Temporal coherence of two alpine lake basins of the Colorado Front Range, USA

1. Knowledge of synchrony in trends is important to determining regional re sponses of lakes to disturbances such as atmospheric deposition and climate change. We explored the temporal coherence of physical and chemical charac teristics of two series of mostly alpine lakes in nearby basins of the Colo rado Rocky Mountains. Using year-to-year variation over a 10-year period, w e asked whether lakes more similar in exposure to the atmosphere behaved mo re similarly than those with greater influence of catchment or in-lake proc esses. 2. The Green Lakes Valley and Loch Vale Watershed are steeply incised basin s with strong altitudinal gradients. There are glaciers at the heads of eac h catchment. The eight lakes studied are small, shallow and typically ice-c overed for more than half the year. Snowmelt is the dominant hydrological e vent each year, flushing about 70% of the annual discharge from each lake b etween April and mid-fury. The lakes do not thermally stratify during the p eriod of open water. Data from these lakes included surface water temperatu re, sulphate, nitrate, calcium, silica, bicarbonate alkalinity and conducti vity. 3. Coherence was estimated by Pearson's correlation coefficient between lak e pairs for each of the different variables. Despite close geographical pro ximity, there was not a strong direct signal from climatic or atmospheric c onditions across all lakes in the study. Individual lake characteristics ov erwhelmed regional responses. Temporal coherence was higher for lakes withi n each basin than between basins and was highest for nearest neighbours. 4. Among the Green Lakes, conductivity, alkalinity and temperature were tem porally coherent, suggesting that these lakes were sensitive to climate flu ctuations. Water temperature is indicative of air temperature, and conducti vity and alkalinity concentrations are indicative of dilution from the amou nt of precipitation flushed through by snowmelt. 5. In Loch Vale, calcium, conductivity, nitrate, sulphate and alkalinity we re temporally coherent, while silica and temperature were not. This suggest s that external influences are attenuated by internal catchment and lake pr ocesses in Loch Vale lakes. Calcium and sulphate are primarily weathering p roducts, but sulphate derives both from deposition and from mineral weather ing. Different proportions of snowmelt versus groundwater in different year s could influence summer lake concentrations. Nitrate is elevated in lake w aters from atmospheric deposition, but the internal dynamics of nitrate and silica may be controlled by lake food webs. Temperature is attenuated by i nconsistently different climates across altitude and glacial meltwaters. 6. It appears that, while the lakes in the two basins are topographically c lose, geologically and morphologically similar, and often connected by stre ams, only some attributes are temporally coherent. Catchment and in-lake pr ocesses influenced temporal patterns, especially for temperature, alkalinit y and silica. Montane lakes with high altitudinal gradients may be particul arly prone to local controls compared to systems where coherence is more ob vious.