Nitrite regeneration in the oligotrophic Atlantic Ocean

Abstract. The recycling of scarce nutrient resources in the sunlit

open ocean is crucial to ecosystem function. Nitrification directs ammonium

(NH4+) derived from organic matter decomposition towards the

regeneration of nitrate (NO3-), an important resource for

photosynthetic primary producers. However, the technical challenge of making

nitrification rate measurements in oligotrophic conditions combined with the

remote nature of these environments means that data availability, and the

understanding that provides, is limited. This study reports nitrite

(NO2-) regeneration rate (RNO2 – the first product of

nitrification derived from NH4+ oxidation) over a 13 000 km

transect within the photic zone of the Atlantic Ocean. These measurements,

at relatively high resolution (order 300 km), permit the examination of

interactions between RNO2 and environmental conditions that may warrant

explicit development in model descriptions. At all locations we report

measurable RNO2 with significant variability between and within

Atlantic provinces. Statistical analysis indicated significant correlative

structure between RNO2 and ecosystem variables, explaining

∼65 % of the data variability. Differences between sampling

depths were of the same magnitude as or greater than horizontally resolved differences, identifying distinct biogeochemical niches between depth

horizons. The best overall match between RNO2 and environmental

variables combined chlorophyll-a concentration, light-phase duration, and silicate concentration (representing a short-term tracer of water column physical instability). On this basis we hypothesize that RNO2 is related to the short-term autotrophic production and heterotrophic

decomposition of dissolved organic nitrogen (DON), which regenerates

NH4+ and supports NH4+ oxidation. However, this did not

explain the observation that RNO2 in the deep euphotic zone was

significantly greater in the Southern Hemisphere compared to the Northern Hemisphere. We present the complimentary hypothesis that observations reflect the

difference in DON concentration supplied by lateral transport into the gyre

interior from the Atlantic's eastern boundary upwelling ecosystems.