Effects of nitrogen on benthic diatom assemblages in high-elevation Central and Eastern Alpine lakes.

We explored patterns of benthic-diatom composition across 62 high-elevation Alpine lakes spanning a wide range of nitrogen (N) concentrations due to atmospheric deposition and background variation in lake and watershed characteristics (e.g., land cover, bedrock geology, lake depth, and elevation). Our goals were to (i) assess the effect of lake-water N concentration on benthic-diatom composition during late summer or fall conditions and (ii) identify policy- relevant response thresholds. The analyses were carried out on an existing data set of diatom and water-chemistry data (EMERGE set), integrated with new data. Multivariate and correlation analyses revealed associations between pH, N concentration, and benthic-diatom composition, but the effects of pH and N were confounded. However, partial correlation analysis allowed us to identify ‘N-responsive diatoms’, i.e., diatom taxa with non-spurious association with N. Focusing on these taxa, we detected a decline in the abundance of taxa preferring low N concentrations and an increase in the abundance of taxa preferring high N concentrations starting at NO 3 concentrations of approximately 5 µmol L -1 . We interpreted this shift as an effect of watershed N saturation due to atmospheric N deposition. Based on the results, we suggest a concentration threshold of 5 µmol NO 3 L -1 during late-summer or fall to prevent change in benthic diatoms in high-elevation Alpine lakes affected by N deposition.