Filtration performance of plankton nets used to catch micro- and mesozooplankton

Zooplankton is of great significance in waters. It acts as an indicator for water quality, links the lower trophic levels like algae in the food chain with higher trophic levels like fish and plays an important, not yet fully known, part in the cycling of biogeochemical key elements. Abundance estimates of zooplankton build the base for the associated fields of research. The most widely used devices to take zooplankton samples to form abundance estimates are up to now Plankton nets. Nets of various sizes and designs are in use which do have different sampling characteristics and in consequence, result in different abundance estimates of zooplankton. To document the different sampling characteristics of nine commonly used plankton nets the hydrodynamics of the catching devices were numerically investigated with computational fluid dynamic (CFD) methods, more precisely „Reynolds averaged Navier-Stokes“ (RANS) methods. The investigation included the 1 m² MOCNESS, CalCOFI 1-m Ring net, Multinet Midi, Bongo net (Ø =60 cm), WP-2 net and the Bongo net (Ø =20 cm) to sample the smaller mesozooplankton and 3 different Apstein nets to sample microzooplankton. Vertical plankton net tows were simulated to determine the filtration efficiency (dimensionless coefficient for the volume flow rate into the net), the filtration pressure (pressure difference between the inside and the outside of the netting) and the pressure distribution in front of the nets. The towing velocity ranged from 0.5 up to 2.3 kn for the nets to catch the smaller mesozooplankton. For the microzooplankton nets the towing velocity was in the range of 0.07 to 0.66 m/s. The findings were combined in technical characteristic sheets available for each investigated net.