DISCUSSION

The results of this study were consistent with previous findings (e.g., Nauwerck 1994) that the water quality of montane lakes was associated with elevation. Increased water temperature, pH, alkalinity, conductivity, concentrations of total phosphorus (without the three lakes receiving turbid glacial effluent) and total Kjeldahl-N, and decreasing concentration of nitrate-N were associated with decreasing elevation.

We believe that the observed changes in lake water quality with decreased elevation were associated with warmer air temperatures, and increased biomass of terrestrial vegetation, soil depth and maturity, dissolved substances, and nutrient availability as described by Aber and Melillo (1991) for temperate forests. The alpine vegetation zone is a cold and desiccated zone covered by ice and snow much of the year. The local environment of these lakes is not moderated extensively by soils and surrounding vegetation. Lake watershed relief tends to be steep, except for lakes perched on ridgetops (Lomnicky 1996). Water retention in the basins is limited, except in frozen form, because soils are thin and predominately mineral or nonexistent (Franklin et al. 1973), and steep relief of the watershed probably facilitates the rapid routing of water down slope. Lake ice-free dates in this vegetation zone occur late in the summer season, if at all.

Air temperatures increase with decreasing elevation as the vegetation progresses from the alpine to the subalpine zone. The subalpine zone is a mixture of meadows and stringers of trees that provide added organic input to lakes in the form of needle litter and woody debris (Franklin et al. 1973; 1988). Increased soil depth and maturity leads to enhanced retention of ground water that increases mineral weathering and nutrient availability to lakes (Gorham 1961). Lakes become ice-free earlier than in the alpine zone. Consequently, subalpine lakes tend to warm earlier than do alpine lakes.

Further decreases in elevation and associated increases in temperature promote the establishment of the forest zone. Warmer temperatures lead to reduced periods of winter snowpack, and increased biomass of vegetation and soil depth and maturity in lake watersheds. Increased soil-water interactions elevate nutrient availability to lake systems (Gorham 1961; Hem 1989). Forest lakes, therefore, are higher in pH, alkalinity, conductivity, total Kjeldahl-N, ammonia-N, and total phosphorus than are subalpine and alpine lakes. These lakes, especially low-forest lakes, also are warmer and become ice-free earlier than lakes in other vegetation zones.

Although increased values of most water-quality variables were associated with decreased elevation and changes in vegetation, some of the results did not fit this pattern. For example, nitrate-N decreased in concentration with decreasing elevation in NOCA lakes. This result was similar to those provided by Nauwerck (1994), which may reflect decreased phytoplankton activity with increased lake elevation (Wetzel 1983). The relatively high concentrations of total phosphorus in the three lakes receiving turbid-glacial outwash may have been related to erosional processes (Brundin 1958). The higher alkalinity and conductivity of low-forest lakes in greenstone basins relative to those in gneiss-granitic basins probably was associated with increased concentrations of calcarious materials (Staatz et al. 1972). The reason that east-slope subalpine lakes were significantly higher in pH and alkalinity and concentrations of total Kjeldahl-N and total phosphorus than were west-slope subalpine lakes remains unclear because the geology and vegetation of the watersheds of these two groups of lakes were comparable. Although some shallow lakes exhibited the highest alkalinities, conductivities, and concentrations of nitrogen and phosphorus, the influence of lake depth remains somewhat unclear because many shallow lakes exhibited low values for these variables, as was the case for nearly all deep lakes.

As expected, ice-out times occurred earlier with decreasing elevation. Warmer air temperatures and decreased snow deposition with decreasing elevation were undoubtedly important in producing this pattern (Lomnicky 1996). The finding that east-slope subalpine lakes iced-out earlier than did west-slope subalpine lakes was consistent with this conclusion because the climate of the east slope was drier and warmer than that of the west slope (Lomnicky 1996).

In summary, this study provides additional evidence that the water qualities of temperate zone montane lakes are influenced to varying degrees by lake depth, geology, climate, and elevation. We believe that the major shifts in vegetation zone and associated increases, as described by Aber and Melillo (1991), in soil depth and maturity, dissolved substances, and nutrient availability with decreasing elevation at NOCA are primary influences on the observed changes in water quality. Explanations for variation in water quality among lakes in the same category remains elementary, however. Additional attributes of the landscape appear to exert control on water quality based on the low r² values from the regression analyses between the water quality variables and elevation, plus the misclassification of about a quarter of the lakes in our discriminant analysis. Further studies of the interrelationships between the morphology of the lakes and the characteristics of their entire watersheds (vegetation, geology, aspect, soil depth and maturity, and climate) are needed to develop better models that describe the limnological variations of montane lakes across the landscape.