maximum concentrations of plankton usually oc- 

 curred between 9 and 22 meters- -2 to 25 times more 

 than at the surface, A few evening collections indi- 

 cated a reversal of this situation; after sunset, plank- 

 ton concentrations were greater at the surface than 

 at greater depths. 



The high concentrations of plankton among the 

 Apostle Islands, in the littoral current flowing east 

 along the northwest shore of the Keweenaw Peninsula, 

 and immediately east of the Peninsula indicate that 

 theproductivity of the area east of the Peninsula may 

 be dependent to a degree upon the nutrients moving 

 in this littoral current from the vicinity of the Apostle 

 Islands. 



The relatively high chemical content and plank- 

 ton concentrations along the soutli shore east of the 

 Keweenaw Peninsula are evidence of a littoral current 

 flowing easterly. Drift-card returns from this study, 

 as well as those of Ruschmeyer, Olson, and Bosch 

 (1957), suggest that a counterclockwise current usu- 

 ally exists in the eastern basin. Occasionally, open- 

 lake waters may be shifted onto the south shore by 

 appropriate wind action. When this movement occurs 

 a large mass of the littoral water must be carried in- 

 to the open lake by the counterclockwise current. 

 Movements of this type may account for the occur- 

 rence of a water mass with the characteristics of 

 littoral water (higher conductivity and plankton) in 

 midlake at station 78 during Cruise V. This littoral 

 current may continue on to the east shore of White- 

 fish Bay, since water with similar characteristics was 

 found along this shore. 



Drift-card studies 



In July 1953, 4,470 drift cards similar to those 

 developed by Olson (1951) were released in the vi- 

 cinity of the Keweenaw Peninsula and Isle Royale, 

 Michigan (table 8; figs. 7-12). By December 1,1957, 

 363 (8. 1 percent) of the cards had been returned; 320 

 (7. 2 percent) were returned in 1953. 



Nearly all cards that were returned showed some 

 evidence of exposure to moisture and a few were so 

 badly soaked that the cards had disintegrated 

 within the plastic envelopes. That water-filled 



envelopes do sink was demonstrated by several re- 

 coveries from the bottom in the clear water around 

 Isle Royale where cards were retrieved from depths 

 up to 10 feet. It must be assumed, therefore, that 

 many envelopes took on sufficient moisture to sink 

 in areas where they could not be recovered. 



A faulty seal on some envelopes contributed to 

 the penetration of water. When envelopes were gen- 

 tly agitated in a "squirrel cage" photographic print 

 washer for several days, some leakage occurred in 

 nearly all of them even though they showed no signs 

 of abrasion. Pinhole perforations around the seal 

 could be detected in envelopes that leaked badly. 

 The abrasion of envelopes reaching shore certainly 

 tended further to aggravate leakage. These factors 

 must have contributed markedly to the relatively low 

 recovery rate. 



Drift missiles floating at or near the surf ace must 

 be influenced predominately by the movement of 

 uppermost water. This layer is in turn strongly affected 

 by the wind, and a good correlation of water movement 

 with wind direction and velocity should be expected. 

 Plastic envelopes used in this study float on the sur- 

 face film when first released, but tend to drift in the 

 upper 1 or 2 inches of water when the plastic ceases 

 to shed water, or if moisture seeps into the envelope. 

 The tendency to float just below the surface film is 

 increased with increases in wind velocity and wave 

 action. Direct observations and experimental re- 

 leases of drift cards in 1954 (in Lake Michigan) have 

 demonstrated that, when floating on the surface film, 

 they can be moved rapidly on the surface film by 

 light breezes, and that they can cover considerable 

 distances in a short time under relatively calm con- 

 ditions. 



Wind conditions at Fort William, Ontario, on 

 the north shore of Lake Superior are given in figure 

 13. The wind track is plotted after the method used 

 by Fry (1956). 



The varied influence of winds upon cards in plas- 

 tic envelopes floating on, in or just below the sur- 

 face film, and under different sea conditions, un- 

 doubtedly was important in dispersing them. The 

 wind must also have affected their speed and direction 



13 



