MicroNESS: an innovative opening–closing multinet for under pack-ice zooplankton sampling

Quantitative sampling of zooplankton communities from ice-covered waters presents many technical and logistic difficulties. Currently available techniques enable only vertical tows through relatively small ice holes, and the filtered volumes are generally low. For these reasons, we developed and tested Micro-Net Environmental Sampling System (MicroNESS), an innovative small-sized opening–closing multinet device carried by a ROV for horizontal and oblique sampling of the under-ice zooplankton community. MicroNESS design ensures a filtration efficiency close to 90 % due to the relevant filtration open area ratio (~4) and exhibited only limited effects of clogging and avoidance for optimal speed of 0.35 ms−1 and towing duration not longer than 14 min with mean filtered volumes in the order of 11 m3. MicroNESS was successfully tested in Terra Nova Bay (Antarctica). A total of 96 tows were carried out to sample the zooplankton in the waters below the 2.6-m-thick pack ice, far from the ice hole, at selected depths (3.5, 4.5, 5.0, 6.0, 7.0, 10.0 and 20.0 m). Overall, more than 70,000 specimens were identified, mostly copepod taxa, which showed a size range from the small Stephos longipes nauplius stage I (0.11 mm) to the large adult females of Calanoides acutus (4.8 mm). The comparison among the ice-associated copepod communities collected by MicroNESS and those of three traditional sampling techniques showed that MicroNESS can provide useful insights about changes in composition, biomass and particle size structure of this key plankton component, especially on a short timescale during ice melting.