Assessing zooplankton advection in the Barents Sea using underway measurements and modelling A. EDVARDSEN 1, *, D. SLAGSTAD 2 , K. S. TANDE 1 AND P. JACCARD 3 1 Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway 2 SINTEF Civil and Environmental Engineering, Department of Coastal and Ocean Engineering, N-7934 Trondheim, Norway 3 Geophysical Institute, University of Bergen, N-5020 Bergen, Norway ABSTRACT The coastal shelf of northern Norway and the Barents Sea is highly advective and mainly affected by the North Atlantic Current and the Norwegian Coastal Current. The oceanographic conditions are an import- ant factor for the spatial and temporal distribution of zooplankton in the region. To quantify zooplankton advection over the western border of the Barents Sea, a Scanfish ⁄ Optical Plankton Counter (OPC), Acoustic Doppler Current Profiler (ADCP), Multiple Opening and Closing Nets and Environmental Sampling System (MOCNESS), and hydrodynamic model were used. This study provides data from two time windows (June and July 1998) by continuous measurements between northern Norway and Bear Island. The zooplankton community structure was obtained by net tows and zo- oplankton abundance fields were mapped by an OPC counting zooplankton in the size range 0.7–14 mm equivalent spherical diameter. A simple zooplankton community structure was found with the copepod Cal- anus finmarchicus (CIII–CV) as the dominant species in this size fraction. Ocean currents were measured by a ship-mounted ADCP and the residual currents were calculated by subtracting the tidal component obtained from a hydrodynamic model. Two measurements con- ducted in June and July 1998 shows a net eastward transport of water of 3.5 and 1.3 Sverdrup over the section. For the same two periods, zooplankton biomass transport is also positive towards east but varies by two orders of magnitude between the two measurements. Key words: Acoustic Doppler Current Profiler, advection, Barents Sea, Calanus finmarchicus, currents, hydrodynamic model, Optical Plankton Counter, temperature INTRODUCTION The coastal shelf of northern Norway and the Barents Sea is a highly productive area, with a food chain in which phytoplankton and zooplankton supports the recruitment of major fish stocks such as herring (Clu- pea harengus), capelin (Mallotus villosus) and cod (Gadus morhua) (Skjoldal and Rey, 1989; Gjøsæter, 1998). Studies have shown that the water properties (salinity and temperature) of the Barents Sea vary considerably between years (Loeng, 1991). The reason for this has been linked to both distant and local atmospheric forcing such as wind and air pressure (A ˚ dlandsvik, 1989), but could also be caused by incidents of local bottom water drainage (Midttun, 1985). This in turn alters the flow of Atlantic and Norwegian coastal water into the Barents Sea and hence the flux of salt and heat into the system. Periods of high inflow tend to increase the ambient tempera- ture while the temperature decreases during low inflow because of heat loss to the atmosphere. The question essential to the present investigation is, how do these fluctuations in the water advection affect fluctuations in zooplankton biomass? Skjoldal et al. (1992) and Skjoldal and Rey (1989) have outlined in detail the possible effects of inflowing water and transport of zooplankton to the Barents Sea. Their line of reasoning was that if periods of high inflow of water to the Barents Sea co-occur with the summer surface aggregation of zooplankton, there will be a potentially strong biomass transport into the Barents Sea. On the other hand, if strong inflow takes place during the winter, when the overwintering populations are found in deep waters of the Norwegian Sea (>500 m depth) or in deep basins on the shelf, the net zooplankton biomass transport will be negligible. In addition, the general contention is that intrusion of warm water into the Barents Sea favours an elevated zooplankton production because of faster growth and, hence, lower mortality. *Correspondence. e-mail: edvard@nfh.uit.no Received 5 January 2001 Revised version accepted 4 December 2001 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 12:2, 61–74, 2003 Ó 2003 Blackwell Publishing Ltd. 61