Nitrification from the lower euphotic zone to the sub-oxic waters of a highly productive British Columbia fjord

Nitrification rates were measured monthly from April to October 2008, at depths ranging from the lower euphotic zone to the sub-oxic waters of Saanich Inlet. Ammonium (NH4+) and nitrite (NO2−) oxidation rates ranged from undetectable to 0.319 and 0.478 μmol L− 1 d− 1, respectively. NH4+ oxidation rates and concentrations were positively correlated at substrate concentrations less than 0.8 μmol NH4+ L− 1. Positive correlations between NH4+ oxidation rates, NO2− concentrations, and NO2− oxidation rates were also observed, highlighting the important role that NH4+ oxidation plays in supporting NO2− oxidation in Saanich Inlet. Despite the apparent dependence of NO2− oxidation rates on NH4+ oxidation rates, the former was still 44% higher than the latter and we concluded that Saanich Inlet NO2− oxidation rates were augmented by fortnightly spring-tide nutrient renewal. From May to October, sub-oxic zone waters were isolated from any significant mixing events, and we estimated that nitrification was responsible for approximately 25% of dissolved oxygen consumption. This estimate is in close agreement with that calculated using Redfield stoichiometry, and as such highlights the accuracy with which nitrification rates can be quantified using incubation techniques. Finally, nitrification rates in the euphotic zone were at times substantial, and we suggest that earlier estimates of new production in Saanich Inlet may have been overestimated by approximately 15%.

Research highlights
► We conducted measurements of nitrification through the water column of Saanich Inlet.
► NH4+ oxidation rates and [NH4+] were positively correlated at [NH4+] less than 0.8 μM.
► NH4+ oxidation rates were positively correlated with [NO2−] and NO2− oxidation rates.
► Nitrification was at times substantial in the euphotic zone.
► New production may have previously been overestimated by approximately 15%.