Zooplankton communities were characterized on the basis of samples taken in summer as vertical net hauls in the central part of lakes. Twenty-eight species of crustaceans were found in the 45 lakes studied. The highest number of species as well as the highest numbers of individuals (per unit of area) usually occurred in the largest deepest lakes with most transparent water.The most common species were: Bosmina longirostris, Tropocyclops prasinus mexicanus, Mesocyclops edax, Diaptomus minutus, Holopedium gibberum, and Cyclops bicuspidatus thomasi. Daphnids were rather scarcely represented.Senecella calanoides, Limnocalanus macrurus, and Diaptomus sicilis were confined to rather deeper lakes, whereas Diaptomus oregonensis was found mostly in smaller shallower lakes and Diaptomus leptopus in very small, but relatively deep lakes. Diaptomus minutus and C. b. thomasi, though widely distributed, showed a preference for deeper lakes, whereas T. p. mexicanus tended towards smaller shallower lakes.In particular lakes, the number of dominants ranged between 1 and 5, but three dominants per lake was the most commonly encountered case. The simplest community was composed of one cyclopoid, though the most common consisted of one cyclopoid, one diaptomid, and one cladoceran.Four types of communities were distinguished, each of them characteristic for a group of lakes of specific size and depth: (I) in the largest and deepest lakes (389–1007 ha of area, 30–117 m maximum depth); (II) in lakes of medium size and depth (6.5–200 ha, 4.8–33 m); (III) in small and very shallow lakes (3.1–28 ha, 2.7–7.3 m); (IV) in very small lakes with medium depth (1.3–3.7 ha, 6.1–12.8 m).The high degree of similarity between the plankton communities of the Experimental Lakes Area (ELA) and southern Ontario, as well as of some Great Lakes, suggest that they all basically belong to the same zoogeographical area.The fact that most species were distributed evenly throughout ELA may indicate the relative uniformity of the area. In ELA, lake morphology is mainly responsible for defining the type of community.
