CONTRIBUTION OF ZOOPLANKTON TO THE TOTAL CADMIUM POOL IN CANADIAN SHIELD LAKES VARYING IN ACIDITY

Crustacean zooplankton > 250-mu-m in size contributed 0.002 to 11% (median of 1.3%) of the total Cd in the epilimnia of 39 Canadian Shield lakes that ranged in pH from 4.7 to 8.3. The contribution of zooplankton to the total Cd pool was consistently low (<< 1%) only in clear water lakes with pH < 5.0. In three intensively studied lakes, the contribution of zooplankton to total aqueous Cd varied with time and depth, the maximum contribution (4 to 6%) occurring in the metalimnion in mid-summer. Changes with depth and time were principally a product of seasonal changes in zooplankton biomass and seasonal succession of species with inherently different tendencies to accumulate Cd. Implications for the study of Cd cycling in lakes are discussed.