// What We're Watching

Alpine lakes — once considered too remote and too cold for algal blooms — are changing. Wind-borne nitrogen from vehicle exhaust, volatilized agricultural ammonia, and dust-bound phosphorus are fertilizing high-elevation watersheds at rates unprecedented in the paleolimnological record. In the Sierra Nevada, decades of nitrogen deposition from the Central Valley have quietly pushed lakes toward a tipping point. In the Cascades, accelerating glacial melt is uncapping phosphorus stored in freshly-exposed mineral soils.

ALPINE-WATCH tracks chlorophyll-a (the photosynthetic pigment that signals algal biomass), Secchi depth (a measure of water clarity going back to the 19th century), water temperature, and nutrient loading across sentinel lakes in three ranges. Data flows from the USGS Water Quality Portal — the same federal repository used by academic researchers — updated twice weekly through the monitoring season.

The threshold that matters most: 10 µg/L chlorophyll-a marks the EPA boundary between mesotrophic and eutrophic status. Below that line, a lake is clear, cold, and functioning. Above it, the ecosystem is under stress — and the next step could be cyanobacteria, fish kills, and closed swimming beaches in places that have never seen them.