Species Composition of Limnetic Zooplankton Communities1

The species composition of limnetic zooplankton communities in 27 Colorado lakes has been determined on the basis of 148 vertical series of plankton‐trap samples. Each such series consisted of three or more evenly spaced samples taken from surface to bottom in the limnetic area. Comparable quantitative data for 42 selected lakes in other parts of the world are also considered, such samples having been taken with the plankton trap, pump, Clarke‐Bumpus sampler, closing net, etc. Momentarily, the great majority of Colorado limnetic communities contained one or two species of copepods, one to three species of cladocerans, and three to seven species of rotifers. Comparable data for other small to medium‐sized lakes of the world were one to three copepods, two to four cladocerans, and three to seven rotifers. From a species standpoint, therefore, these limnetic communities are remarkably simple. Depending on the complexities of cyclic and seasonal abundance, however, the individual species making up these communities change from one time of the year to another. The most common and widely distributed North American species are contained in the following : Cyclops, Mesocyclops, Diaptomus (Copepoda); Daphnia, Bosmina, Diaphanosoma, Ceriodaphnia, Chydorus (Cladocera); Keratella, Polyarthra, Filinia, Kellicottia, Conochilus, Asplanchna, Synchaeta (Rotatoria). It is unusual to find more than one species in the same genus in a limnetic community at the same time, at least when ordinary quantitative sampling methods are used. But when two species in the same genus do occur together, one is usually 20 or more times as abundant as the other. These and other phenomena are discussed with particular reference to the “niche” concept. In spite of the cyclic waxing and waning of populations of individual zooplankton species, the great majority of limnetic communities are characterized at any one time by one numerically dominant species of copepod, one dominant cladoceran, and one dominant rotifer. Thus, adequate sampling in “typical” lakes shows that, on the average, 80 per cent of all limnetic copepods belong to one species, 78 per cent of all cladocerans belong to one species, and 64 per cent of all rotifers to one species.