specific data. The cruises were made in 

 1955. Lauff (1957) published surface and 

 subsurface temperatures obtained at 30 sta- 

 tions in Grand Traverse Bay on July 30 and 

 31, 1954. 



Ayers et al. (1958) presented in 

 detail the limnological data (transparen- 

 cies, temperatures, chemicals, currents) 

 collected at 174 Lake Michigan stations by 

 the synoptic survey method (multiple-boat 

 single-date coverage of entire lake). The 

 4 synoptic cruises covered 8 pair of round 

 trips across the lake on June 28 and 29 and 

 7 pair on August 9 and 10, 1955. The au- 

 thors discussed the results of each synoptic 

 cruise. Two hundred and forty-two (242) 

 temperature soundings were taken from a 

 bathythermograph and checked by a reversing 

 thermometer. The discussion usually covered 

 the range, average, maximum, and minimum of 

 the surface temperatures of the major sec- 

 tions and tlie isotherms that were also mapped 

 throughout the lake. Vertical temperatures 

 were also shown on charts. The text empha- 

 sized the details of the thermoclines (sharp 

 thermal gradients, discontinuity layers) but 

 also contained discussions of the isotherms 

 of the epilimnion and hypolimnion and the 

 temperatures of the bottom. 



STATIONS OFF 

 ST. JOSEPH-SOUTH HAVEN 



Thirteen (13) series of temperatures 

 were taken off St. Joseph and South Haven 

 in 1930 and 1931 (table 2). Depths of the 

 3 designated shallow stations ranged from 

 52 to 62 meters (171 to 203 ft.) and of the 

 2 deep stations from 85 to 94 meters (279 

 to 308 ft.) and averaged 55 and 90 meters 

 (180 and 295 ft.) respectively. 



In 1931 the surface temperatures of 

 both deep and shallow waters increased 

 progressively from about 5° to 6° (41° to 43° 

 F.) on May 12 to 23" (73° F.) on July 1. 

 The 1930 records were 20° and 20.9° (68° 

 and 69.6° F.) in August and 13° (55° F.) in 

 October. Although the deep-station surface 

 temperatures were comparatively low until 

 June 9, they later rose rapidly and finally 

 exceeded the shallow-water surface tempera- 

 ture. 



The subsurface temperatures of both 

 years followed at various depths the trend 

 of the surface temperatures. They had in- 



creased at all levels on May 27, 1931, but 

 thereafter only down to a maximum of 30 

 meters (98 ft.), although on June 18 and 

 July 1, 1931, and August 19, 1930, upwell- 

 ings had brought cold water to upper levels 

 as high as 10 to 15 meters (33 to 49 ft.) 

 but only at the shallow stations. When the 

 temperatures of the upper levels changed, 

 those below shifted in the opposite direc- 

 tion, remained constant, or fluctuated at 

 different depths. The subsurface waters 

 warmed most rapidly at the shallow stations, 

 and on the same dates their temperatures 

 were almost always higher than those of the 

 corresponding levels of the deep areas. 



Five-degree (41° F.) water was con- 

 fined almost entirely to depths of 40-45 

 meters (131-148 ft.) during May-August, 

 except at certain deep stations when it oc- 

 curred above 5 meters (16 ft.) on May 12. 

 and below 60 meters (197 ft.) on October 23. 

 In 1931 the bottom temperature rose slightly 

 in the shallow waters from 4.3° to 4.7° 

 (39.7° to 40.5° F.) in May and then remained 

 practically constant through July 1. In 

 August 1930 it increased from 4.7° on the 

 7th to 5.3° on the 19th (40.5° to 41.5° F.). 

 In the deep waters the bottom temperatures 

 of both years ranged only from 4.0° to 4.2° 

 (39.2° to 39.6° F.). 



The earliest thermocline was observed 

 in the shallow water on June 9, 1931, at 

 depths of 30-33 meters (98-108 ft.), but 

 none had formed at the deep station. By 

 June 18 the cold water had risen and wiped 

 out this thermocline. On July 1 a thermo- 

 cline was again recorded but this time at 

 both deep and shallow stations at approxi- 

 mately the same level cind temperatures. An 

 upwelling had also shifted a thermocline to 

 shallower water in August 1930. Because of 

 such movements the subsurface temperatures 

 above the bottom did not always increase at 

 all depths with the advance of the season. 

 In the fall the temperatures begin to ap- 

 proach uniformity at all depths. This fact 

 was evidenced by the October 23 records 

 when the temperatures were virtually uniform 

 down to at least 45 meters (148 ft.), and a 

 thermocline had formed between 50 and 55 

 meters (164 and 180 ft.). 



The depth intervals covered by the 

 thermoclines ranged from 3 to 6 meters (10 

 to 20 ft.) and averaged 4 meters (13 ft.). 

 Their thermal gradients varied from 5° to 

 9° (9° to 16° F.) and averaged 7° (12° F.). 



