The impact of artificial circulation on phytoplankton community in a freshwater eutrophic lake

The present research was carried out in Lake Nainital, Uttarakhand, India from 2006 to 2008 with the objective to examine the impacts of artificial mixing of the lake on phytoplankton community. Hypolimnetic aeration of the lake is being done with two compressors attached with thirty discs placed on sediments at different locations of the lake. The aeration was started in September, 2008 and the whole water mass of the lake circulated within 10 days. Initially all phytoplankton disappeared due to lake mixing but they appeared soon. The total phytoplankton standing crop during aeration was much lower than that of corresponding months of pre-aeration. Due to aeration the Cyanophyceae almost disappeared while the chlorophyceae reduced and Bacillariophyceae increased in number. Concentration of chlorophyll-a was also reduced due to aeration. Our observations suggested that high intensity aerators could inhibit the growth and development of algae especially Cyanophyceae in lakes undergoing restoration by aeration. The percent composition of chlorophyceae and that of Bacillariophyceae was increased due to lowered pH and release of competition from Cyanophyceae.