Natural Lake Water Temperatures in the Nearshore Waters of Southeastern Lake Michigan

Abstract

A five-year study of the lake-water temperatures of southeastern Lake Michigan at four locations ranging in depth from 2 to 40 feet and at a distance of 300 to 3375 feet from shore has produced the conclusion that natural lake-water temperature fluctuations greater than 3° F are a common event, except during the month of January. The greatest and most frequent daily temperature fluctuations occurred during the months of June through September, with the observed maximum daily range of 27° F occurring on 20 July 1970. Direct wind-induced upwelling of cold deep offshore waters is probably responsible for the natural daily fluctuations in excess of 12° F, while daily temperature fluctuations less than 12° F are probably the result of a combination of factors. The range between the maximum and minimum recorded monthly temperatures was substantial. However, the average maximum and minimum average temperatures at different depths did not fluctuate greatly from year to year. The deeper stations consistently showed higher maximum and minimum average temperatures for the months of January through March. The shallower stations consistently showed higher maximum and minimum monthly average temperatures during the months of June through September. The similarity between the monthly average maximum and minimum lake-water temperatures at stations of different depths is interpreted as the result of frequent mixing of the waters in this portion of southeastern Lake Michigan. The data indicate that the natural lake-water temperatures vary over a large range on both a daily and monthly basis. Because the existing aquatic community maintains itself within this naturally wide temperature range, it's questionable whether or not the introduction of thermal effluents will markedly influence the aquatic system in this part of Lake Michigan.