Seasonal algal succession and cultural eutrophication in a north temperate lake

A year-round study of the algal composition of a previously uninvestigated north temperate, dimictic lake revealed the abundance of many pollution-tolerant forms, including such toxin-producing blue-greens as Anabaena and Aphanizomenon. A significant hypolimnetic oxygen deficit (0.4 ppm, 3.5% saturation) in late summer and the rate of oxygen depletion from 1 August to 19 September (0.13 mg/ml/day) further indicate eutrophic conditions. Of the three groups considered in this study (Chlorophyta, Bacillariophyta and Cyanophyta), the Cyanophyta were less diverse in terms of relative numbers of genera but produced the largest blooms. Chlorophyta had the greatest number of genera. Diatoms dominated in winter and spring, Chlorophyta in summer and fall and blue-greens in late summer, fall and winter. Spirogyra and Oscillatoria were the most ubiquitous members of the algal flora. Important perennials included Oscillatoria, Spirogyra, Closterium, Fragilaria, Meridion, Tabellaria and Cymbella. No unialgal blooms ever occurred. The accelerated rate of eutrophication in recent years is due primarily to excess nutrient loading resulting from input of raw domestic sewage. The completion of a sanitary sewer system is expected to alleviate the excessive nutrient loading and thereby slow the eutrophication process.