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. F ish 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 intermedi ate depths, with deeper maxima detected for suprabenthos (between 802 and 1 322 m) than for zooplankton (between 402 and 802 m). Fish and decapod bioma ss were weakly correlated with each other, and with the suprabenthos-zoopla nkton biomass, fluorescence and light transmission. Significant correlation s were, however, detected between the suprabenthos and zooplankton and thei r possible food-sources deduced from fluorometer and Light transmission dat a, with different patterns depending on the distribution of each compartmen t (trophic level) in the water column and the sediment-water interface. Wit hin a single taxon (crustaceans), size distribution for suprabenthos-zoopla nkton and megafaunal species followed opposite temporal patterns in our stu dy. Among suprabenthos-zooplankton species, smaller specimens (recruitment) were detected in October 1996, coinciding with the highest fluorometer sig nals in the water column. The dominant phytoplankton/phytodetritus consumer s (the euphausiid Euphausia krohni and the mysid Boreomysis arctica) showed clearer recruitment peaks during October than those species preying upon m eiofaunal taxa (the amphipod Rhachotropis caeca, and the isopod Munnopsurus atlanticus). These results suggest that the influence of food input signal s progressively decreases with increasing trophic level, and therefore the indirect effect of superficial production on top trophic levels is difficul t to establish at least for the short time scales of our oceanographic surv eys.