Factors influencing distribution of aquatic macrophytes in a small northwestern Ontario lake

Abstract

In October, 1968, 5.0 Kilograms of wild rice (Zizania aquatica L.) seed were planted at 11 selected sites in Grassy Lake, in an attempt to introduce a self-sustaining crop. Some success was obtained at all sites, subject to the various effects of wind and wildfowl. As a sound understanding of the aquatic environment was necessary before further research could be carried out in connection with the wild rice, the temperature relations, of primary importance in all limnological investigations, were chosen as the subject of this study. Temperatures in air, water, and soils were measured by a series of 55 thermocouple units, mounted on 4 standards established in line across the shore zone. Sixteen sets of weekly and diurnal readings illustrate a pattern of uniform temperature in the vertical water column, and the lack of any evidence of seasonal thermal stratification in the water mass. Turbidity in varying degrees was present at all times, and with increased adsorption of solar radiation in the upper layers of the lake, mean water temperatures remained higher than mean air temperatures. The distribution of heat in the surface layers of the water was found to be relatively uniform, and temperature checks made throughout the depth of the lake supported the findings at the thermocouple station. Heat energy was found to be readily transferred to the bottom sediments, and mean temperatures for the first 20 centimetres of the substrate remained within a few centigrade degrees of the temperature of the vertical water column. Grassy Lake, situated near the southern tip of Sibley Peninsula, on the north shore of Lake Superior, is exposed to the force of the wind, and comparative anemometer readings give strong support to this fact. The wind is believed to be instrumental in the constant turnover and mixing of the lake waters, and to a considerable extent in governing the distribution of the aquatic macrophytes in the region of the shore.