Natural swimming holes, at the crossroad between conservation and recreation

Natural swimming holes (NSH) are commonly the only habitats with water throughout the year in semi-arid climates, thus becoming refuges for freshwater biota. However, human crowds also gather in NSH during the warm season, threatening biodiversity.Our goal was to analyse the drivers of recreation (e.g., distance to roads, river flow) in NSH, assess the relationship between visitation and impacts on water quality (dissolved organic carbon, ammonium, caffeine, and the sunscreen agent 2-ethylhexyl 4-methoxycinnamate) and bacterial biodiversity (as indicator of biodiversity), and to develop a method to determine the carrying capacity (i.e. maximum number of visitors per day to avoid an impact in biodiversity) of each NSH.Results suggest that recreation impacts both water quality and biodiversity, and that changes in biodiversity were especially fast when increasing recreation from none to low.Although the study is limited by the low number of case-studies, and the consideration of only bacterial biodiversity, we believe that recreation should be banned or severely restricted in those NSH with high conservation value, whereas a recreation cap using river flow and upstream water quality should be used for those NSH with lower conservation value.