Summer distribution patterns and biomass estimates of macrozooplankton and micronekton in the nordic seas

Abstract

Abstract The vertical and horizontal distribution patterns of zooplankton and micronekton 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 masses and the distribution of planktivorous fish such as herring, Clupea harengus. Zooplankton biomass typically revealed a bimodal vertical distribution with high values in the surface layer and at 200-600 m depth. This subsurface maximum contained, among others, several species of macrozooplankton and micronekton such as krill, pelagic shrimps and 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 in 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 Pasiphaea spp., the squid Gonatus fabricii and jellyfish Periphylla periphylla were distributed throughout the study area. Of the mesopelagic fishes only the lantemfish Benthosema glaciale showed a wide distribution whereas Maurolicus muelleri and Notolepis rissoi were restricted to the warm Atlantic water masses. Themisto libellula was the dominant amphipod in the subarctic and Arctic waters of the Nordic Seas. Based on trawl catches in 1994 the total biomass of krill and amphipods was estimated at 50 and 110 million tons wet weight respectively. Biomass estimates of other groups varied from 0.25 to II million tons wet weight. Key Words: Distributionbiomasskrillamphipodsjellyfishsquidshrimpsmesopelagic fishNordic Seas