Abstract
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.
Highlights
Oligotrophic gyres and their transition regions are the largest biome on Earth, covering approximately 60 % of its surface (Eppley and Peterson, 1979), and are expanding as a result of the warming climate (Polovina et al, 2008)
Biogeochemical models typically apply a specific nitrification rate of 0.02 to 0.10 d−1 (Moore et al, 2002; Wang et al, 2006; Christian et al, 2002; Jiang et al, 2003), comparable to the value of 0.011–0.113 d−1 derived from the present study assuming 10–100 nmol L−1 NH+4, a range consistent with historical measurements (Clark et al, 2008; British Oceanographic Data Centre data archives; https://www.bodc.ac.uk/)
Through statistical analysis we demonstrated that ∼ 65 % of variability within RNO2 was explained through co-variability with environmental variables, identifying a role for chlorophyll-a concentration, light duration, and silicate
Summary
Oligotrophic gyres and their transition regions are the largest biome on Earth, covering approximately 60 % of its surface (Eppley and Peterson, 1979), and are expanding as a result of the warming climate (Polovina et al, 2008). Biogeochemical processes in these regions have the potential to influence global elemental cycles. This biomass supports the ecosystem and contributes over half of global organic carbon export (Emerson et al, 2001) Persistent stratification in these systems impedes nutrient inputs to the sunlit region from the deep ocean, constraining microbial growth (Eppley and Peterson, 1979; Moore et al, 2013). The two-stage oxidation of NH+4 via nitrite (NO−2 ) to nitrate (NO−3 ), takes place in the photic zone of open oceans (Santoro et al, 2013), where it directly competes with photosynthetic cells for the NH+4 resource
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