A submersible net-pump for quantitative zooplankton sampling; comparison with conventional net sampling

A new sampling system for quantitative zooplankton studies is described. It consists of a centrifugal pump and a plankton net combined in a single submersible unit. The performance of the net-pump was compared to vertical hauls with a 100-µm WP-2 net on two occasions characterized by large differences in zooplankton biomass. The filtration efficiency of the WP-2 net decreased steadily, due to clogging, from ca. 100% at the onset of sampling (25 m) to about 50% at the surface. Clogging in the net-pump was not observed even after prolonged sampling. In vertical hauls, the capacity of the net-pump (420 1 · min-1) was sufficient to catch even adult Calanus finmarckicus quantitatively. Used in the stationary mode at fixed depths, most adult C. finmar chicus escaped from the pump intake. The volume filtered by the net-pump was only a fraction of the volume filtered by the WP-2 net. However, the total variation (variation among samples + subsample error) was identical (May) or markedly lower (October) f...