Temporal changes of deep-sea mesozooplankton abundance in the temperate NE Atlantic and estimates of the carbon budget

Full-depth spring and summer vertical profiles of mesozooplankton numbers and biomass were obtained with a 1 m(2) multiple opening/closing net and environmental sensing system (MOCNESS) from the BIOTRANS (biological vertical transport and energetics in the benthic boundary layer of the deep sea) study area (47 degrees N, 20 degrees W) in the temperate NE Atlantic in 1992. Mesozooplankton abundance was high in spring and less in summer in the upper 750 m. Between 750 and 1050 m differences between these seasons could not be detected. In the upper bathypelagic zone, between 1050 and 2250 m, where abiotic fluctuations are minor, both mesozooplankton biomass and numbers were significantly higher in summer compared to spring. Samples from summer 1989 fitted this pattern. Calanoid copepods of the genus Metridia were the main contributors; most of the major zooplankton groups, though playing a subsidiary role, also showed a significant increase in summer in the upper bathypelagic zone. The increase probably was due to the large transient input of detrital material, which regularly occurred in the course of the phytoplankton spring bloom in the area investigated and may have stimulated the onset of reproduction in the bathypelagic zone. Temporal changes in mesozooplankton abundance could not be detected below 2250 m depth. Metabolic carbon requirements of mesozooplankton, calculated from ETS (electron transport system) data, increased in the bathypelagic zone (1000 to 4250 m) from 1.61 mg C m(-2) d(-1) in spring to 4.12 mg C m(-2) d(-1) in summer. The carbon respired by the bathypelagic micro- and mesozooplankton in summer, based on an assumed spring bloom area of 50 000 km(2), was 893 t C d(-1) as a minimum estimate, which was higher than in spring by a factor of 2.6.