Nitrite cycling experiments across the upper and lower boundaries of the Primary Nitrite Maximum in the Eastern Tropical North Pacific

The processes involved in nitrite cycling in the upper ocean are often studied using in situ measurements from incubations and/or isotope values in conjunction with knowledge of isotope systematics. Near the primary nitrite maximum (PNM), bacteria, archaea and phytoplankton are all potentially involved in nitrite cycling. Estimates of nitrite production and consumption near the PNM are not conclusive regarding which microbial groups and which processes are critical to the ubiquitous accumulation of measurable nitrite. The appearance, magnitude and profile shape of the PNM feature varies locally and regionally, suggesting that the balance of nitrite processes responds to changes in environmental conditions with depth and regime (eg. coastal vs offshore).

Here we simultaneously examine multiple microbial processes (ammonia oxidation, nitrite oxidation, nitrate reduction, nitrite reduction, and phytoplankton uptake) using ¹⁵N incubations measured across the upper and lower boundaries of the PNM within the Eastern Tropical North Pacific Ocean (which also hosts a secondary nitrite maximum in the oxygen-deficient thermocline). In addition to measuring rates under ambient conditions, corresponding experimental manipulations were made to simulate changes in light and nitrogen availability.

Experiments analyzed to date show variable rates with depth, and clear response to source water, light level and nitrate addition. Highest ammonia oxidation rates are seen in the dark incubation treatments (max 86 nM day^-1). Ammonia oxidation rates declined as the light level increased for all source communities of microbes. The addition of nitrate resulted in a decrease in the measured ammonia oxidation rate at all light levels for the offshore station. However, other source waters showed minimal change in ammonia oxidation with nitrate addition. Correlation of increased ammonia oxidation rates with nitrate uptake rates may support phytoplankton-based control on ammonia oxidation rates via competition for ammonia.