooplankton, dominated by cladocerans (Daphnia sp.),was lowest in the inlet wetland and highest in the last wetland. Total nitrogen and phosphorus concentration was (14 mg/l and 5mg/l respectively) at the wastewater inlet point and decreased in the first wetland (7 mg/l and 2mg/l respectively). Fecal streptococci was highest at the inlet point (1033 ± 351 ufc/100 mL) and decreased to 1 ± 1 ufc/100 mL before entering in the RAMSAR wetland. In conclusion, during the wetlands circuit (i) phytoplankton reduce the total phosphorous and nitrogen concentration, (ii) then phytoplankton is controlled by zooplankton decreasing drastically the input of nutrient and biomass into the RAMSAR wetland, (iii) fecal bacteria decreases three orders of magnitude. Thus, the negative impact from wastewater treatment plant is reduced. The waterbirds, one of the major tourists attractive of this wetland, benefit from food and water supply in dry years, guaranteeing the possibility of bird watching during high season.