Study area
The present study was performed on 22 permanent and temporary ponds located in a lowland area (900 m. a. s. l.) area (ca. 230 km2) in the northwest part of Spain (Figure 1). The study sites were characterized by a marked variability in environmental conditions, such us morphometry, nutrient content and mineralization (Table 1). The predominant land uses in the study area are arable land and pasture. The climate is Mediterranean dry moderate with mean temperatures in summer and winter of 18 °C and 3.2 °C, respectively, and a mean precipitation in summer and winter of 84.5 mm and 173 mm, respectively (1976-2015; data provided by the Spanish Met Agency-AEMET; http://www.aemet.es). The majority of ponds are shallow (0.2-1.5 m), are fed by groundwater and rainfall, and they experience a high reduction in water volume during summer.