Field and Experimental Winter Limnology of Three Colorado Mountain Lakes

Physical, chemical, and plankton conditions were studied during two winters in three Colorado mountain lakes. Black Lake is mesotrophic, Pass Lake is highly oligotrophic, and Tea Lake is shallow, eutrophic, and pondlike. The lower waters of Black and Pass lakes absorb heat from the basin so that their winter temperatures are above 4.0°C and as high as 5.4°C. The substrate and bottom waters reach equilibrium in late February or March. Thereafter the lower waters cool toward 4.0°C. Black and Pass lakes had thick snow covers and no photosynthesis for 5—7 months, but Tea Lake had little snow and intermittent photosynthesis during all winter months. Black and Pass lakes are summer—oligotrophic and winter—eutrophic. They became so highly anaerobic by March and April that trout populations died. Such winter kills are thought to occur frequently in many small mountain lakes. All three lakes had negligible winter populations of diatoms and green and blue—green algae, but the populations of μ—algae attained winter maxima of 1—14.5 million cells per liter. Such maxima had no consistent seasonal pattern. The importance of winter algal heterotrophy is discussed. Winter copepod and cladoceran populations were negligible; densities seldom exceeded one adult per liter. Rotifer populations were usually dense, commonly exceeding 500 per liter, especially during December and January before the onset of severe anaerobiosis in Black and Pass lakes, and during all months in Tea Lake. Seston varied much more widely than during the months of open water. The seston of large lake—water samples stored in the dark at 3°C for 30 days ranged from a 20% decrease to a 350% increase over the original seston content. Winter plankton had an average respiratory rate in situ of about twice that of the plankton in corresponding water samples kept in a dark refrigerator at 3°C. Water samples treated with antibiotics had a lower plankton respiratory rate during 30 days in a dark refrigerator at 3°C than did refrigerated control samples. After an interval of 60 days, however, comparable samples treated with antibiotics had a respiratory rate higher than that of controls. It is postulated that the inhibition of bacteria by antibiotics makes nutrients available to yeasts, molds, mutant antibiotic—resistant bacteria, and μ—algae (?), which, after a lag phase, grow and collectively attain a high respiratory rate after the longer experimental interval. These results on seston and plankton show that the metabolism of small, enclosed experimental water samples is highly variable and poorly understood, especially in the dark at low temperatures.