FAUNA OF THE ALEUTIAN ISLANDS AND ALASKA PENINSULA 389 



formed: "The few aquatic plants, such as Potamogeton perfolia- 

 tum, Myriophyllum spicatum, Sparganium hyperboreum, and 

 Ranunculus tricophyUiis , Hippuris vulgaris and Isoetes Braunii 

 maritima, usually occur single or in patches." We found that the 

 dominant organism in plankton of type 2 pools is Chydorus 

 sphaericus. Amphipods are usually present in the weeds. 



Figure 16. — A fresh-water pool of type 3 (large and barren), about 0.2 x 1.5 

 miles. Semisopochnoi Island, August 23, 1938. 



Type 3: Large, barren lakes (fig. 16). Scant vegetation around 

 shore ; clean sand and rubble bottom ; windswept. The largest 

 examples are about 2 miles long. Only about 10 of the 75 islands 

 have lakes of this type. The temperature of the water in three 

 lakes at least 1 mile long was measured in August and was found 

 to be 56° F., 57° F., and 58° F. respectively. The plankton is 

 uniformly sparse; in fact, hauls made in the lake at Unalaska 

 Village in June and July were discarded for lack of a discernible 

 catch. Again, in a lake measuring 1 by 2 miles, on Unimak Island, 

 a haul was made in late August with negative results. 



The surface temperature of fresh-water bodies in the Aleutian 

 Islands fluctuates greatly from day to day because of the shallow- 

 ness of the water and the open surroundings (see table, p. 390). 

 The lowest temperature recorded was 44° F. on September 10; 



