Some ecological effects of artificial circulation on a small eutrophic lake with particular emphasis on phytoplankton II. Kezar lake experiment, 1969

A small eutrophic New Hampshire lake was artificially circulated (mixed) from 28 May to 15 September, 1969, inclusive, to impede the annual bloom of a noxious blue-green alga; yet Aphanizomenon flos-aquae bloomed immediately after mixing was commenced. The bloom collapsed in early July; it was succeeded by heavy growth of predominantly chlorophycean taxa. In an in vitro experiment Aphanizomenon flos-aquae did reattain bloom proportions when the influence of artificial circulation was removed. Other phytoplankton exhibited population pulses only when the dominance of cyanophycean and chlorophycean taxa were in transition. Mixing maintained uniform vertical populations of all phytoplankton. Changes in water transparency attended fluctuations in phytoplankton abundance. An isothermic condition was maintained over the test period, which increased the lake's heat budget, and most chemical nutrients were distributed isometrically in the water column. Increased concentrations were exhibited by Ca, Cl, Cu, K, Mg, SiO₂ and Zn. Sodium was not affected by mixing. Levels of Fe, Mn, Zn, phosphate and ammonia, nitrate, and organic nitrogen were influenced by phytoplankton. Mixing could not maintain orthograde profiles of dissolved O₂ and CO₂ when dense populations of phytoplankton prevailed. Variations in chlorophyll-a followed closely changes in phytoplankton abundance. Its degradation to pheo-pigments appeared to be less for a bloom of Aphanizomenon flos-aquae than during dense growth of chlorophycean taxa. Rates of photosynthesis were considerably greater when the latter algae were predominant. Extracellular release of organic carbon usually increased with depth; it amounted to 19.4 percent of the total carbon fixed in the euphotic zone.