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High-elevation aquatic ecosystems are highly vulnerable to climate change, yet relatively few records are available to characterize shifts in ecosystem structure or their underlying mechanisms. Using a long-term data set on seven alpine lakes (3126 to 3620 m) in Colorado, USA, we show that ice-off dates have shifted 7 days earlier over the past 33 years and that spring weather conditions—especially snowfall—drive yearly variation in ice-off timing. In the most well studied lake, earlier ice-off associated with increases in water residence times, thermal stratification, ion concentrations, dissolved nitrogen, pH, and chlorophyll a. Mechanistically, low spring snowfall and warm temperatures reduce summer stream flow (increasing lake residence times) but enhance melting of glacial and permafrost ice (increasing lake solute inputs). The observed links among hydrological, chemical, and biological responses to climate factors highlight the potential for major shifts in the functioning of alpine lakes due to forecasted climate change.

Permafrost landforms and regional reconstructions periglacial landforms glacial and fluviglacial landforms marine and lacustrine landforms polygenetic landforms cold climate slopes rock glaciers.

Preliminary evidence is presented from plant phenology, plant productivity and enhanced snowpack, soil bacterial populations, and hydrology and stream water chemistry to demonstrate some ways in which water availability is a limiting factor in the alpine tundra of Colorado. We also consider the hypothesis that responses to perturbations will tend to facilitate ecosystem changes toward a state commensurate with the present climate. It is demonstrated that the water budget concept is effective, and has considerable potential, in aiding the understanding of ecological conditions of the alpine tundra.

This paper presents the model around which a geomorphic study of the impact of artificially augmented snowfall on alpine terrain has been designed. It consists of a two-order hierarchy which can be...

Precipitation acidity was monitored continuously from July to September 1983 at a well-instrumented research site located near the Continental Divide in central Colorado. Rainfall amount, temperature, pH, and specific conductance were determined using a prototype device consisting of a modified wet-only precipitation collector with appropriate sensors. Data were recorded automatically with microprocessor control by preset time intervals. Weekly wet-only composite precipitation samples were also collected. Precipitation totaled 190 mm during the period, all as rain. Sixty-five percent of the continuous pH measurements were between 3.40 and 4.10, with an average of pH 3.80. Values of pH greater than 5.00 were rare. Composite samples were significantly less acidic, and averaged pH 4.36. Precipitation pH typically decreased rapidly as a storm began, was lowest near or slightly after the time of maximum rainfall intensity, then increased slightly. Paradoxical low pH and low specific conductance were recorded, and it is possible that some initial continuous pH measurements were too low. Acid loading varied widely among storms, and may have resulted largely from short bursts of high-intensity, low-pH rainfall.