Nitrogen subsidies in glacial meltwaters have altered planktonic diatom communities in lakes of the US Rocky Mountains for at least a century

Abstract

We examined sedimentary diatom profiles from alpine lakes in the US Rocky Mountains to assess when glacially fed lakes started receiving nitrogen subsidies and whether that timing varies across regions. We focused on lake sediment cores from Glacier National Park and compared them to previously published work from the Greater Yellowstone Ecosystem. The only contemporary feature of these glacially fed lakes that differs from snowmelt-fed lakes is nitrate concentration, with glacially fed lakes having about 40 times higher nitrogen than snow-fed lakes; lake thermal structure and temperatures do not differ between lake types. Increases in Asterionella formosa, a strong indicator of nitrogen enrichment, occurred much earlier in glacially fed compared to snow-fed lakes across both regions. Responses to nitrogen enrichment in glacially fed lakes started at least a century ago, and in some cases, many centuries ago, whereas they occurred after 1970 in the snow-fed lakes. Furthermore, diatom assemblages in glacially fed lakes showed declines in species richness over time and greater community turnover compared to snow-fed lakes. Multiple lines of evidence indicate that throughout the late Holocene glacially fed lakes in both regions of the Rocky Mountains have followed different ecological trajectories than snow-fed lakes as a result of increasing nitrogen concentrations.