Changes in the composition and biomass distribution of deep-living zooplankton over wide gradients of depth (400-2300m) and longitude ( 180km) have been analyzed in the Balearic Basin (western Mediterranean), seeking the environmental variables responsible for these changes. Zooplankton tends to aggregate at different levels of the water column (forming Deep Scattering Layers, DSL) and in the Benthic Boundary Layer (BBL). Macrozooplankton biomass and composition were analyzed along a transect performed in July 2010 in midwater (between 350 and 450m) and near the bottom (at 5-200mab), over soundings of 450-2263m, including the top of Valencia Seamount (at 40° 25' N-02° 42' E, 1076m). Zooplankton changed significantly in composition at the mesoscale ( 180km) in both the DSL and the BBL. Siphonophores and calanoid copepods were the most dominant deep zooplankton taxa, calanoids reaching higher abundance in the BBL (1761-5177 individuals/1000m3) than in the DSL (1568-1743 individuals/1000m3). There was a significant increase in near-bottom zooplankton biomass over the middle slope, at 1000-1300m, linked to an increase in scyphozoans and siphonophores (Lensia spp. and Abylopsis tetragona) with peaks of 1.5-2.0gWW/1000m3. The peak of near-bottom zooplankton at 1000-1300m coincided with the lowest temperatures (13.08°C) and maximum O2 concentration (4.40ml/l) near the bottom and below 1000m with higher records in near-bottom turbidity. Gelatinous zooplankton are the main prey in the diet of the demersal fish Alepocephalus rostratus in the western Mediterranean, fish responsible for the peak of megafauna biomass reported at around 1200-1400m in the deep Mediterranean and at similar depths in other oceanic areas (e.g. the NW Atlantic). We suggest that deep-sea environmental conditions can govern peaks of near-bottom zooplankton, as well as influence the structure of the demersal fish community. © 2012 Elsevier B.V.

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