During the past two years this Survey has made chemical and biological investigations 011 a number of lakes in Wisconsin. About 90 lakes have been studied, representing the chief h k e clist,ricts of the state. About 60 have been visited only once a,nd this T V ~S during the month of August, 1907. O n i t few hkes observations have been made regularly dnring all seitsons of the two yea,rs; and more than 20 have been studied from t,he t.ime of the spring over- t,urii of the water till t,he fall overturn, thus making a complete series of observations for the summer, which is t.he season charttrterized by the greatest ant! most mpid changes both chemical a i d biological.
These lakes vary very widely in size, the smallest being only good sized ponds, while the largest has an area of about 39 square kilometers. The depth, also, varies from 3 to 4 meters in the shallowest to 72 meters in the deepest. About 60 of the lakes lie in the noi.thenstern part of the state, 30 in the southea'stern part and a t,hird a,nd smaller group toward t.he northwestern part. The water of t,he southeastern lakes, with one exvept,ion, contains muc,h calcium and magnesium in solution (50 t,o 100 or inore parts per million) and the water is therefore ,,hard". The lakes of the northeastern region contains only a littlc cal-&in and magnesium, perhaps avera,ging one-tenth of the amount in the southeastern lakes. Their water is therefore ,soft". The water of the third g o u p of lakes, with one exception, contains an intermediate amount of calcium and inagnesiuin.
In those lakes which have a. (1ept.h of 8 meters or inore, three distinct temperature regions exist for a longer or shorter period during the summer, generally 4 months or inore. There is amn upper st,ratuin which is wttnned by the sun a.nd is kept in circulation by the wind so that it has a rather uniform temperature. This is the warmest part of the lake a,iid in the vario-us lakes it ranges in thickness from 4 or 5 m. to about 12 in. depending upon the size of the lake and its protection from winds. There is cz lower or bottom stratum of vool-water whose temperature changes very little during the summer. The thickness of this stratum depends largely, of course, upon the depth of the lake, and its temperature depends on the mea and the depth. Between these two regions there is a transition st,ratuni in which the temperature changes rapidly fTom that of the warm water above to that of the cool water below. Thisstratum is known as the thermocline or ,,Spiiingschicht", and is generally from