Efficiency of reactive nitrogen removal in a model Mediterranean high-mountain lake and its downwater river ecosystem: Biotic and abiotic controls.

High-mountain lakes and rivers are usually oligotrophic and strongly influenced by atmospheric transport processes. Thus, wet deposition of reactive N species (Nr), mainly in the form of nitrate (NO), is a major source of N input in these high-mountain ecosystems. Bacterial denitrifiers are thought to be largely responsible for reduction of NO to nitrous oxide (NO) and molecular dinitrogen (N) as main biological pathway of N removal in these ecosystems. Nitrifiers, through the oxidation of ammonium to NO, can also be a source of NO and NO. However, there is uncertainty regarding the abiotic and biotic factors controlling Nr elimination from aquatic ecosystems at different altitudes and seasons. We examined the efficiency of Nr removal as NO and N (total removal) or N only (clean removal) in a model lake and its downwater river ecosystem (Sierra Nevada, Spain) representative of Mediterranean high-mountain freshwater ecosystems along an altitudinal gradient during the warm period of the year. Denitrification activity and the abundance of nitrifiers and denitrifiers in sediments were measured at thaw, mid ice-free and late ice-free periods. We found the efficiency of total and clean removal of Nr increased from the downwater river to the high-mountain lake. Regardless of the location, the efficiency of total removal of Nr decreased over the ice-free period whereas that of clean removal of Nr peaked at mid ice-free period. The efficiency of total removal of Nr was mainly controlled by the abundance of archaeal nitrifiers and bacterial denitrifiers. Abiotic (ammonium and NO concentration) and biotic (mainly nosZI-type denitrifiers) factors drove changes in the efficiency of clean removal of Nr. Our results suggest that abiotic and biotic factors can control the efficiencies of Nr removal in Mediterranean high-mountain lakes and their downwater rivers, and that these efficiencies increase with altitude and vary over the ice-free period.