Planktonic diatom fluctuations in a Northern Arizona Mountain Lake

Upper Lake Mary, located in the San Francisco mountain volcanic field of northern Arizona (2,082 m), is a long, shallow eutrophic lake. Nutrient levels are relatively high during most of the year, particularly nitrate-nitrogen and total silica. The shallow nature of the lake and long fetch allow prevailing winds to continually redistribute nutrients throughout the water column. Sixteen physico-chemical parameters were measured at regular intervals throughout the year and correlated with the seasonal dynamics of 18 diatom species. Seasonal silica averaged 5.2 mg/l and showed an inverse correlation to seasonal diatom populations. Total diatom populations maintained a complex successional sequence and remained at relatively high levels except during cold mid-winter months. Six diatom species played a major role in the seasonal successional sequence with Melosira granulata var. angustissima the dominant species in the lake.