Biological Survey — Erie-Xiagaka Watershed 53 



4tli cruise sliowed eouclusively tliat this was the ease. For at 

 this time all trace of the cold bottom water at these eastern sta- 

 tions had disappeared. 



To return to the third cruise : At stations outside of the 

 Buffalo region and Long Point bay the creeping in of this cold 

 bottom water had set up strong density gradients and consequently 

 retarded mixing. Nevertheless it was fractionally warmer at the 

 bottom at stations 14 and 17 than it had been two weeks earlier. 

 At station 17 the existence of the surprising thermocline was again 

 noted. This time the discontinuity layer lay between 17 and 18 

 meters instead of between 11 and 15 meters as earlier. The top 

 15 meters of water were quite thoroughly mixed. The temperature 

 at 15 m. was 20.5° ; at 17 m. it Avas 12.9° ; and at 18 m. only 6.4°. 

 The finding of this phenomenon again removed any doubt as to the 

 accuracy of the previous readings. 



It is interesting to note that while the surface density gradient 

 had been reversed over what its direction was on the first cruise, 

 the forces which were holding the water layers inclined at so sharp 

 an angle to the horizontal on the south side of Long Point had 

 changed apparently neither in magnitude nor in direction. Water 

 of low temperature was still found at station 17 many meters 

 higher than was water of the same temperature at the adjacent 

 station 15. 



Cruise 4 Avhicli took ]:)lace on Augnst 22, 23, and 24 was a fishing 

 trip and temperatures were taken only at stations 1 to 9 in order 

 to check up on the movement of the bottom water. 



The fifth cruise began on August 28 and lasted, with inter- 

 ruptions due to strong winds, until September 3. The mean sur- 

 face temperature was 21.6° at the 19 stations which were occupied 

 and tlie individual divergences from this mean so small that no 

 chart has been prepared. 



The surface had cooled slightly since the third cruise and was 

 now about the temperature of a month earlier. Nor was the maxi- 

 mum temperature, 22.8° at station 11, as high as the maximum of 

 cruise 3. Evidently the peak of the summer warming was reached 

 some time between the 15th of August and the 1st of September 

 and with the increasing declination of the sun and the greater in- 

 tensity and frequency of strong winds which now^ began, surface 

 cooling went on faster and faster. Already water in mistable equi- 

 librium had appeared. On the morning of September 1 which was 

 raw and cold an inverted temperature gradient was observed at 

 station 13 extending from top to bottom and at station 14 between 

 the top and 20 meters. Convectional overturning must have been 

 going on rapidly at these stations. 



The upper 20 meters was now thoroughly mixed and at station 

 15 the same reading 21.3° was obtained at 0, 10, and 20 metres. 

 Just below 20 meters, liowever, a pronounced thermocline was 

 evident in all stations having depths exceeding this figure and 

 while time would not permit of more detailed investigation at sta- 

 tion 17, evidences of the former concentrated discontinuity layer — 

 somewhat attenuated — were found even though the water at 18 

 meters was several degrees warmer than it was two weeks before. 



