Vertical flux and biogeochemical turnover regulate nutrient limitation of net organic production in the North Pacific Gyre

Water samples collected at a station in the North Pacific Gyre near Hawaii precisely define the temporal average vertical profile of dissolved inorganic and organic N and P (DON and DOP). DON and DOP concentrations decrease with depth. As shown by other studies, the regression of NO 3 − (with or without NH 4 − ) vs. PO 4 3− yields an approximately Redfield slope and a negative nitrogen intercept. If DON and DOP are included in the regression equation, the intercept approaches 0. Vertical flux ratios of dissolved materials are calculated with a one‐dimensional diffusion model. Downward mixing of DON offsets about 6% of the upward NO 3 − flux; downward DOP mixing offsets only about 4% of the upward PO 4 3− flux. Net production of particulate organic matter in the euphotic zone is N limited because of slow biochemical turnover of dissolved organic N relative to that of dissolved organic P and to downward mixing.