Determination of total N ₂ fixation rates in the ocean taking into account both the particulate and filtrate fractions

Abstract. Using the ¹⁵N₂ tracer method and high-sensitivity δ¹⁵N analytical systems, we determined N₂ fixation rates for ocean samples by dividing them into particulate (>0.7 μm) and filtrate (<0.7 μm) fractions. While N₂ fixation in the filtrate fraction had been ignored in previous studies, we found a significant N₂ fixation rates in the filtrate fraction in our study. The areal N₂ fixation rates in the western North Pacific Ocean estimated from the particulate fraction varied from <1 to 160 μmol N m^-2 d⁻¹, and those rates estimated from the filtrate fraction ranged from <0.5 to 54 μmol N m^-2 d⁻¹. Thus, N₂ fixation in the filtrate fraction accounts for on average 50% (ranging from <10% to 84%) of the total N₂ fixation rates. If these results are confirmed generally in the ocean, the new total N₂ fixation flux, which includes fixation in the filtrate fraction, possibly doubles the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.