Zooplankton samples, usually together with morphometric, physical, and chemical data, were collected from 696 lakes in glaciated eastern North America between 1969 and 1978. Geographical distributions of the 44 Crustacea and 4 Diptera species suggest six broad categories: 23 species appear to have no barriers to dispersal throughout the area; 7 have restricted ranges, possibly because of either recent evolution or competitive exclusion; 3 may be gradually invading the area from centres to the south or west; 1 is a euryhaline estuarine form with a limited freshwater distribution; 4, possibly 5, are found almost exclusively in areas of former glacial lakes or spillways and may be classified as "glacial opportunists"; 9 species have distributions which appear to be linked in some unspecified way to calcium–magnesium water hardness.

The zooplankton community structure of 696 lakes in Pleistocene glaciated eastern North America indicated to us that glacial influence was the primary determinant of community structure. Using this criterion we successfully discriminated 82% of the lakes between glacial lakes, to which Diaptomus sicilis, Limnocalanus macrurus, Mysis relicta, and Senecella calanoides were restricted, and nonglacial lakes, to which Cyclops scutifer, Eubosmina longispina, Daphnia pulex, D. catawba, and other species were characteristic. A third large group of common species was distributed throughout the study area with no particular relation to glacial lake influence. No clear patterns in community structure with respect to size of zooplankters were discerned. Although some species distributions or abundances were consistent with the well-known predatory and competitive interactions among zooplankters typical of small lakes and ponds, for example, the negative correlation between the predatory Mysis relicta and Eubosmina longispina, most were not. We suggest that this arose partly because the study area is large and heterogeneous and partly because the biological interactions observed in small lakes and ponds may be less intense in the more physically complex environment of large lakes.Key words: zooplankton, community structure, glacial opportunists, multivariate analysis, zoogeography, size-efficiency hypothesis

Considerable variation in cephalothorax length of adult female Diaptomus minutus and D. oregonensis occurs among lakes of glaciated eastern North America during spring and early summer. Moderately significant Spearman rank correlations existed between D. minutus length, and Secchi transparency depth (negative) and true water color (positive). Similar significant correlations were not found for D. oregonensis. In these lakes water transparency is largely a negative function of color derived from humic substances. It is hypothesized that waters of low transparency provide a measure of protection to D. minutus against visually feeding fish predators. It can thus undergo a modest gain in size, thereby reducing vulnerability to invertebrate predation, increasing potential fecundity and (possibly) allowing it to feed on larger food particles.

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