Underestimation of microzooplankton is a macro problem: One size fits all zooplankton sampling needs alterations

Microzooplankton (rotifers, copepod nauplii, and dreissenid veligers) are an important but overlooked part of zooplankton communities and aquatic food webs, particularly in the Great Lakes. Most studies that do include microzooplankton data are not describing the full picture due to inappropriate sampling methodology. We compared the traditional macrozooplankton sampling method (64-μm mesh plankton net) to a microzooplankton method using a 20-μm mesh screen in various habitats in Lake Michigan. The macrozooplankton method significantly underestimated total rotifer density by an order of magnitude, veliger density by nearly an order of magnitude, and copepod nauplii density by threefold. Combining macrozooplankton method estimates for cladocerans and copepods with estimates of rotifer, nauplii, and veligers from the microzooplankton method samples showed rotifers contributed 51% of total mean zooplankton biomass, refuting the past notion that rotifers contribute little to overall zooplankton biomass. Our study demonstrates that the traditional one-size fits all sampling approach used in the majority of zooplankton monitoring studies in the Great Lakes significantly underestimates microzooplankton abundance and its relative importance. Biassed information on Great Lakes zooplankton community composition has ramifications beyond a basic understanding of Great Lakes food webs. The lack of accurate data on microzooplankton abundance suggests that prey resources available to Asian carp in Lake Michigan have been greatly underestimated along with the likelihood these invasive species could become established. The dual sampling approach must become the norm rather than the exception for zooplankton research in the Great Lakes and other freshwater systems.