Implications of nitrogen‐rich glacial meltwater for phytoplankton diversity and productivity in alpine lakes

We compared phytoplankton diversity and productivity over various time scales in a set of lakes in the central Rocky Mountains of North America (fed by both glacial and snowpack meltwaters GSF and by snowpack alone SF) to better understand the influence of nitrogen‐rich glacial meltwater on the structure and function of phytoplankton. Nitrate concentrations in GSF lakes were on average 44 times higher than in SF, even though only 0.01–0.59% of the catchment area of GSF lakes was covered by glaciers. In three lakes of each type, we determined the vertical distribution of algae, epilimnetic primary productivity rates, and nutrient‐limitation patterns. Phytoplankton richness and community structure were compared between lake types in the nutrient enrichment experiments, as well as in the tops (i.e., modern) and bottoms (circa 1850) of sediment cores from four lakes of each type. Average primary productivity rates were five times higher in GSF lakes, but vertically integrated chlorophyll concentrations did not differ. However, algal biomass was higher in the epi‐ and metalimnion of GSF lakes, while it was equivalent in the hypolimnion of both lake types. Nutrient‐limitation patterns differed between lake types, with GSF lakes limited by phosphorus and most SF lakes co‐limited by nitrogen and phosphorus. Modern diatom species richness was lower in GSF compared with SF lakes, whereas differences between lake types were not apparent during the mid‐19th century. Key differences in the structure and function of GSF compared with SF lakes imply that GSF lakes will take a different ecological trajectory if glaciers disappear.