A novel approach to estimate active carbon flux using the micronekton biomass spectra

Micronekton play a critical role in global carbon cycling, actively transporting carbon between the surface and deep ocean during diel migrations. Carbon transport is mediated through respiration, gut flux, excretion and mortality of migrating organisms. Because marine ecological processes are strongly size dependent, it was proposed that active carbon transport can be measured from biomass spectra. The micronekton community was sampled during the Micronekton Intercalibration Experiment (MIE-1) in October of 2004 off the south west coast of Oahu. Sampling was conducted during both day and night in the epipelagic (0 – 120 m) and mesopelagic (550 – 650 m) layers, using three micronekton sampling gears (Cobb trawl, Isaacs-Kidd Midwater trawl, Hokkaido University Frame Trawl) and an acoustic echosounder (hull-mounted, dual-frequency, split-beam Simrad EK60). Micronekton species composition and size spectra varied depending on the depth and time of day. We estimated total migratory micronekton abundance and biomass to be ~487 ind.m-² and ~6,014 mgC m-², respectively, assuming that ~16% of micronekton remained within the epipelagic zone during the day. A biomass/production size dependent model based on the biomass spectra theory was developed to predict active carbon transport of micronekton. The model was robust against changes in respiration, excretion and mortality, but relatively sensitive to changes in the gut flux. The model estimated that vertically migrating micronekton exported approximately 88.5 mgC m-² day -¹ to ~450 m depth in this region. This estimate was substantially higher than the majority of past estimates, and comparable to recent total global carbon export estimates. The model output suggests that micronekton play a key role in active carbon cycling, and that their contribution to carbon cycling may have been underestimated. Therefore, behavioural and physiological responses of these organisms to changes in oceanic conditions may affect the efficiency and strength of the biological pump potentially reducing atmospheric CO₂ sink to the deep-ocean.Science, Faculty ofGraduat