NATIONAL OCEANOGRAPHIC PROGRAM—1965 413 
Some degree of thermal stratification occurs in all of the lakes 
during the warmer months. They are almost homothermous throughout most of 
the winter and spring The effect of deep mixing is apparent in Lake 
Superior where awoiunSIaneS water around 2° C. occurs to depths of LOO 
fathoms. The lakes differ chiefly in the rate at which they exchange heat 
and in the stability of thermal stratification. The deep waters of all 
the lakes probably remain close to 4° C. throughout the year. Well de- 
fined and persistent thermal stratification is usually established in Lake 
Erie by mid-June and Lakes Huron, Michigan, and Ontario by the end of 
June. The epilimnion extends to about 8 fathoms in Lakes Huron, Michigan, 
and Ontario, and to 11 fathoms in Lake Erie during the period of maximum 
stratification in August. Lake Superior usually does not exhibit any well 
defined stratification until mid-July. The lower limit of the epilimnion 
is around 8 fathoms. Thermal stratification is not uniform from area to 
area and may be transitory. The metalimnion is usually poorly developed. 
Solid ice sheets in winter are generally limited to sheltered areas, 
embayments, channels and along shore. Off-shore ice usually occurs as 
floes. Jakes Erie, Ontario, and Superior may have 80 to 95 percent of 
their surfaces ice covered in severe winters. Lakes Huron and Michigan 
normally remain open though shore ice may extend lakeward 8-10 miles. In 
eect severe winters all of the lakes may be fully ice covered 
1962-63). 
The Great Lakes consist of bicarbonate waters. Total alkulinity 
ranges from 46 parts per million (ppm Caco- ) in Lake Superior to 113 ppm 
in Lake Michigan. ‘The pH ranges from 8.0 to 8.5 for most of the waters, 
except Lake Superior (pH 7.4). Sulfate concentrations are greater than 
chlorides in the lakes above Lake Erie and they are almost equal in Lakes 
Erie and Ontario. The proportions of calcium, magnesium, and sodium in 
the upper lakes are about 10:3:1. Potassium usually averages around 1 ppm 
in all the lakes. Silica fluctuates around 2-3 ppm, although only trace 
amounts occur at times in the highly productive waters of Lake Erie. Con- 
centrations of total phosphorus range from less than 5 parts per billion 
in Lake Superior to 13 ppb in the other lakes except Erie. The phosphorus 
content in the latter is about six times greater. The dissolved-oxygen 
content of most of the lake waters is near saturation, even at the great- 
est depths, and supersaturation is conmon. Dissolved-oxygen concentrations 
of less than 1 ppm have been found in Lake Erie. These low values have 
been detected in the bottom waters in an areca of more than 1,000 square 
miles of the central basin. 
Our knowledge of the currents of the lakes have increased considerably 
within the past few years. . Nevertheless, current pattern determination 
remains fragmentary. Most studies describe surface currents but none of 
them have extended over long snough periods to support conclusions regard-. 
ing seasonal patterns. 
Several methods have been developed to study currents and water 
masses of the lakes. Drift missiles have been used for many years, 
