Vertical Fluxes of Carbon, Nitrogen and Phosphorus at Station ALOHA

James Christian¹, Marlon Lewis¹ and David Karl²

¹Department of Oceanography, Dalhousie University, Halifax, NS, Canada B3H 4J1

²School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

Abstract

The hypothesized "biological pump" mechanism for removing carbon from the euphotic zone to the deep ocean requires different rates of recycling of C, N and P in sinking particles. At Station ALOHA the C:N and C:P ratios in sinking particles increase with depth while the ratios in the upward mixing flux decrease. A new expression describing the sinking flux as a function of depth is introduced, which is more realistic physically than the earlier hyperbolic formulations and permits solubilization/remineralization length and time scales to be estimated. Remineralization time scales are found to be consistent with microbiological measurements made on particles collected in unpoisoned traps, and length scales are greater for C than for N and P, consistent with the existence of a biological pump.

Assuming that the system is nutrient-limited and therefore that total upward and downward fluxes of N or P are equal permits us to estimate vertical eddy diffusivity (K_z) and the net export of carbon from the system. Values of K_z are within the range of previously published estimates for similar environments (0.05-0.5 cm^-2 s^-1). Approximately 20% of the air-sea carbon flux at Station ALOHA, and a similar fraction of annual new production, is estimated to be exported to the deep ocean (sigma-theta > 27) by particle sinking. Eddy diffusivities are lower and net carbon export greater if P is assumed to be the controlling element; this may reflect the importance of dinitrogen fixation in this ecosystem.