An Artificially Induced Planktothrix rubescens Surface Bloom in a Small Kettle Lake in Southern Ontario Compared to Blooms World-wide

Abstract

ABSTRACT To combat hypolimnetic anoxia and sediment phosphorus release in a small, mesotrophic kettle lake on the Oakridge Moraine north of the Metropolitan Toronto, Southern Ontario, oxygenation and aeration was applied to the hypolimnion alternately during the summer of 1998 until mid-November and then to the entire water column until the end of December. This treatment coincided with the proliferation of a toxic strain of the purple cyanobacterium, Planktothrix rubescens, from almost undetectable values to bloom conditions under ice in the following winter and spring. Although small numbers of P. rubescens have been detected during several years before the treatment, prolonged artificial mixing in the fall and winter of 1998 distributed numerous filaments throughout the water column and to the surface when light was suitably low for these algae to survive and grow. Algae were supported by simultaneous entrainment and mixing of nutrients from the enriched bottom water. Such blooms of P. rubescens and related bluegreens have been found in many lakes with comparable characteristics and during similar episodes like those of the study lake. Lakes were typically stratified, mesotrophic hardwater lakes, with phosphorus levels that have recently been increasing to levels above 20 μg L−1. Blooms occurred during periods of low light and enhanced mixing, in several cases after treating the lake with whole-lake aeration and mixing. Recommendations to prevent such blooms in Lake Wilcox are (1) the discontinuation of artificial mixing during periods of natural destratification in the fall and winter, (2) the prevention of further eutrophication, and (3) the installation of an in-lake treatment, such as hypolimnetic withdrawal, to decrease internal phosphorus loading from anoxic sediment surfaces.