Abstract
ABSTRACT The populations of fingernail clams, family Sphaeriidae, of eight backwater lakes of the upper Mississippi River were compared over an 18-year period based upon 284 grab samples. Lakes ranged in size from 2 to 255 hectare surface area, mean depths from 0.2 to 0.9 meters, and mean water exchange times from 0.5 to almost 57 hours. The benthos of these lakes has been dominated by four taxa: Sphaeriidae, Hexagenia sp, Chironomidae, and Oligochaeta. Populations of Sphaeriidae during the mid-1970's showed substantial between-lake variations, but differences between years were not significant (p>0.05). By the 1989–91 period there had been a significant (p<0.05) overall decline in Sphaeriidae populations from these lakes. By the 1989–91 period the early summer populations of Sphaeriidae declined to only about 9 percent of their mid-1970 levels, which accounted for most of the 38 percent decline in total benthic population size. Anaerobic conditions near the bottom waters of one lake (Green Lake) could ac...
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