Synergistic effects of multiple stressors force mountain lakes into unprecedented ecological state
Despite seeming remote and pristine, mountain lakes in Rocky Mountain National Park have been subjected to elevated nitrogen (N) deposition for over 50 years because of atmospherically-transported regional pollution originating from industrial and agricultural activities. This chronic N deposition has induced changes in diatoms, a group of algae that are sensitive indicators of ecosystem disturbance. However, recently we have observed increases in filamentous green algae on the lake shores of The Loch and Sky Pond, two lakes in the Loch Vale Watershed, Rocky Mountain National Park, as well as planktonic chlorophyll a concentrations indicative of mesotrophic conditions. We collected sediment cores from these two lakes and conducted geochemical and pigment analyses to ask (1) whether contemporary assemblages of phototrophic microbes are different now than in the past, (2) if there has been an increase in primary production, and (3) if the changes are similar between two lakes in the same watershed. We found that chlorophytes have increased by 200-300% in both lakes in recent decades while diatom taxa have shifted from the benthos to planktonic habitats, with decreases in overall diversity. In recent decades, warming trends have accelerated with implications for seasonal snowpacks, lake ice duration, and lake thermal structure. Combined with chronic N deposition, these factors appear to have moved these historically ultra-oligotrophic systems into a new ecosystem state, possibly unique in the context of the Anthropocene.
Click here to read our open-access paper, published in Proceedings of the Royal Society: B.
Photo credit: John Hammond
Ecological stoichiometry of the mountain cryosphere
From an ecological perspective, ice-dominated ecosystems place harsh controls on life including cold temperatures, limited nutrient availability, and often prolonged darkness due to snow cover for much of the year. Despite these limitations, glaciers and perennial snowfields still support diverse ecological communities. The availability and mass balance of key elements (carbon, nitrogen, phosphorus) are known to influence the population dynamics of organisms, and ultimately shape the structure and function of ecosystems worldwide. While considerable attention has been devoted to patterns of existing biodiversity in mountain cryosphere-influenced ecosystems, the ecological stoichiometry of these habitats has been less studied. In this review, we synthesized existing knowledge of ecological stoichiometry, nutrient availability, and food webs in the mountain cryosphere. Then we compared nutrient availability in mountain cryosphere habitats to comparable habitats on well-studied continental ice sheets. Finally, we discussed how ongoing climate warming will alter nutrient and trophic dynamics in mountain glacier-influenced ecosystems. Our review paper is published in open-access format at Frontiers in Ecology and Evolution!
Understanding drivers of mountain lake phytoplankton across space and time
In light of recent evidence from sediment cores that shows increases in productivity of mountain lakes in Rocky Mountain National Park, we sought out to understand what drives productivity in other lakes in the region. We used machine learning models to identify important predictors of phytoplankton biomass in 30 Rocky Mountain lakes. Across multiple temporal scales, nutrients and water temperature were influential in explaining variation in phytoplankton, with peak biomass occurring when water temperature was at its maxima and N:P at its minima. We found that snow water equivalent (SWE) of the previous winter and spring explained inter-annual variation in phytoplankton biomass through its influence on water chemistry and water retention time, but our models indicated that summer drought coupled with warmer than average air temperatures caused high phytoplankton biomass even in years with normal SWE. This work is now published in Ecology.