Temporary uncoupling of the marine nitrogen cycle: Accumulation of nitrite on the Bering Sea shelf

An unprecedented pool of nitrite (2–5.6 µM) was observed in the well-oxygenated, ammonium-rich bottom waters of Bering Sea middle shelf in fall 2005 on two simultaneous oceanographic cruises. This nitrite pool was located in a transition zone that separated the ice-derived cold pool to the north from warmer waters to the south. The transition zone was influenced by on-shelf flow. The nitrite pool was transitory; it was not apparent 11 days after it was first observed. Several origins of the pool were considered including: truncation of sedimentary denitrification and/or curtailment of anammox (these are dominate pathways in the nitrogen cycle of the Bering Sea), nitrite release from light-limited phytoplankton (light and nutrient conditions were favorable for this mechanism), and truncation of water column nitrification (there was a small decrease of ammonium in the vicinity of the nitrite pool). The occurrence of this pool suggests a temporary uncoupling of the marine nitrogen cycle.