Seasonal Distribution, Constitution, and Abundance of Zooplankton in Lake Erie

Zooplankton samples, collected by vertical hauls from near bottom to the surface, were obtained from 28 stations along the length of Lake Erie in October 1967. A partial similar series, confined largely to the Western Basin because of ice conditions, was collected in January 1968. The results of the analyses are tabulated as numbers of individuals per m3, and are discussed. Comparisons are made with results previously published for a comparable series of stations visited in July 1967.As in July, distinct differences were to be seen among the three major basins (Western, Central, and Eastern) in the October results. These differences are summarized. In July an expected decrease in zooplankton, such as would be predicted from lake morphology, amount of runoff from rivers, etc., occurred from west to east in the lake. In October, however, the zooplankton of the Western Basin was extremely impoverished, and the expected distribution of biomass was reversed. It is shown, on the other hand, from work done by others on samples taken from the same series of stations in October, that phytoplankton, chlorophyll a, and seston exhibited the expected distribution, emphasizing the danger of judging an ecosystem by the examination of only limited components of the community and at single periods of the year.In July the bulk of the zooplankters consisted of large cladocerans (especially daphnids) and copepods. In October there were much larger populations of rotifers and of small cladocerans (Bosmina, Chydorus). Copepods were relatively sparse. The January samples were characterized by greater numbers and a greater variety of diaptomids than in October, and the rotifers remained very important. All of the January samples contained many partially decayed microcrustaceans. This was interpreted as indicating a seasonal die-off associated with the very severe weather of the period.