where _R j is the measured resistance, c is the tem- 

 perature coefficient of conductance at t_, and At_ the 

 difference in temperature between 18* C. and tem- 

 perature of test measurement. 



The determinations for calcium, magnesium, 

 nitrogen, sodium, phosphorus, and silica content 

 show that not only are the concentrations of the var- 

 ious chemicals very low, but the variation in con- 

 centration is slight with time as well as from one 

 area to another. In fact, the ratio of calcium, mag- 

 nesium, and sodium (12:3:1) varied little among 

 seasons and areas of the lake. Few freshwater lakes 

 have this consistency in chemical content. Con- 

 centrations of calcium were so uniform from area to 

 area during most of the year, except spring, that the 

 analyses for this element were of little value for 

 identifying water masses. The same was true in 

 some degree for elements other than phosphorus and 

 specific conductance. Consequently, it was necessary 

 to consider several measurements as well as other 

 factors in identifying water masses. 



Concentrations of all chemicals as well as 

 plankton were usually low in the central open-lake 

 area. Waters along the north shore from Isle Royale 

 to Duluth had similar characteristics. This similarity 

 may have been due to a counterclockwise cunent 

 that carried open- lake water into the western end of 

 the lake (Ruschmeyer, Olson, and Bosch 1957). The 

 phosphorous content of the water increased slightly, 

 butsignificantly after it had flowed past Duluth. These 

 slightly higher phosphorous concentrations occurred in 

 all of the water samples collected along the south 

 shore from Duluth to the Apostle Islands. 



The dry weight of plankton per cubic meter in 

 the Apostle Island region was 4 to 5 times greater 

 than at stations within the western arm of the lake. 

 Perhaps the increase in phosphorous, as well as other 

 nutrients from wastes discharged into the lake near 

 Duluth, was responsible for this greater abundance 

 of plankton. 



Plankton data 



All of the 1952 and most of the 1953 plankton 

 collections were made by Clarke-Bumpus samplers 



with No. 10 mesh (aperture 0. 158 mm) nets. A few 

 samples were taken in 1953 with No. 20 mesh (aper- 

 ture 0.076 mm) nets. Horizontal tows usually were 

 made at the surface, although a few were at greater 

 depths. The Clarke-Bumpus samplers were calibrated 

 by towing them a known distance. A few vertical 

 tows were made with a 1/2-meter, No. 20 mesh net 

 in 1953. 



The wet volume, and dry, ash, and organic 

 weight were determined for plankton samples col- 

 lected in 1953. Wet volume was determined by meas- 

 uring the total volume of the sample and subsequently 

 allowing the plankton in a 10 ml subsample to settle 

 out in a graduated centrifuge tube. The dry, ash, and 

 organic weights of 10 ml portions were obtained by 

 the method described by Welch (1948). The data were 

 multiplied by a factor, determined by the flow of 

 water through the nets, so that all measurements are 

 recorded in milliliters or milligrams per cubic meter 

 of water. 



These data have certain limitations that restrict 

 interpretation. Surface tows alone are unreliable 

 for a study of seasonal abundance, and sampling at 

 any one station was not extensive enough to allow 

 suitable comparison of one area with another. Never- 

 theless, certain conclusions can be drawn which may 

 serve as a guide to future studies in Lake Superior. 



If the standing crop of plankton can be considered 

 as indicative of productivity, it may be concluded that 

 the area southeast of the Keweenaw Peninsula and east 

 to Grand Marais, Michigan, is 2 to 3 times more pro- 

 ductive than the open-lake area northeast of the 

 Peninsula. Other areas of comparable productivity 

 are the waters of the Apostle Islands and the littoral 

 zone along the northwest shore of the Keweenaw 

 Peninsula. 



The dry-weight data for the area southeast of the 

 Keweenaw Peninsula indicate two plankton maxima 

 in this region during 1953, one in early summer and 

 another in early fall. The smallest concentrations 

 occurred in May. 



Samples collected at the surface and from various 

 depths at 8 stations showed that during daylight the 



12 



