Structure of the community of deep-water near-bottom macroplankton and micronekton from the Galicia Bank (NE Atlantic) and relationships with environmental drivers

Abstract

The composition of the near-bottom macroplankton and micronekton, with especial emphasis on eucarid and peracarid crustaceans and fish, of the Galicia Bank (NW Spain) was analysed in Jul-Aug 2010 and 2011 at 730-2230 m. WP2 nets, equipped with an opening-closing system and 0.5 mm mesh-size, and beam trawls (10 mm mesh size) were used. Environmental variables (temperature, salinity, oxygen) were collected at near-bottom. Relationship between abundance patterns and environmental variables was explored by GLMs/Spearman/CCA. Macroplankton community was estructured as a function of depth, with different species at summit (to c. 800 m) and deeper, over the seamount flanks. Tendencies related with diel cycle and year, i.e. higher abundance of the euphausiid Euphausia krohni in 2010, were recorded. The macroplankton was dominated by the hyperiid Themisto gaudichaudi. The mysid Eucopia hanseni was also important at the summit, while the gammarid Cyphocaris anonyx was dominant in the flanks. The micronekton community was homogenous to 1100 m, with a different community at 1400 ¿ 1800 m. Systellaspis debilis was important at all depths. Gnatophausia zoea was also dominant in shallow stations while Acantephyra purpurea and Meganyctiphanes norvegica were found deeper. Trends in abundance and diveristy were related with oxygen concentration. Total zooplankton biomass, i.e. including all gelatinuos taxa was higher in the summit (3.75 gWW/1000 m3) and over flanks (3.05 gWW/1000 m3) at 1420-2230 m, with minimum biomass at intermediate depths (890-1080 m, 2.02 gWW/1000 m3), coinciding with the Mediterranean Outlow Water (MOW) with minimals in O2 content (4.2 ml/l) at c. 1000 m. Biomass of near-bottom macroplankton at Galicia Bank is similar to BBL biomass (in samplings done in July 2010) found in the Balearic Basin (western Mediterranean) at comparable depths (3.23 gWW/1000 m3, Cartes et al. 2013), and clearly below biomass found in Avilés Canyon (6.16-15.19 gWW/1000 m3 ; authors unpl. data)