MICHIGAN ACADEMY OF SCIENCE. 
123 
cc. at 15 meters with no change at (he 10 meter level resulted in the 
lower level having a slightly greater quantity of oxygen. The oxygen at 
15 meters fell to 0.3 cc. On the 30th there was no change in the surface 
content but that of the 5 meter level had increased 0.8 cc. bringing it 
up to about 5.3 cc. per liter. However, on the 1st of August the 5 
meter level showed again a little less than 4.5 cc. per liter; and the 
oxygen at 15 meters had become almost negligible and only traces of 
the gas could be found at the bottom after July 24th; most of the 
analyses showing no oxygen at all at the bottom. 
The oxygen in Lake Douglas has a summer distribution similiar to 
that found by Birge in Lake Mendota. The lake is deep enough at 
least in North and South Fish-Tail Bays for the formation of a well 
marked thermocline (see Fig. 3). Since the water below the thermo- 
cline is not in circulation so as to come in contact with the air it has 
no source of oxygen during the summer months unless brought in per- 
haps by springs. The analyses indicate so little oxygen below the 
thermocline as to make improbable the carrying of any large amount 
of oxygen to the lower strata in this manner. It is probable then that 
the oxygen secured by the lower waters at the time of the vernal over- 
turn and before the thermocline is well established constitutes its sum- 
mer supply. There is a constant removal of the oxygen due to the liv- 
ing organisms; at the same time oxygen is furnished the water through 
plants, but also oxygen is removed by the decomposition of organic 
matter. The vast number of organisms which sink to the bottom is 
according to Birge the most effective agency in depleting the lower 
waters of oxygen. This depletion is shown to occur progressively as 
the summer passes by the analyses and affects the strata below 12 meters 
to the greatest extent since that, depth marks the upper limit of the 
thermocline. Above the thermocline convection and wind currents tend 
to keep the water aereated to such an extent as to prevent anything like 
its complete depletion. 
The excess of oxygen which is shown by the fact that on some days 
and at some levels the water was supersaturated is undoubtedly due in 
large part to the activity of algae and other chlorophyll bearing plants. 
Such an excess is shown in the table for July 23rd. The excess of 
oxygen produced in this manner can only accumulate in calm weather 
and as there was usually quite strong winds blowing over the Lake 
as often as every twenty-four hours there was little opportunity for a 
large excess accumulation. The tendency for the water above the thermo- 
cline become supersaturated decreased so that after the 3rd week in 
July only the surface water was supersaturated. 
Observations were made on the water contained in shallow regions 
densely populated with weeds of various kinds and it was found that 
in these places the oxygen content was usually greater at a depth of 
1 to 2 meters and that the oxygen was present in greater quantities than 
at the same level in the deeper water. The analyses for July 24th in- 
dicate these facts. 
Analyses of water taken at 0 a. m. and at 4 :30 p. m. on the same 
day showed a slightly increased amount of oxygon at depths 6 or S 
meters in the samples taken at 4 :30 p. m. Sufficient data were not ac- 
quired on this point to give accurate quantitative results. 
