Factors Regulating Nitrification in the Arctic Ocean: Potential Impact of Sea Ice Reduction and Ocean Acidification

• Published in: Global biogeochemical cycles 2019-08, Vol.33 (8), p.1085-1099
• Main Authors: Shiozaki, Takuhei, Ijichi, Minoru, Fujiwara, Amane, Makabe, Akiko, Nishino, Shigeto, Yoshikawa, Chisato, Harada, Naomi
• Format: Article
• Language: English
• Subjects: Acidification; Ammonium; Ammonium compounds; Arctic Ocean; Ecosystems; Halocline; Ice; Ice environments; Light; Light intensity; Light levels; Luminous intensity; Nitrification; nitrogen cycling; Ocean acidification; Oceans; pH control; pH effects; Photons; Reduction; Satellite data; Sea ice; sea ice reduction; Shelves; Substrates
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2018GB006068

Nitrification is susceptible to changes in light and pH and, thus, could be influenced by recent sea ice reductions and acidification in the Arctic Ocean. We investigated the sensitivity of nitrification to light, pH, and substrate availability in a natural nitrifier community of the Arctic Ocean. Nitrification was active near the bottom of the shelf region (250 m). In pH control experiments, nitrification rates significantly declined when the pH was manipulated to be 0.22 lower than the controls. However, nitrification was relatively insensitive to changes in pH compared to changes in light. Light control experiments showed that nitrification was inhibited by a light intensity above 0.11 mol photons m−2 day−1, which was presumably the light threshold. A light intensity greater than the light threshold extended to the shelf bottom and upper halocline layer, limiting nitrification in these waters. Satellite data analyses indicated that the area where light levels inhibit nitrification has increased throughout the Arctic Ocean due to the recent sea ice reduction, which may lead to a declining trend in nitrification. Our results suggest that stronger light levels in the future Arctic Ocean could further suppress nitrification and alter the composition of inorganic nitrogen, with implications for the structure of ecosystems. Key Points Arctic nitrification is primarily regulated by the light environment and may have experienced a decreasing trend over the last two decades Nitrification rate significantly decreased with reduced pH, but the light effect had a greater influence A decrease in nitrification could alter the composition of inorganic nitrogen and consequently impact ecosystem structure