Algal blooms in the alpine: investigating the coupled effects of warming and nutrient deposition on mountain lakes

While 20th century atmospheric nitrogen (N) deposition has been strongly linked to changes in diatom assemblages in high-elevation lakes, contemporaneous changes in other algae suggest additional causes. Using proxies preserved in lake sediments, we explored the origin and magnitude of changes in an alpine and subalpine lake from the end of the Little Ice Age in the 19th century to ca. 2010. We found dramatic changes in algal community structure. Diatom analyses revealed a pronounced shift from majority benthic to planktonic diatoms ca. 1950, coincident with the rise of atmospheric N deposition. Pigments representing benthic green algae have increased 200-300% since ca. 1950; diatom pigments suggest stable or slightly declining populations. Cyanophytes and cryptophytes are not abundant in the sediment record, but there has been a slight increase in some taxa since ca. 1950. While some changes began ca. 1900, the shifts in nearly all indicators of change accelerate ca. 1950 commensurate with many human-caused changes to the Earth system. In addition to N deposition, there have been marked recent increases in aeolian deposition to western mountains that contributes phosphorus. Strong increases in summer air (0.7 °C per decade) and surface water (0.2-0.5°C per decade) temperatures since 1983 have direct and indirect consequences for high elevation ecosystems. While our links between the causes of changes and the responses of mountain lake primary producers are inferred, the drivers and their responses are indicators of changes in the Earth system that have been used to define the Anthropocene. Algal communities (or assemblages) in historically unproductive mountain lakes are shifting, and these changes are taking place commensurate with increasing water temperatures and nutrient availability. However, the mechanisms promoting chlorophytes over bacillariophytes and the implications for ecosystem function are not well understood. We tested the effect of nutrient enrichment on the relative abundance of algal taxonomic groups in a field experiment. We also tested the interactive effects of nutrients and temperature on ecological function of chlorophyte-dominated benthic communities in a laboratory experiment. Nutrient enrichment of both nitrogen and phosphorus favored chlorophytes and led to the highest overall algal biomass. In the absence of nutrient enrichment, the relative abundance of bacillariophytes was significantly greater than chlorophytes and cyanobacteria. Nitrogen assimilation increased significantly, but net ecosystem production decreased, with warming temperatures. Collectively, our results show how chronic N deposition, permafrost thaw, P deposition, and a warming climate interact to alter both the structure and function of mountain lake algal communities. Climate change is altering biogeochemical, metabolic, and ecological functions in lakes across the globe. Historically, high-elevation lakes in temperate regions have been unproductive due to brief ice-free seasons, a snowmelt-driven hydrograph, cold temperatures, and steep topography with low vegetation and soil cover. Observed increases in high elevation lake productivity in the Southern Rocky Mountains over the past decade led us to ask: what are the drivers behind increasing primary productivity? We tested the relative importance of winter and summer weather, watershed characteristics, and water chemistry as drivers of phytoplankton dynamics. Boosted regression tree models were applied using data from 28 high-elevation lakes in Colorado to examine spatial, intra-seasonal, and inter-annual drivers of variability in lake phytoplankton, using chlorophyll a as a proxy. Similar to previous studies, we found that phytoplankton biomass was inversely related to the maximum snow water equivalent (SWE) of the previous winter. However, even in years with average SWE, summer precipitation extremes and warming enhanced phytoplankton biomass. Peak phytoplankton biomass consistently coincided with the warmest water temperatures and lowest nitrogen to phosphorus ratios. While links between declining snowpack, lake temperature, nutrients, and organic matter dynamics are increasingly recognized as critical drivers of change in high elevation lakes, this study identifies additional processes that will influence lake productivity as the climate continues to change. Continued changes in the timing, type, and magnitude of precipitation in combination with other global change drivers (e.g., nutrient deposition) may have consequences for production in high elevation lakes, potentially shifting these historically oligotrophic lakes toward new ecosystem states.