Full-depth zooplankton profiles over the deep bathyal of the NE Atlantic

Vertical profiles of zooplankton (size range less than 5 mm) were taken with a 1 m2 MOCNESS (333 pm mesh aperture) from the > 4000 m water column at 2 sites in the NE Atlantic. In the upper 1000 m, the profiles tended to exhibit high variability and were influenced by diurnal migrations, but below, the distributions were nearly similar. The standing crop in the upper 400 m was remarkably low, possibly due to a bloom of salps and their grazing pressure on the phytoplankton crop. The nonlinear decrease of the normalized zooplankton abundance and biomass below 1000 m in the area investigated could be approximated by a power function. The improved regression 'model' revealed no differences between the slopes when compared to biomass data from other marine locations given by various authors. T h s indicates that the processes of vertical material flux seem to be similar in many bathypelagic systems of the open s e a The y-intercepts, however, showed differences due to either a higher surface productivity in some ocean areas or differences in sampling methods and evaluation of the material. Possible causes which may have led to less of a decrease in zooplankton abundance at greater depths (below 2500 m) such as food supply, resuspension, upward flux, fauna1 changes and trophic interactions are discussed.