Comparison of nets and pump sampling gears to assess zooplankton vertical distribution in stratified lakes

Abstract

The efficiency of a cantilevered bridle net was tested in comparison with a Wisconsin net and a pumping system to sample zooplankton organisms in three water layers (epi-, meta- and hypolimnion) of three Canadian Shield lakes. Variations among samplers were compared to variations due to within-lake vertical distribution of zooplankters and among lake variations. For each lake and water layer, we also assessed the efficiency of the three methods according to the catches of zooplanktonic taxa. The highest percentages of variation were generally due to lake or water-layer effects; interaction between sampling gears and water layers was above 50% for most taxa, except cladoceran. Sampling methods explained more variation than the lake effect for some zooplankton taxa, indicating that using different sampling devices could potentially alter the among-lake variation interpretation of zooplankton abundance. The pumping system captured higher densities of animals per taxa than the cantilever and the Wisconsin nets. The cantilever net generally captured mobile taxa more efficiently (Polyarthra vulgaris, copepods, Daphnia sp., Diaphanosoma brachyurum and chaoborids) than the Wisconsin net and the pumping system, but its efficiency varied among water layers.