SUMMER DISTRIBUTION PATTERNS AND BIOMASS ESTIMATES OF MACROZOOPLANKTON AND MICRONEKTON IN THE NORDIC SEAS

The vertical and horizontal distribution patterns of zooplankton and m icronekton were mapped during three research cruises in summers 1993, 1994 and 1995 by pelagic trawl and MOCNESS plankton net sampling. The distribution patterns have been related to the distribution of water m asses and the distribution of planktivorous fish such as herring, Clup ea harengus. Zooplankton biomass typically revealed a bimodal vertical distribution with high values in the surface layer and at 200-600 m d epth. This subsurface maximum contained, among others, several species of macrozooplankton and micronekton such as krill, pelagic shrimps an d mesopelagic fish. The dominant krill species Thysanoessa inermis, T. longicaudata and Meganyctiphanes norvegica are widely distributed in the Nordic Seas, extending from the coastal areas of southern Norway i n the south to the subarctic and Arctic water masses in the northwest. Though widespread, highest abundances of M. norvegica were restricted to the warmer Atlantic waters. The pelagic shrimps Sergestes and Pasi phaea spp., the squid Gonatus fabricii and jellyfish Periphylla periph ylla were distributed throughout the study area. Of the mesopelagic fi shes only the lanternfish Benthosema glaciale showed a wide distributi on whereas Maurolicus muelleri and Notolepis rissoi were restricted to the warm Atlantic water masses. Themisto libellula was the dominant a mphipod in the subarctic and Arctic waters of the Nordic Seas. Based o n trawl catches in 1994 the total biomass of krill and amphipods was e stimated at 50 and 110 million tons wet weight respectively. Biomass e stimates of other groups varied from 0.25 to 11 million tons wet weigh t.