Deep-sea macroplankton distribution (at 400 to 2300 m) in the northwestern Mediterranean in relation to environmental factors

Changes in the composition and biomass distribution of deep-living zooplankton over wide gradients of depth (400–2300 m) and longitude (~ 180 km) 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 450 m) and near the bottom (at ~ 5–200 mab), over soundings of 450–2263 m, including the top of Valencia Seamount (at ~ 40° 25′ N–02° 42′ E, 1076 m). Zooplankton changed significantly in composition at the mesoscale (~ 180 km) 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/1000 m3) than in the DSL (1568–1743 individuals/1000 m3). There was a significant increase in near-bottom zooplankton biomass over the middle slope, at 1000–1300 m, linked to an increase in scyphozoans and siphonophores (Lensia spp. and Abylopsis tetragona) with peaks of 1.5–2.0 gWW/1000 m3. The peak of near-bottom zooplankton at 1000–1300 m coincided with the lowest temperatures (13.08 °C) and maximum O2 concentration (4.40 ml/l) near the bottom and below 1000 m 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 1200–1400 m 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.

► Near-bottom zooplankton peaked at 1000-1300 m by increase of jellyfish and copepods. ► The peak fitted with the lowest T and maximum O2 concentration near the bottom. ► Depth-trends recorded over 450-2263 m were found in previous surveys seasonaly. ► Copepods reached higher abundance in the BBL than at midwater (DSL). ► At the DSL zooplankton seemed to avoid the core of Levantine intermediate waters.