Some ecological effects of artificial circulation on a small eutrophic lake with particular emphasis on phytoplankton

A small eutrophic New Hampshire lake was artificially circulated from July 16 to September 12, 1968. Artificial circulation destratified Kezar Lake completely; the stability of stratification was reduced to zero when the lake became isothermous. Mixing caused an increase in the heat budget. Water transparency also increased after mixing.Inverse clinograde distributions of Fe, Mn, ammonia-N, CO₂, alkalinity and conductivity were ameliorated after mixing by reoxygenation of stagnant bottom water. The chemical nutrients Ca, Mg, K, Cl, and SiO₂ were little influenced, but a marked increase in total-P occurred when artificial circulation transferred suspended organic detritus into the water column from agitated profundal muds. The effects of mixing on Na, Cu, Zn, NO₂-N, NO₃- N, organic-N and orthophospate are also discussed. Most chemical nutrients were distributed isometrically in the water column after mixing. The supply of chemical nutrients was sufficient to support large populations of phytoplankton.During stagnation a dense bloom ofAphanizomenon flos-aquae occurred. Mixing caused a uniform vertical distribution of this alga and its large population eventually dissipated. The phytoplankton then became dominated by chlorophycean taxa. The variations in chlorophyll-a followed closely changes in phytoplankton abundance. Chlorophyll-a levels are shown to be typical of other eutrophic lakes. Primary production in surface waters decreased markedly subsequent to destratification, but it increased at lower depths in agreement with vertical expansion of the euphotic zone.