Nitrate assimilation and regeneration in the Barents Sea: insights from nitrate isotopes
<p>While the entire Arctic Ocean is warming rapidly, the Barents Sea in particular is experiencing significant warming and sea ice retreat. An increase in ocean heat transport from the Atlantic is causing the Barents Sea to be transformed from a cold, salinity-stratified system into a warmer,...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-01-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/18/637/2021/bg-18-637-2021.pdf |
| Summary: | <p>While the entire Arctic Ocean is warming rapidly, the
Barents Sea in particular is experiencing significant warming and sea ice
retreat. An increase in ocean heat transport from the Atlantic is causing
the Barents Sea to be transformed from a cold, salinity-stratified system
into a warmer, less-stratified Atlantic-dominated climate regime.
Productivity in the Barents Sea shelf is fuelled by waters of Atlantic
origin (AW) which are ultimately exported to the Arctic Basin. The
consequences of this current regime shift on the nutrient characteristics of
the Barents Sea are poorly defined. Here we use the stable isotopic ratios
of nitrate (<span class="inline-formula"><i>δ</i><sup>15</sup></span>N-<span class="inline-formula">NO<sub>3</sub></span>, <span class="inline-formula"><i>δ</i><sup>18</sup></span>O-<span class="inline-formula">NO<sub>3</sub></span>) to determine the
uptake and modification of AW nutrients in the Barents Sea. In summer
months, phytoplankton consume nitrate, surface waters become nitrate
depleted, and particulate nitrogen (<span class="inline-formula"><i>δ</i><sup>15</sup></span>N-PN) reflects the AW
nitrate source. The ammonification of organic matter in shallow sediments
resupplies N to the water column and replenishes the nitrate inventory for
the following season. Low <span class="inline-formula"><i>δ</i><sup>18</sup></span>O-<span class="inline-formula">NO<sub>3</sub></span> in the northern Barents Sea
reveals that the nitrate in lower-temperature Arctic waters is <span class="inline-formula">></span> 80 % regenerated through seasonal nitrification. During on-shelf nutrient
uptake and regeneration, there is no significant change to <span class="inline-formula"><i>δ</i><sup>15</sup></span>N-<span class="inline-formula">NO<sub>3</sub></span> or <span class="inline-formula"><i>N</i><sup>*</sup></span>, suggesting that benthic denitrification does not impart an
isotopic imprint on pelagic nitrate. Our results demonstrate that the
Barents Sea is distinct from other Arctic shelves where benthic
denitrification enriches <span class="inline-formula"><i>δ</i><sup>15</sup></span>N-<span class="inline-formula">NO<sub>3</sub></span> and decreases <span class="inline-formula"><i>N</i><sup>*</sup></span>. As
nutrients are efficiently recycled in the Barents Sea and there is no
significant loss of N through benthic denitrification, changes to Barents
Sea productivity are unlikely to alter N availability on shelf or the
magnitude of N advected to the central Arctic Basin. However, we suggest
that the AW nutrient source ultimately determines Barents Sea productivity
and that changes to AW delivery have the potential to alter Barents Sea primary
production and subsequent nutrient supply to the central Arctic Ocean.</p> |
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| ISSN: | 1726-4170 1726-4189 |