Intercomparison of five nets used for mesozooplankton sampling

Abstract

Intercomparison of nets commonly used for mesozooplankton sampling in the Black and Mediterranean seas was attempted within the SESAME (Southern European Seas: Assessing and Modelling Ecosystem Changes) project. Five nets were compared: three Juday nets equipped with 150 μm, 180 μm and 200 μm mesh size, a Nansen net (100 μm mesh size) and a WP2 (200 μm mesh size). Replicated samples were taken at one station in the western Black Sea offshore waters in April 2009. The samples were analyzed at species level (except for meroplankton), stages (for copepods) and body length was measured for all organisms. A decrease of total abundance values was observed with increasing mesh size, due to the significantly higher numbers of animals smaller than 1 mm in the samples obtained using fine mesh size rather than with coarser nets. Few comparisons were revealed significant as regards the abundance of 1-2 mm long animals, while no significance was detected for specimens larger than 2 mm. The above differences resulted in discripancies between nets as regards species and stage composition. Biomass values did not differ significantly between nets, due to the important contribution of the large animals (Calanus euxinus) to total biomass. The smallest and the largest animals revealed high variability between replicates collected using the Nansen, Juday- 200 μm and WP2 nets. Correction factors were calculated for the conversion of abundance values between each pair of nets. The differences observed between nets regarding abundance and biomass, community taxonomic composition and size structure, as well as the estimated correction factors, provide useful information for the harmonization of data obtained using the above nets in the Black Sea.