121 



T U A X Sr A R E'XC Y. 



The transparency of the water was determined by means of a Seechi's 

 disk, abont iifteen centimeters in diameter. The depth at which this disk 

 just disappeared from view varied from 2.1 meters as a minimum to 2.5 

 meters as a maximum. 



TEMPERATURES. 



The tliermophone and deep sea thermometer were not in Avorkiug 

 order so that the temperatures had to be tal^en bj^ means of a pump and 

 hose. Tliis metliod, of course, is subject to considerable error and the 

 results were found to be of comparatively little value except to determine 

 the location and extent of the thermocline, so that the distribution of the 

 plankton with regard to this region, might be studied. 



The temperature observations were made in the deepest part of the 

 lake, and they consisted of eight sets in July and ten sets in Aiigust. 

 During July there was very little wind so that the upper stratum of water 

 was not disturbed to any great extent. As a result this stratum accu- 

 mulated considerable heat dm-ing this period. The surface temperature, 

 taken at a depth of eight or ten centimeters, averaged 28.0° C. for the 

 eight sets of observations, with 31.2° C. as a maximum. 



During August, however, the wind was much stronger and the upper 

 stratum of water was much more thoroughly stirred up. As a result, the 

 average surface temperature for the ten sets of observations was 25.0° 

 with a maximum of 2G.0.° 



Tlie thermocline consisted of a stratum of water three meters in thick- 

 ness. The difference in temperatiire between the top of this stratum and 

 tlie bottom of it varied between 9.0° and 12.0°. In July it extended 

 from four meters to seven meters, and in August from five meters to eight 

 meters. The downward movement was doulitless due to the stronger 

 winds prevailing in August. 



The change in liottom temperature during the two months was very 

 slight, 7.5° being the minimum and 8.0° the maxinuun. 



METHODS. 



The plankton observations as well as the temperature observations 

 were made in the deepest part of the lake and by the pump method. An 

 ordinary pitcher pump, 1-inch garden hose, and a plankton net whose 



