388 NORTH AMERICAN FAUNA 61, FISH AND WILDLIFE SERVICE 



or, if present, consisting- of sparse patches of emergent Hippuris 

 and Car ex; bottom consisting of clean volcanic sand or occasional 

 silt. This type is by far the most abundant. Many of the clean 

 pools do not support plankton because they overflow and are 

 flushed out by each rain. The water in type 1 pools usually is 

 slightly tea-colored, like that of sphagnum bog pools on the main- 

 land. We discovered that it was possible to determine whether a 

 pool contained enough plankton to warrant taking a haul by 

 looking for aquatic bugs and beetles. A pool highly productive 

 of plankton is generally well populated with aquatic insects. The 

 clear pools are characterized by the presence of Diaptomus, often 

 in such numbers that a tow over a course of 300 feet may net a 

 haul of 50 cubic centimeters of these red-bodied crustaceans. One 

 such haul on Sanak Island consisted almost entirely of Diaptomus 

 shoshone var. wardi, D. ashlandi, and D. eiseni. Cyclops serrulatus 

 is also commonly present in type 1 pools. 



Figure 15. — A fresh-water pool of type 2 (small and weedy). Atka Island, 



August 13, 1937. 



Type 2: Small, weedy pools (fig. 15). Shallow depressions in 

 the tundra, 50-100 feet in diameter, with oozy silt bottoms. This 

 type is not common. Hulten (1937) concludes that the vegetation 

 in Aleutian lakes is so sparse that real associations are hardly 



