Nutrient export from largely pristine catchments (Serranía de Cuenca, Central Spain)

Abstract

We report here the summer nutrient (organic carbon, nitrogen and phosphorus) export from pristine catchments of the Serranía de Cuenca (Central Spain). These mountains are mostly encompassed by forested and rangeland landscapes and appear to have important groundwater outputs. On the basis of instantaneous sampling for nutrient chemistry, including in situ records of discharge in these largely non-gauged areas, nutrient fluxes downstream were estimated. Long-term (1982-2010) annual yields of total nitrogen and phosphorus downstream were also estimated from official data using discharge and concentrations at three larger sub-catchments, with the aim of relating them to the effects of global warming. Average nutrient concentrations in these streams were rather low. The fraction of organic nitrogen content was relatively high, as in other forested catchments. Variability of concentrations changed from one catchment to another, but was wide. Organic carbon, total nitrogen and total phosphorus export downstream ranged from one- for organic carbon and nitrogen to three orders of magnitude for phosphorus, but these values lie within the lower quartile of reported export worldwide. There were no statistically significant relationships between discharge and nutrient concentrations. Water retention by lakes and reservoirs upstream decreased the variability of nutrients, particulate organic carbon and total nitrogen, but total phosphorus retention was unaffected. The catchment surface area and land use were unrelated with the phosphorus export. In-stream biological processes appeared to be negligible for nutrient export. Contrary to the N export the annual P flux export was partly explained by discharge (and hence rainfall) in the long-term, which seemingly suggests a global warming effect for P. Our results suggest that pristine catchments experience a complex behaviour of nutrient export that deserves further research, and should be more detailed to see if groundwater input plays a significant role.