Sampling and filtration efficiency of two commonly used plankton nets. A comparative study of the Nansen net and the Unesco WP 2 net

The paper presents the results from a study in which two commonlyused plankton nets, the Nansen net and the WP 2 net, were compared.The study was performed at six stations in the Baltic Sea andthe samples were collected during seasons of both high and lowparticle abundance. The comparison included both qualitativeand quantitative filtration capacity of the nets under differentenvironmental conditions. Different mesh sizes were also tested.The results show that the Nansen net has an efficiency of 50–70%compared to the WP 2 net under favourable conditions. When conditionsare unfavourable, i.e. during periods of high particle abundanceor during long hauls, the efficiency may be as low as 25–30%.The mesh size of the nets proved to be of decisive importancefor the qualitative composition of the samples. Most of thesmaller size fractions such as rotifers, copepod nauplii andyoung copepodites passed through the 160 and 200 µm meshes.The 90 µm nets sampled these fractions well but with thedrawback of retaining vast amounts of phytoplankton at certaintimes of the year.