234 research outputs found
Net Community Production and Carbon Exchange From Winter to Summer in the Atlantic Water Inflow to the Arctic Ocean
The eastern Fram Strait and area north of Svalbard, are influenced by the inflow of warm Atlantic water, which is high in nutrients and CO2, influencing the carbon flux into the Arctic Ocean. However, these estimates are mainly based on summer data and there is still doubt on the size of the net ocean Arctic CO2 sink. We use data on carbonate chemistry and nutrients from three cruises in 2014 in the CarbonBridge project (January, May, and August) and one in Fram Strait (August). We describe the seasonal variability and the major drivers explaining the inorganic carbon change (CDIC) in the upper 50 m, such as photosynthesis (CBIO), and air-sea CO2 exchange (CEXCH). Remotely sensed data describes the evolution of the bloom and net community production. The focus area encompasses the meltwater-influenced domain (MWD) along the ice edge, the Atlantic water inflow (AWD), and the West Spitsbergen shelf (SD). The CBIO total was 2.2 mol C mâ2 in the MWD derived from the nitrate consumption between January and May. Between January and August, the CBIO was 3.0 mol C mâ2 in the AWD, thus CBIO between May and August was 0.8 mol C mâ2. The ocean in our study area mainly acted as a CO2 sink throughout the period. The mean CO2 sink varied between 0.1 and 2.1 mol C mâ2 in the AWD in August. By the end of August, the AWD acted as a CO2 source of 0.7 mol C mâ2, attributed to vertical mixing of CO2-rich waters and contribution from respiratory CO2 as net community production declined. The oceanic CO2 uptake (CEXCH) from the atmosphere had an impact on CDIC between 5 and 36%, which is of similar magnitude as the impact of the calcium carbonate (CaCO3, CCALC) dissolution of 6â18%. CCALC was attributed to be caused by a combination of the sea-ice ikaite dissolution and dissolution of advected CaCO3 shells from the south. Indications of denitrification were observed, associated with sea-ice meltwater and bottom shelf processes. CBIO played a major role (48â89%) for the impact on CDIC.publishedVersio
Cold-Water Coral Reefs in the Langenuen Fjord, Southwestern Norway â A Window into Future Environmental Change
Ocean warming and acidification pose serious threats to cold-water corals (CWCs) and the surrounding habitat. Yet, little is known about the role of natural short-term and seasonal environmental variability, which could be pivotal to determine the resilience of CWCs in a changing environment. Here, we provide continuous observational data of the hydrodynamic regime (recorded using two benthic landers) and point measurements of the carbonate and nutrient systems from five Lophelia pertusa reefs in the Langenuen Fjord, southwestern Norway, from 2016 to 2017. In this fjord setting, we found that over a tidal (12 °C) than the mean conditions and high CT concentrations of 20 ”mol kgâ1 over the suggested threshold for healthy CWC reefs (i.e., >2170 ”mol kgâ1). Combined with hindcast measurements, our findings indicate that these shallow fjord reefs may act as an early hotspot for ocean warming and acidification. We predict that corals in Langenuen will face seasonally high temperatures (>18 °C) and hypoxic and corrosive conditions within this century. Therefore, these fjord coral communities could forewarn us of the coming consequences of climate change on CWC diversity and function
Long-Term and Seasonal Trends in Estuarine and Coastal Carbonate Systems
Coastal pH and total alkalinity are regulated by a diverse range of local processes superimposed on global trends of warming and ocean acidification, yet few studies have investigated the relative importance of different processes for coastal acidification. We describe long-term (1972-2016) and seasonal trends in the carbonate system of three Danish coastal systems demonstrating that hydrological modification, changes in nutrient inputs from land, and presence/absence of calcifiers can drastically alter carbonate chemistry. Total alkalinity was mainly governed by conservative mixing of freshwater (0.73-5.17mmolkg(-1)) with outer boundary concentrations (similar to 2-2.4mmolkg(-1)), modulated seasonally and spatially (similar to 0.1-0.2mmolkg(-1)) by calcifiers. Nitrate assimilation by primary production, denitrification, and sulfate reduction increased total alkalinity by almost 0.6mmolkg(-1) in the most eutrophic system during a period without calcifiers. Trends in pH ranged from -0.0088year(-1) to 0.021year(-1), the more extreme of these mainly driven by salinity changes in a sluice-controlled lagoon. Temperature increased 0.05 degrees Cyr(-1) across all three systems, which directly accounted for a pH decrease of 0.0008year(-1). Accounting for mixing, salinity, and temperature effects on dissociation and solubility constants, the resulting pH decline (0.0040year(-1)) was about twice the ocean trend, emphasizing the effect of nutrient management on primary production and coastal acidification. Coastal pCO(2) increased similar to 4 times more rapidly than ocean rates, enhancing CO2 emissions to the atmosphere. Indeed, coastal systems undergo more drastic changes than the ocean and coastal acidification trends are substantially enhanced from nutrient reductions to address coastal eutrophication.Peer reviewe
Shell density of planktonic foraminifera and pteropod species Limacina helicina in the Barents Sea: Relation to ontogeny and water chemistry
Planktonic calcifiers, the foraminiferal species Neogloboquadrina pachyderma and Turborotalita quinqueloba, and the thecosome pteropod Limacina helicina from plankton tows and surface sediments from the northern Barents Sea were studied to assess how shell density varies with depth habitat and ontogenetic processes. The shells were measured using X-ray microcomputed tomography (XMCT) scanning and compared to the physical and chemical properties of the water column including the carbonate chemistry and calcium carbonate saturation of calcite and aragonite. Both living L. helicina and N. pachyderma increased in shell density from the surface to 300 m water depth. Turborotalita quinqueloba increased in shell density to 150â200 m water depth. Deeper than 150 m, T. quinqueloba experienced a loss of density due to internal dissolution, possibly related to gametogenesis. The shell density of recently settled (dead) specimens of planktonic foraminifera from surface sediment samples was compared to the living fauna and showed a large range of dissolution states. This dissolution was not apparent from shell-surface texture, especially for N. pachyderma, which tended to be both thicker and denser than T. quinqueloba. Dissolution lowered the shell density while the thickness of the shell remained intact. Limacina helicina also increase in shell size with water depth and thicken the shell apex with growth. This study demonstrates that the living fauna in this specific area from the Barents Sea did not suffer from dissolution effects. Dissolution occurred after death and after settling on the sea floor. The study also shows that biomonitoring is important for the understanding of the natural variability in shell density of calcifying zooplankton.publishedVersio
Hydrography, inorganic nutrients and chlorophyll a linked to sea ice cover in the Atlantic Water inflow region north of Svalbard
Changes in the inflow of Atlantic Water (AW) and its properties to the Arctic Ocean bring more warm water,
contribute to sea ice decline, promote borealisation of marine ecosystems, and affect biological and particularly
primary productivity in the Eurasian Arctic Ocean. One of the two branches of AW inflow follows the shelf
break north of Svalbard, where it dominates oceanographic conditions, bringing in heat, salt, nutrients and
organisms. However, the interplay with sea ice and Polar Surface Water (PSW) determines the supply of
nutrients to the euphotic layer especially northeast of Svalbard where AW subducts below PSW. In an effort to
build up a time series monitoring the key characteristics of the AW inflow, repeat sampling of hydrography,
macronutrients (nitrate, phosphate and silicate), and chlorophyll a (chl a) was undertaken along a transect
across the AW inflow at 31âŠE, 81.5âŠN since 2012 â first during late summer and in later years during early
winter. Such time series are scarce but invaluable for investigating the range of variability in hydrography and
nutrient concentrations. We investigate linkages between late summer hydrographic conditions and nutrient
concentrations along the transect and the preceding seasonal dynamics of surface chl a and sea ice cover in the
region north of Svalbard. We find large interannual variability in hydrography, nutrients and chl a, indicating
varying levels of nutrient drawdown by primary producers over summer. Sea ice conditions varied considerably
between the years, impacting upper ocean stratification, light availability and potential wind-driven mixing,
with a strong potential for steering chl a concentration over the productive season. Early winter measurements
show variable efficiency of nutrient re-supply through vertical mixing when stratification was low, related
to autumn wind forcing and sea ice conditions. While this re-supply elevates nutrient levels sufficiently for
primary production, it likely happens too late in the season when light levels are already low, limiting the
potential for autumn blooms. Such multidisciplinary observations provide insight into the interplay between
physical, chemical and biological drivers in the marine environment and are key to understanding ongoing
and future changes, especially at this entrance to the central Arctic Ocean
Seasonal dynamics of the marine CO2 system in Adventfjorden, a West Spitsbergen fjord
Time series of the marine CO2 system and related parameters at the IsA Station, by Adventfjorden, Svalbard, were investigated between March 2015 and November 2017. The physical and biogeochemical processes that govern changes in total alkalinity (TA), total dissolved inorganic carbon (DIC) and the saturation state of the calcium carbonate mineral aragonite (ΩAr) were assessed on a monthly timescale. The major driver for TA and DIC was changes in salinity, caused by river runoff, mixing and advection. This accounted for 77 and 45%, respectively, of the overall variability. It contributed minimally to the variability in ΩAr (5%); instead, biological activity was responsible for 60% of the monthly variations. For DIC, the biological activity was also important, contributing 44%. The monthly effect of airâsea CO2 fluxes accounted for 11 and 15% of the total changes in DIC and ΩAr, respectively. Net community production (NCP) during the productive season ranged between 65 and 85 g C mâ2, depending on the year and the presence of either Arctic water or transformed Atlantic water (TAW). The annual NCP as estimated from DIC consumption was 34 g C mâ2 yrâ1 in 2016, which was opposite in direction but similar in magnitude to the integrated annual airâsea CO2 flux (i.e., uptake of carbon from the atmosphere) of â29 g C mâ2 yrâ1 for the same year. The results showed that increased intrusions of TAW into Adventfjorden in the future could possibly lower the NCP, with the potential to reduce the CO2buffer capacity and ΩAr over the summer season.publishedVersio
Methane release from open leads and new ice following an Arctic winter storm event
We examine an Arctic winter storm event, which led to ice breakâup, the formation of open leads, and the subsequent freezing of these leads. The methane (CH4) concentration in underâice surface water before and during the storm event was 8â12 nmol Lâ1, which resulted in a potential seaâtoâair CH4 flux ranging from +0.2 to +2.1 mg CH4 mâ2 dâ1 in open leads. CH4 ventilation between seawater and atmosphere occurred when both open water fraction and wind speed increased. Over the nine days after the storm, sea ice grew 27 cm thick. Initially, CH4 concentrations in the sea ice brine were above the equilibrium with the atmosphere. As the ice grew thicker, most of the CH4 was lost from upper layers of sea ice into the atmosphere, implying continued CH4 evasion after the leads were iceâcovered. This suggests that wintertime CH4 emissions need to be better constrained
Diffusive and advective cross-frontal fluxes of inorganic nutrients and dissolved inorganic carbon in the Barents Sea in autumn
The Atlantic Water, entering the Arctic through the Barents Sea and Fram Strait, is the main source of
nutrients in the Arctic Ocean. The Barents Sea is divided by the Polar Front into an Atlantic-dominated domain
in the south, and an Arctic-dominated domain in the north. The Polar Front is a thermohaline structure,
which is topographically-steered at sub-surface, and influenced by the seasonal sea ice edge near the surface.
Exchanges of nutrients between the inflowing Atlantic Water and the surrounding waters are key for the
primary production in the Barents Sea. In October 2020, we measured nutrients (nitrate, phosphate and
silicic acid), dissolved inorganic carbon (DIC), ocean stratification, currents and turbulence in the vicinity
of the Polar Front in the Barents Sea within the framework of the Nansen Legacy project, allowing estimates
of horizontal and vertical advective fluxes and turbulent fluxes of nitrate and DIC. We studied the autumn
situation when primary production was declining. We found a substantial transfer of nitrate and DIC across
the Polar Front from the Atlantic domain to the Arctic domain. Up to one quarter of the replenishment of
the nitrate in the mixed layer during winter could be attributed to vertical mixing during wind events, shared
approximately equally between advective and turbulent fluxes. The vertical turbulent fluxes bring nutrients
from the subsurface Atlantic Water to the surface. We also identified an export of nitrate and DIC from the
Barents Sea to the Nordic Seas occurring along the eastern shelf of Svalbard. Our study shows the role of
vertical fluxes in fall and winter to precondition for the following spring bloom
Ocean acidification state variability of the Atlantic Arctic Ocean around northern Svalbard
The Svalbard shelf and Atlantic Arctic Ocean are a transition zone between northward flowing Atlantic Water and ice-covered waters of the Arctic. Effects of regional ocean warming, sea ice loss and greater influence of Atlantic Water or âAtlantificationâ on the state of ocean acidification, i.e. calcium carbonate (CaCO3) saturation (Ω) are yet to be fully understood. Anomalies in surface layer Ω for the climatically-vulnerable CaCO3 mineral aragonite (ÎΩ) were determined by considering the variability in Ωaragonite during late summer each year from 2014 to 2017 relative to the four-year average. Greatest sea ice extent and more Arctic-like conditions in 2014 resulted in ÎΩ anomalies of â0.05 to â0.01 (up to 45% of total ÎΩ) as a result of lower primary production. Conversely, greater Atlantic Water influence in 2015 supplied the ice-free surface layer with nitrate, which prolonged primary production to drive ÎΩ anomalies of 0.01 to 0.06 (up to 45% of total ÎΩ) in more Atlantic-like conditions. Additionally, dissolution of CaCO3 increased carbonate ion concentrations giving ÎΩ anomalies up to 0.06 (up to 52% of total ÎΩ). These processes enhanced surface water Ω, which ranged between 2.01 and 2.65 across the region. Recent sea ice retreat in 2016 and 2017 (rate of decrease in ice cover of âŒ4% in 30 days) created transitional Atlantic-Arctic conditions, where surface water Ω varied between 1.87 and 2.29 driven by ÎΩ anomalies of â0.10 to 0.01 due to meltwater inputs and influence of Arctic waters. Anomalies as low as â0.12 from reduced CaCO3 dissolution in 2016 further supressed Ω. Wind-driven mixing in 2017 entrained Atlantic Water with low Ω into the surface layer to drive large ÎΩ anomalies of â0.15 (up to 58% of ÎΩ). Sea-ice meltwater provided a minor source of carbonate ions, slightly counteracting dilution effects. Ice-free surface waters were substantial sinks for atmospheric CO2, where uptake of 20.5 mmol mâ2 dayâ1 lowered surface water Ω. âAtlantificationâ could exacerbate or alleviate acidification of the Arctic Ocean, being highly dependent on the numerous factors examined here that are intricately linked to the sea ice-ocean system variability.publishedVersio
Methane release from open leads and new ice following an Arctic winter storm event
We examine an Arctic winter storm event, which led to ice breakâup, the formation of open leads, and the subsequent freezing of these leads. The methane (CH4) concentration in underâice surface water before and during the storm event was 8â12 nmol Lâ1, which resulted in a potential seaâtoâair CH4 flux ranging from +0.2 to +2.1 mg CH4 mâ2 dâ1 in open leads. CH4 ventilation between seawater and atmosphere occurred when both open water fraction and wind speed increased. Over the nine days after the storm, sea ice grew 27 cm thick. Initially, CH4 concentrations in the sea ice brine were above the equilibrium with the atmosphere. As the ice grew thicker, most of the CH4 was lost from upper layers of sea ice into the atmosphere, implying continued CH4 evasion after the leads were iceâcovered. This suggests that wintertime CH4 emissions need to be better constrainedMethane release from open leads and new ice following an Arctic winter storm eventacceptedVersio
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