Trophic structure of a bathyal benthopelagic boundary layer community south of the Balearic Islands (southwestern Mediterranean)

During 2 deep-sea oceanographic cruises carried out in October 1996 and May 1998, megafauna (fish and decapod crustaceans), suprabenthos, zooplankton, and environmental variables from the water column (fluorescence and light transmission) were simultaneously collected between 210 and 1752 m depth. Fish were the dominant megafaunal taxon in biomass along the slope; decapod crustaceans were co-dominants at intermediate depths (between 402 and 710 m). Suprabenthos and zooplankton attained their maximal biomass at intermediate depths, with deeper maxima detected for suprabenthos (between 802 and 1322 m) than for zooplankton (between 402 and 802 m). Fish and decapod biomass were weakly correlated with each other, and with the suprabenthos-zooplankton biomass, fluorescence and light transmission. Significant correlations were, however, detected between the suprabenthos and zooplankton and their possible food-sources deduced from fluorometer and Light transmission data, with different patterns depending on the distribution of each compartment (trophic level) in the water column and the sediment-water interface. Within a single taxon (crustaceans), size distribution for suprabenthos-zooplankton and megafaunal species followed opposite temporal patterns in our study. Among suprabenthos-zooplankton species, smaller specimens (recruitment) were detected in October 1996, coinciding with the highest fluorometer signals in the water column. The dominant phytoplankton/phytodetritus consumers (the euphausiid Euphausia krohni and the mysid Boreomysis arctica) showed clearer recruitment peaks during October than those species preying upon meiofaunal taxa (the amphipod Rhachotropis caeca, and the isopod Munnopsurus atlanticus). These results suggest that the influence of food input signals progressively decreases with increasing trophic level, and therefore the indirect effect of superficial production on top trophic levels is difficult to establish at least for the short time scales of our oceanographic surveys.