Preliminary studies on the vertical distribution of size-fractions in the zooplankton community in Lindåspollene, western Norway

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Abstract
Zooplankton collected with Clarke-Bumpus plankton samplers equipped with 180 µm mesh-size nets in 10 m depth-strata and with 2–3 hours time intervals in Lindåspollene 2–3 October 1979 and 12–13 May 1981 revealed distinct differences with regard to the vertical distribution of size-fractions in the plankton. Thus, the major part of the biomass of the > 1000 µm size-fraction in October 1979 was concentrated in the deepest 20 m throughout the sampling period, but a smaller part of the size-fraction, composed particularly of Sagitta elegans and Aglantha digitale, performed distinct vertical migrations. In May 1981 the same size-fraction was concentrated particularly in the upper 20 m, and no diel vertical migration could be ascertained. However, the species S. elegans and A. digitale displayed diel vertical migrations similar to those observed in October 1979. The major part of the biomass of the 180–500µm and the 500–1000 µm size-fractions was concentrated in the upper 20 m in October 1979, and a weak diel vertical migration was apparent in the upper 20 m only. In May 1981 the same size-fractions performed distinct vertical migrations in the upper 40 m of the water column. The differences in vertical distribution and in the contribution of the various size-fractions to the total biomass is discussed with reference to the change in the hydrographic conditions between the two sampling periods. A series of samples obtained with Clarke-Bumpus samplers equipped with 75 µm mesh-size nets in 5 or 10 m depth strata during 10–11 June 1981 showed that the major 20 taxa of the zooplankton differed with regard to vertical distribution, but diel vertical migrations could not be detected for any of the taxa. The biomass of the 75–250 µm size-fraction was a little higher than the biomass of the > 250 µm size-fraction.