Vertical fluxes of carbon, nitrogen, and phosphorus in the North Pacific Subtropical Gyre near Hawaii

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 in the subtropical North Pacific Ocean (22° 45′N, 158°W), the C/N and C/P ratios in sinking particles increase with depth, while the ratios in the upward mixing flux decrease. An exponential model of the sinking flux as a function of depth is used, which permits remineralization length and time scales (more properly called solubilization scales) to be estimated. Time scales (2–5 days) are consistent with microbiological decomposition, and length scales are greater for C (368 m) than for N and P (249 and 248 m), consistent with the existence of a biological pump for carbon. Assuming that the system is nutrient limited (total upward and downward fluxes of N or P are equal) permits us to estimate vertical eddy diffusivity (Kz) and the net export of carbon from the system. Values of Kz range from 5×10−6 to 4×10−5 m2 s−1 and are consistent with independent estimates for similar environments. We estimate that approximately 20% of the air‐to‐sea carbon flux at Station ALOHA and a similar fraction of new carbon production are exported to the deep ocean (σθ>27) by particle sinking. Eddy diffusivities are lower and net carbon export greater if phosphorus is assumed to be the controlling element. This has implications for the role of dinitrogen fixation in the subtropical North Pacific.