Nitrate assimilation and regeneration in the Barents Sea: insights from nitrogen isotopes

Tuerena, Robyn E.; Hopkins, Joanne; Ganeshram, Raja S.; Norman, Louiza; Vega, Camille de la; Jeffreys, Rachel M.; Mahaffey, Claire

doi:10.5194/bg-2020-293

Cited by 2 publications

(3 citation statements)

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“…Observed deep mixing in spring 2018 was sufficient for seasonal nitrate replenishment, indicating that the east-west gradient in nutrient concentrations may strengthen in coming years. Further declines in nitrate supply to the western Fram Strait will reduce NPP and further alter phytoplankton δ 15 N with implications for Arctic food web studies (de la Vega et al, 2021). It is important to note that we draw contrasts between distinct water masses on either side of the Fram Strait, and acknowledge that the transition between these water masses is unlikely to be linear.…”

Section: Future Prospects For Fram Strait Nutrient Dynamicsmentioning

confidence: 94%

An Arctic Strait of Two Halves: The Changing Dynamics of Nutrient Uptake and Limitation Across the Fram Strait

Tuerena

Hopkins

Buchanan

et al. 2021

Global Biogeochemical Cycles

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Arctic primary production has increased by >50% in the last two decades, fueled by increased light and nutrient availability, but whether or not this trend continues will depend on a sustained nutrient supply to phytoplankton (Arrigo & van Dijken, 2015;Lewis et al., 2020). Currently, nitrogen (N) is the main limiting nutrient to phytoplankton in the Arctic Ocean (Krisch et al., 2020;Mills et al., 2018). As warming continues, the degree of N limitation may change as the water mass properties supplied from the Atlantic and Pacific are altered (Hatún et al., 2017;Woodgate, 2018). Stable isotope measurements of nitrate and organic nitrogen can be used to provide insights into past and present N limitation to phytoplankton (Francois Abstract The hydrography of the Arctic Seas is being altered by ongoing climate change, with knockon effects to nutrient dynamics and primary production. As the major pathway of exchange between the Arctic and the Atlantic, the Fram Strait hosts two distinct water masses in the upper water column, northward flowing warm and saline Atlantic Waters in the east, and southward flowing cold and fresh Polar Surface Water in the west. Here, we assess how physical processes control nutrient dynamics in the Fram Strait using nitrogen isotope data collected during 2016 and 2018. In Atlantic Waters, a weakly stratified water column and a shallow nitracline reduce nitrogen limitation. To the west, in Polar Surface Water, nitrogen limitation is greater because stronger stratification inhibits nutrient resupply from deeper water and lateral nitrate supply from central Arctic waters is low. A historical hindcast simulation of ocean biogeochemistry from 1970 to 2019 corroborates these findings and highlights a strong link between nitrate supply to Atlantic Waters and the depth of winter mixing, which shoaled during the simulation in response to a local reduction in sea-ice formation. Overall, we find that while the eastern Fram Strait currently experiences seasonal nutrient replenishment and high primary production, the loss of winter sea ice and continued atmospheric warming has the potential to inhibit deep winter mixing and limit primary production in the future. Plain Language SummaryThe Fram Strait is the main gateway of the Arctic Ocean. In the east, warm, salty waters from the Atlantic flow north into the Arctic basin, and in the west, cold, fresh waters flow south from the central Arctic into the North Atlantic. We examined how changes to the availability of nutrients (which are essential for algae to grow) may limit algae growth in the Fram Strait, both as a result of changes to their source and also how easily the upper ocean mixes nutrients from depth. In the eastern Fram Strait, there is a high availability of nitrate, one of the main nutrients to support algae growth, and winter mixing sustains nutrient supply and biological production in recent decades. However, in the western Fram Strait, the outflowing surface waters do not easily mix with deeper waters and are depleted in nitrate, and nut...

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Section: Future Prospects For Fram Strait Nutrient Dynamicsmentioning

confidence: 94%

An Arctic Strait of Two Halves: The Changing Dynamics of Nutrient Uptake and Limitation Across the Fram Strait

Tuerena

Hopkins

Buchanan

et al. 2021

Global Biogeochemical Cycles

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“…In support of limited range in baseline δ 15 N value at both low and high latitudes within the BS, seawater nitrate δ 15 N had stable values of c. 5.1 for both Atlantic and Arctic water in the BS (Tuerena et al, 2020). Tuerena et al (2020) found no clear trend in δ 15 N values for nitrate or for particulate nitrogen with increasing latitude in the BS. This apparent stability in BS in lower TP δ 15 N values is in contrast to persistent latitudinal gradients in δ 15 N values in the North Sea and at West-Greenland (Jennings et al, 2008;Hansen et al, 2012;MacKenzie et al, 2014).…”

Section: Variability In Trophic Positions and δ 15 N Values Related T...mentioning

confidence: 87%

“…producer in the BS (Sakshaug et al, 1994), and nitrate is the major source of nitrogen for phytoplankton during the spring/summer bloom in the BS (Tamelander et al, 2009). In support of limited range in baseline δ 15 N value at both low and high latitudes within the BS, seawater nitrate δ 15 N had stable values of c. 5.1 for both Atlantic and Arctic water in the BS (Tuerena et al, 2020). Tuerena et al (2020) found no clear trend in δ 15 N values for nitrate or for particulate nitrogen with increasing latitude in the BS.…”

Section: Variability In Trophic Positions and δ 15 N Values Related T...mentioning

confidence: 87%

Comparison Between Trophic Positions in the Barents Sea Estimated From Stable Isotope Data and a Mass Balance Model

Pedersen

2022

Front. Mar. Sci.

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The trophic position concept is central in system ecology, and in this study, trophic position (TP) estimates from stable-isotopes and an Ecopath mass-balance food web model for the Barents Sea were compared. Two alternative models for estimating TP from stable isotopes, with fixed or scaled trophic fractionation were applied. The mass-balance model was parametrized and balanced for year 2000, was comprised of 108 functional groups (Gs), and was based on biomass and diet data largely based on predator stomach data. Literature search for the Barents Sea Large Marine Ecosystem revealed 93 sources with stable isotope data (δ15N values) for 83 FGs, and 25 of the publications had trophic position estimated from nitrogen stable isotopes. Trophic positions estimated from the mass-balance model ranged to 5.1 TP and were highly correlated with group mean δ15N values, and also highly correlated with the original literature estimates of trophic positions from stable isotopes. On average, TP from the mass-balance model was 0.1 TP higher than the original literature TP estimates (TPSIR) from stable isotopes. A trophic enrichment factor (TEF) was estimated assuming fixed fractionation and minimizing differences between trophic positions from Ecopath and TP predicted from δ15N values assuming a baseline value for δ15N calculated for pelagic particulate organic matter at a baseline TP of 1.0. The estimated TEF of 3.0‰ was lower than the most commonly used TEF of 3.4 and 3.8‰ in the literature. The pelagic whales and pelagic invertebrates functional groups tended to have higher trophic positions from Ecopath than from stable isotopes while benthic invertebrate functional groups tended to show an opposite pattern. Trophic positions calculated using the scaled trophic fractionation approach resulted in lower TP than from Ecopath for intermediate TPs and also a larger TP range in the BS. It is concluded that TPs estimated from δ15N values using a linear model compared better to the Ecopath model than the TPs from scaled fractionation approach.

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Nitrate assimilation and regeneration in the Barents Sea: insights from nitrogen isotopes

Cited by 2 publications

References 62 publications

An Arctic Strait of Two Halves: The Changing Dynamics of Nutrient Uptake and Limitation Across the Fram Strait

An Arctic Strait of Two Halves: The Changing Dynamics of Nutrient Uptake and Limitation Across the Fram Strait

Comparison Between Trophic Positions in the Barents Sea Estimated From Stable Isotope Data and a Mass Balance Model

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