Data on zooplankton abundance and biovolume were collected in concert with data on the biophysical environment at 16 stations along a transect north of Svalbard, crossing the Atlantic Water inflow into the Arctic Ocean, in January 2014. The data were sampled along vertical profiles by a Laser Optical Plankton Counter (LOPC; ODIM-Brooke Ocean Rolls Royce Canada Ltd., Herman et al. 2004) and a Conductivity-Temperature-Depth Sensor (CTD, Seabird 19plusV2, Seabird Electronics Inc., USA), both mounted on a sturdy rosette frame. Sampling depth was from surface to ca. 10 m above the bottom, but restricted to 1000 m at maximum, only down profiles are used. Based on the LOPC data, abundance (individuals/m3) and biovolume (mm3/m**3) were calculated as described in the LOPC Software Operation Manual [(Anonymous, 2006), http://www.brooke-ocean.com/index.html]. LOPC data were regrouped into 49 size groups of equal log10(body volume) increments, see Edvardsen et al. (2002, doi:10.3354/meps227205). LOPC data quality was checked as described in Basedow et al. (2013, doi:10.1016/j.pocean.2012.10.005). CTD data were screened for erroneous (out of range) values and then averaged to the same frequency as the LOPC data (2 Hz). All data were processed using especially developed scripts in the python programming language. The LOPC is an optical instrument designed to count and measure particles (0.1 to 30 mm equivalent spherical diameter) in the water column, see Herman et al., (2004, doi:10.1093/plankt/fbh095). The size of particles as equivalent spherical diameter (ESD) was computed as described in the manual (Anonymous, 2006), and in more detail in Checkley et al. (2008, doi:10.4319/lo.2008.53.5_part_2.2123) and Gaardsted et al. (2010, doi:10.1111/j.1365-2419.2010.00558.x).

Two data files area available: Abundance and Biovolume. Both files have the same file structure. Data are saved as tab-separated ascii files (.txt) with 57 columns. The first 8 columns contain the station number, an id number, date/time (ISO 8601), latitude (degree N, decimals), longitude (degree E, decimals) and hydrographical data (depth (m), temperature (degree Celsius), salinity). The following 49 columns contain abundance (individuals/m3) or biovolume (mm3/m3), respectively, within the 49 size groups. Size limits (in mm ESD) and biovolume limits (in mm*3) of the 49 classes are given. The first value gives the minimum size of the first size group (0.100), the second value the minimum size of the next size group (0.112), so that all particles in the first size group have an ESD >= 0.100 mm and < 0.112 mm.