Stukel, M. R., Biard, T., Krause, J. W., & Ohman, M. D. (2018). Large Phaeodaria in the twilight zone: Their role in the carbon cycle.
Association for the Sciences of Limnology and Oceanography, .
Abstract: Advances in in situ imaging allow enumeration of abundant populations of large Rhizarians that compose a substantial proportion of total mesozooplankton biovolume. Using a quasi-Lagrangian sampling scheme, we quantified the abundance, vertical distributions, and sinking‐related mortality of Aulosphaeridae, an abundant family of Phaeodaria in the California Current Ecosystem. Inter‐cruise variability was high, with average concentrations at the depth of maximum abundance ranging from < 10 to > 300 cells m−3, with seasonal and interannual variability associated with temperature‐preferences and regional shoaling of the 10°C isotherm. Vertical profiles showed that these organisms were consistently most abundant at 100�150 m depth. Average turnover times with respect to sinking were 4.7�10.9 d, equating to minimum in situ population growth rates of ~ 0.1�0.2 d−1. Using simultaneous measurements of sinking organic carbon, we find that these organisms could only meet their carbon demand if their carbon : volume ratio were ~ 1 μg C mm−3. This value is substantially lower than previously used in global estimates of rhizarian biomass, but is reasonable for organisms that use large siliceous tests to inflate their cross‐sectional area without a concomitant increase in biomass. We found that Aulosphaeridae alone can intercept > 20% of sinking particles produced in the euphotic zone before these particles reach a depth of 300 m. Our results suggest that the local (and likely global) carbon biomass of Aulosphaeridae, and probably the large Rhizaria overall, needs to be revised downwards, but that these organisms nevertheless play a major role in carbon flux attenuation in the twilight zone.
Stukel, M. R., Aluwihare, L. I., Barbeau, K. A., Chekalyuk, A. M., Goericke, R., Miller, A. J., et al. (2017). Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction.
Proc Natl Acad Sci U S A, 114(6), 1252–1257.
Abstract: Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from 238U:234Th disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg Cm-2d-1) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front was mechanistically linked to Fe-stressed diatoms and high mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional approximately 225 mg Cm-2d-1 was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.