Mesozooplankton and Micronekton Active Carbon Transport in Contrasting Eddies

Abstract

were sampled along the eastern coast of Australia. Depth stratified mesozooplankton and micronekton were collected using a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and an International Young Gadoid Pelagic Trawl (IYGPT) equipped with an opening/closing codend. Sampling was undertaken at the center and edge of a frontal cold-core eddy (F-CCE Center and Edge) in 2015, and at the center of a cold-core eddy (B-CCE) and two warmcore eddies (R-WCE and WCE) in 2017. We assess the diel vertical structure, biomass, and downward active carbon transport by mesozooplankton and micronekton in eddies. Total water column mesozooplankton and micronekton biomass did not vary substantially across water masses, while the extent and depth of diel vertical migration did. Using in situ measurements of temperature and measurements of mesozooplankton and micronekton abundance and biomass, we estimated the contribution of respiration, dissolved organic carbon (DOC) excretion, gut flux, and mortality to total downward active carbon transport in each water mass. Overall, active carbon transport by mesozooplankton and micronekton below the mixed layer varied substantially across water masses. We corrected estimates of micronekton migratory biomass and active carbon transport assuming 50% net efficiency. In the R-WCE mesozooplankton remained within the mixed layer during the day and night; only 50% of the total micronekton population migrated below the mixed layer contributing to carbon transport, equating to 2.89 mg C m −2 d −1. Mesozooplankton actively transported 16.1 and 8.0 mg C m −2 d −1 at the F-CCE Center and Edge, respectively. Mesozooplankton and micronekton active carbon transport in the B-CCE were 5.4 and 0.74 mg C m −2 d −1 , and in the WCE 88 and 13.4 mg C m −2 d −1. Differences in carbon export were dependent on food availability, temperature, time spent migrating, and mixed layer depth. These findings suggest that under certain conditions mesoscale eddies can act as important carbon sinks.

Both Dissolved Oxygen and Chlorophyll Explain the Large-Scale Longitudinal Variation of Deep Scattering Layers in the Tropical Pacific Ocean

“…Micronekton, organisms with a length in the range of 2-20 cm, have a stronger ability to migrate and wider migration range than zooplankton. Micronekton are one of the dominant components of mesopelagic biomes in the marine system (Catul et al, 2010;Kwong et al, 2020). Micronekton migrate to the surface (0-200 m) to feed at night and descend to the mesopelagic layers (200-1,000 m) to escape predators after sunrise, which is referred as diel vertical migration (DVM).…”

Cloud shadows drive vertical migrations of deep-dwelling marine life

“…Furthermore, zooplankton alter their DVM in response to chemicals produced by fish, called kairomones ( 12 , 13 ), and to internal circadian rhythms that can cue migration during periods of polar midnight sun ( 14 ). DVM is not only important to understanding aquatic trophic dynamics, but it also impacts nutrient cycles worldwide, as organic matter consumed at the surface by migrators is transported to depth, metabolized, and released as dissolved inorganic carbon (respired), dissolved organic matter (excreted), and particulate organic matter (egested) ( 15 – 18 ).…”

“…Kaartvedt et al ( 19 ) showed that mesopelagic fish swam upward beneath a passing storm, Frank and Widder ( 20 ) found that a transient influx of turbid water enhanced light attenuation and induced an upward migration of crustaceans and salps, and other studies show that cloudiness can influence vertical migration behavior on diel timescales ( 21 – 23 ). McLaren ( 24 ) proposed that during the day, zooplankton may be “resting” in deeper, cooler waters to save energy, and studies estimating migrator excretion and respiration rates at depth assume a constant rate of metabolic activity throughout daylight hours based largely on temperature and animal size ( 15 , 18 ). In situ observations of inactivity by animals at depth are for mesopelagic fishes that are large enough to be observed by submersibles ( 24 ), while less information is available for zooplankton.…”

Subsurface Eddy Facilitates Retention of Simulated Diel Vertical Migrators in a Biological Hotspot

“…Retention of zooplankton within the subsurface eddy over PDC may also play a critical role in carbon cycling within the region. Mesoscale eddies have been hypothesized to be important carbon sinks, and that the role these features play in carbon cycling is highly dependent on the zooplankton communities they retain (Christiansen et al., 2018; Kwong et al., 2020). More work is necessary to determine if the subsurface eddy over PDC has any impact on the local carbon cycle and if it retains zooplankton communities that allow it to act as a carbon sink.…”