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

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 97 nmol L-1 d(-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 6.0 mmol m(-2) d(-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 similar to 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. (C) 2013 Elsevier B.V. All rights reserved.