Euphotic zone nitrification in the NE subarctic Pacific: Implications for measurements of new production

Abstract

Ammonium (NH4+) oxidation rates, nitrate (NO3−) uptake rates and dissolved nutrient concentrations were measured throughout the euphotic zone along the Line P oceanographic transect in the NE subarctic Pacific, on three cruises (winter, spring and late-summer) in 2009. NH4+ oxidation was found to proceed throughout the euphotic zone and rates ranged from undetectable to 97nmolL−1d−1. Evidence was found to suggest that both light and NH4+ concentrations played a role in regulating NH4+ oxidation rates, with rates increasing as light decreased and NH4+ concentrations increased. The observation that NH4+ oxidation was a ubiquitous process throughout the euphotic zone demonstrates that not all NO3− present in this region of the water column should be considered new nitrogen, and has important implications for estimates of new production in the NE subarctic Pacific. Depth-integrated NO3− uptake rates ranged from 0.65 to 6.0mmolm−2d−1, and we found that euphotic zone NH4+ oxidation could have supported between 3 and 100% of the phytoplankton NO3− demands along Line P. Taking potential euphotic zone NO3− regeneration into account resulted in new production estimates being reduced by an average of ~50%. During spring and late-summer, the degree by which new production was reduced, when euphotic zone NO3− regeneration was taken into consideration, was tightly anti-correlated to total NO3− uptake rates. This indicates that the implications of euphotic zone nitrification for estimates of new production are greatest in the less productive regions of the oceans.