A comparative analysis of zooplankton field collection and sample enumeration methods

Understanding differences in zooplankton abundance depends on comparing similar data. Scientists employ numerous methods to collect and count freshwater zooplankton, potentially leading to difficulties when comparing studies. As scant information concerning specific biases exists, we quantified how (1) net mesh size and (2) counting techniques influence zooplankton size and abundance estimates. We compared mesh sizes by collecting zooplankton with four vertically hauled nets differing only in mesh size. We examined mesh-size effects on filtration efficiency across a suspended sediment gradient by comparing the known (quantified with a flowmeter) and assumed (from site depth) sample volumes. Finally, eight samples were counted with each of four methods to compare estimates of zooplankton abundance among techniques that differ in the minimum number of individuals counted. We found that only 64-mu m mesh nets effectively capture small-bodied zooplankton (small cladocerans, copepod nauplii, and rotifers, mean size = 0.13-0.30 mm) whereas all nets effectively collected larger-bodied zooplankton (mean size > 0.48 mm). All three > 100 mu m-mesh nets were > 90% efficient, regardless of suspended solids abundance. In contrast, the 64-mu m mesh net was at best 60% efficient; hence, actual volume sampled must be determined to avoid underestimation. We found counting-method-specific shortcomings, particularly for those in which few individuals were counted. We recommend, then, that a moderate number of individuals are counted for many finely resolved taxonomic categories or many individuals are counted for fewer, broader taxonomic categories. Consequently, to directly compare zooplankton densities between studies, scientists need a clear understanding of field collection and laboratory enumeration methods.