Regional lake typology in northern Colorado, U.S.A.

Summary

Northcentral Colorado contains more than 400 lakes featured by an exceptionally wide range of limnological conditions. Four altitudinal limnological zones are characterized: the plains zone, which includes lakes at elevations of 1480 to 1700 m, the foothills zone (1700 to 2500 m), the montane zone (2500 to 3200 m), and the barren alpine zone (above 3200 m).

Within the montane zone two lake types are distinguished: (1) drainage lakes, which have permanent inlets and outlets, and (2) semidrainage lakes, which have no inlet streams and an outlet for only a short time in an occasional year.

Within the plains zone two lake types are also sharply differentiated: (1) non-alkali lakes with total residues of less than 500 mg per liter, and (2) highly productive alkali lakes with total residues in excess of 500 mg per liter.

The fundamental limnological characteristics of these six groups of lakes are discussed with particular reference to their chemical features, especially as compared with regional lake typology studies in other parts of the world.

From high to low altitudes, the northcentral Colorado lakes contain progressively greater quantities of dissolved salts, total organic matter, calcium, and bound carbon dioxide.

Semidrainage lakes of the montane zone appear to be “nitrogen-accumulators”, since they contained an average of 1.137 mg of nitrate-nitrogen per liter as compared with 0.168, 0.063, and 0.098 mg per liter for the montane drainage lakes, alpine lakes, and alkali plains lakes, respectively. No correlation was found between nitrate-nitrogen and lake productivity. The significance of rain and snow as sources of nitrogen in lakes is emphasized.

Phosphate-phosphorus showed a striking correlation with altitude, from an average of 0.0029 mg per liter for alpine lakes to 0.0143 for alkali lakes of the plains. Semidrainage lakes contained amounts similar to those found in drainage lakes of the montane zone. Except at very high and very low concentrations, there was no correlation between phosphate-phosphorus and lake productivity.