The European Arctic Corridor is the main gateway to the Arctic Ocean, where 80% of both in- and outflow takes place. One of the most prominent feature of the European Arctic Corridor is the northward flowing North Atlantic Waters entering the Arctic Ocean through two main branches, which separate around 70°N. One branch keeps flowing northward toward Fram Strait while the second one turns eastward into the Barents Sea. Over the last three decades, in a context of amplified warming and sea-ice loss, substantial changes have been documented in the North Atlantic Water inflow with a twofold increase of its volume occupation and a northward shift of the polar front structure in the Barents Sea. The North Atlantic Water inflow largely controls the physical and sea-ice conditions of the region In addition, almost 50% of the total Arctic primary production takes place in the European Arctic Corridor, which is of major importance for fisheries. Recent warming of the European Arctic Corridor related to the North Atlantic Water inflow has been suspected to trigger poleward intrusions of temperate phytoplankton and species from higher trophic levels. By carrying biomass and nutrients produced elsewhere, bioadvection has recently been proposed as an 'essential mechanism' for ecosystem dynamics in the Arctic Ocean. Although the actual role of advection has already been identified as a potential driver altering zooplankton dynamics, it has never been assessed quantitatively from observations for phytoplankton in the European Arctic Corridor, or more generally in the Arctic Ocean.
In a paper published in the journal Nature Communications on 10 April 2020, Laurent Oziel of the Ocean and Ecosystem Sciences Division at Fisheries and Oceans Canada, the joint Takuvik International Research Laboratory of Université Laval and the Centre National de la Recherche Scientifique, and the Laboratoire d’Océanographie de Villefranche-sur-Mer of Sorbonne Université, Alberto Baudena, also of the Laboratoire d’Océanographie de Villefranche-sur-Mer, Mathieu Ardyna also of the Laboratoire d’Océanographie de Villefranche-sur-Mer, and of the Department of Earth System Science at Stanford University, Philippe Massicotte and Achim Randelhoff, also of the Takuvik International Research Laboratory, Jean-Baptiste Sallée, again of Sorbonne Université, Randi Ingvaldsen of the Institute of Marine Research, Emmanuel Devred, also of the Ocean and Ecosystem Sciences Division at Fisheries and Oceans Canada, and Marcel Babin, again of the Takuvik International Research Laboratory, present the results of an investigation into how ocean currents control the spatial dynamics of a specific Coccolithophore bloom-forming species, Emiliania huxleyi.
The European Arctic Corridor. Bathymetry and surface circulation. The Atlantic currents are in red, the Arctic or Polar Waters are in blue and the Coastal Waters are in green. The southern Barents Sea Polar Front is illustrated in black dashed line and separates the Atlantic Waters from the colder and fresher waters from the North. Oziel et al. (2020).
Emiliania huxleyi is usually associated with the surface layer of the temperate North Atlantic Waters in summer, typically in a post-spring bloom context characterized by low nutrients, low light and strong stratification. Since Emiliania huxleyi does not form winter resting spores, this tracer of Atlantic ecosystem is generally considered to be a summer visitor in the Barents Sea, unlike neritic Diatoms species. A combination of bottom-up (i.e. winter darkness, cold temperatures and intense vertical mixing) and top-down controls (i.e. zooplankton grazing, viral lysis) prevent Emiliania huxleyi from year-to-year survival in the north-easternmost parts of the Barents Sea. Because of their high abundance in Emiliania huxleyi (115 000 000 cells per litre), coastal regions and fjords of the Norwegian Sea have been suspected to be the source of Emiliania huxleyi for the whole European Arctic Corridor.
Using a Lagrangian tracking approach based on satellite derived current fields, Oziel et al. explored how the advection of Emiliania huxleyi cells from these upstream coastal temperate regions shapes the distribution of the massive Emiliania huxleyi blooms in the Barents Sea (via the 'seeding effect'). By combining satellite-derived altimetry with ocean-color Particulate Inorganic Carbon (a Coccolithophore biomass proxy) estimates, Oziel et al. demonstrate a major role of bio-advection in phytoplankton transport along the European Arctic Corridor.
An individual Emiliania huxleyi cell. Stig Haugen/Institute of Marine Research.
To document the interannual and decadal variability of surface North Atlantic Water currents, Oziel et al. first performed an statistical analysis of Sea Level Anomalies from 1993 to 2016, on which the seasonal signal was removed. The analysis showed that the two variables accounted for more than 80% of the non-seasonal variability. A dipole structure emerged, centered on the North Atlantic Waters path between the Barents and Norwegian Sea shelves (east of 5°E) and the center of the Norwegian Sea (west of 5°E). The associated time-series showed a linear positive trend over the last 24 years with high energy at the interannual and decadal time scale. This trend toward a more positive phase corresponds to an increase of the sea level gradient across the North Atlantic Waters path, hence intensifying North Atlantic Waters currents, while negative phases are associated with weaker North Atlantic Waters currents.
Statistical analysis of Monthly averaged Sea Level Anomalies in the European Arctic Corridor. The sum of the first two modes account for more than 80% of the total variability. Oziel et al. (2020).
To reveal the impact of changing sea level on surface velocity fields, absolute surface geostrophic velocities, derived from Absolute Dynamic Topography fields were mapped during the extreme negative (1993) and positive (2015) phases of the analysis. These two contrasting years illustrate the ongoing strengthening of the ocean surface circulation in the European Arctic Corridor associated with the Atlantic inflow. Surface geostrophic current linear trends showed that surface currents changed in most of the Atlantic pathway In the Norwegian Sea, the increase in the Atlantic surface current speed reached 2 mm per second per year, corresponding to a relative increase ranging from 30% on the shelf to 100% in the basin. The surface currents in the eastern Barents Sea-Atlantic corridor also significantly increased by about 6 mm per second per year, an increase in flow rate of about 14%. The positive phase associated with stronger north-eastward North Atlantic Waters surface currents since year 2000 reflects an increase in advection at or near the surface. This result is consistent with the recent trends in North Atlantic Waters current velocities observed upstream in the Nordic Seas or modelled in the Barents Sea.
Surface absolute geostrophic velocities during the extremums of the timeseries which are, respectively, reached in (a) December 1993 (minimum) and (b) 2015 (maximum) with (c|) the corresponding absolute linear trend of the entire time-series (all months) over the 1993–2016 period. Areas covered by sea ice (sea-ice concentration over 15%) or with insufficient data coverage for the trend (less than 50%) are in dark gray. Oziel et al. (2020).
The interannual fluctuations as well as the long-term trend observed in the time-series of surface geostrophic velocities are mainly attributed to the dynamics of the North Atlantic Subpolar Gyre, and to the atmospheric forcing of the North Atlantic Oscillation, which are both tightly coupled. This suggests that the increase in surface advection in the European Arctic Corridor is likely due to a natural multidecadal oscillation related to the upstream synoptic oceanic circulation and atmospheric forcing, which could, in turn, drive long-term climatic change in the Barents Sea.
The first hypothesis that ocean currents control the summer spatial distribution of Emiliania huxleyi in the Barents Sea was tested using a Lagrangian model. Oziel et al. advected virtual particles (considered as the inoculum of Emiliania huxleyi cells), using observations of surface geostrophic velocities, from their expected overwintering location in March (defined by sea surface temperatures greater than 4°C and distance from coast no more than 180 km). This model revealed that the spatial distribution of the virtual particles at the end of the advection period matched the extent of Emiliania huxleyi blooms (evidenced by satellite-derived Particulate Inorganic Concentration), with 80% of tracked particles ending up within 50 km of an Emiliania huxleyi blooming location. The year-to-year robustness of the matchup between virtual particles and Particulate Inorganic Concentration clearly supports the fact that the North Atlantic Waters surface currents shape the location and extent of Emiliania huxleyi blooms in the European Arctic Corridor.
Poleward expansion of Emiliania huxleyi (EHux) in the European Arctic Corridor. Comparison between 1998 (a) and 2015 (b). The initialisation (inoculum) of virtual particles in March are illustrated by brown dots. During 6 months, particles drift with the Norwegian Atlantic Current (red arrows) as the ocean seasonally warms as illustrated by the northward expansion of the 4 °C isotherm. In August, the particles end up in positions indicated by the red dots. In the background, remotely sensed Particulate Inorganic Concentration indicating Coccolithophore biomass in summer (July–August–September) is shown in blue colors. Areas with no data are in dark gray. Oziel et al. (2020).
It is noteworthy that the particles reached further north and east in the European Arctic Corridor in 2015 than in 1998, in agreement with Emiliania huxleyi blooms. The north-eastward expansion of the Emiliania huxleyi bloom, expressed here as the distance reached by the leading-edge increased on average by 325 km during the last 19 years as indicated by ocean-color observations, in close agreement with previous estimations (324 km, 40–50°E, 1989–2016) and with the estimates from Oziel et al.'s model.
The 4 °C surface isotherm is considered for Emiliania huxleyi as the lowest temperature required for sufficient growth to allow bloom formation. However, the Emiliania huxleyi blooms do not seem to follow this isotherm in summer and appear to be constrained by another factor. For example, the north-eastward expansion of the winter 4°C isotherm, which delimits the areal extent of the inoculum in Oziel et al.'s model, could contribute to reduce the distance between the Emiliania huxleyi winter location and the Arctic domain. To test this hypothesis, Oziel et al created two additional models to examine both the role of currents and winter temperature on the poleward expansion of Emiliania huxleyi. In the second model Oziel et al. constantly initialised the virtual particles at the same inoculation region for all years, using a climatological mean temperature field to determine the inoculation region. Hence, the second model exclusively reflected the role of currents on the interannual variability of the advected particles. In contrast, in the third model, the constant inoculation region varied from one year to another, but Oziel et al used a climatological mean current to advect the particles. In this way, the interannual variability of the advected particles due to interannual variability of the inoculation region was quantified. The interannual Particulate Inorganic Concentration leading-edge location was highly correlated with the leading-edge from the second model, suggesting a stronger control of the bloom expansion by currents than by winter temperature. On the decadal scale, currents were found responsible for the 56% (240 km) increase in the long-term leading-edge expansion against 44% (186 km) for winter temperature, when compared to the first model. This significant increasing trend in current velocities was also revealed by the greater distance covered by the virtual particles reaching the Barents Sea, which increased by 110 km on average since 1993.
Shifting position of the leading-edge Emiliania huxleyi bloom distribution. Shifting position from ocean-colour Particulate Inorganic Concentration (a), and the three models (b)–(d) for the last 19-years (1998–2016). The comparison between the first model (EXP1), (b) with the second (EXP2), (c) and the third (EXP3), (d) aims at estimating the relative contribution of currents (EXP2, constant temperature) vs. temperature (EXP3, constant currents) on the total Emiliania huxleyi poleward expansion (EXP1, varying temperature and currents). The right panel is a schematic illustration of the poleward expansion of the Emiliania huxleyi with the winter 4°C isotherm (lowest temperature for a ‘regular’ Emiliania huxleyi growth) in blue and the summer bloom position (northern boundary) in red. The two extreme years 1998 (dashed) and 2015 (solid) are represented. Arrows indicate the contribution from temperature and/or currents keeping the same color code. Oziel et al. (2020).
Increasing water temperature has previously been assumed to be the main driver of the spatial distribution of Emiliania huxleyi blooms in the Barents Sea. Oziel et al.'s results demonstrate that the primary driver of the Emiliania huxleyi dynamics (i.e., spatial distribution and timing) could be, in fact, stronger surface currents, which in turn intrinsically shape the temperature field and frontal structures. Oziel et al. show that oceanic currents (i.e. their intensity and fluctuations) drive the spatial distribution of the bloom, its interannual variability and more than 50% of the long-term poleward expansion of Emiliania huxleyi bloom in the Barents Sea. More importantly, from 2006 and onward, the contribution of water temperature to the expansion of Emiliania huxleyi blooms becomes negligible, and its poleward expansion is entirely due to the accelerating currents.
Emiliania huxleyi is largely studied for its significant role in marine geochemical cycles, as illustrated by its sensitivity to ocean acidification, its effect on carbon dioxide pressure and carbon dioxide uptake, and its role on carbon export by providing calcite ballast effect. In Oziel et al.'s study Emiliania huxleyi was used as an indicator of Atlantic ecosystems. By expanding poleward and doubling its areal extent in the Barents Sea, Emiliania huxleyi attests to the ongoing 'Atlantification' of the Arctic Ocean. Both arctic and Atlantic domains have distinct ecological signatures, and the latter is undeniably 'invading' the former. Advected with the surface currents, Emiliania huxleyi will have to survive during the travel (for example avoiding grazing by zooplankton, and subduction under the polar mixed layer in the Fram Strait) until finding more favourable blooming conditions in the Barents Sea in summer. The fate of Emiliania huxleyi in the Barents Sea is therefore of major importance as it determines the potential 'seeding effect' of Emiliania huxleyi in the Arctic regions. Emiliania huxleyi seems to be adapted to the low light, low nutrient, oligotrophic and highly stratified conditions of the North Atlantic Waters in summer such that its expansion, growth and blooming in the Arctic Ocean will be limited at some point by those constraints (bottom-up) but also by the grazing pressure (top-down).
Despite its adaptation to low light conditions, Emiliania huxleyi still requires sufficient light levels to sustain the energy-demanding calcification of its coccoliths. Such conditions are met in highly stratified oceans where Emiliania huxleyi can accumulate in the surface layer. At high latitudes (more than 81°N), even with sea surface temperature above 3 or 4 °C, the survival of Emiliania huxleyi would require adaptation to rapidly decreasing solar radiations in late summer. Emiliania huxleyi's fate thus mainly relies on its ability to drift, with the appropriate timing, to highly stratified and temperate areas that allow it to stay in the surface euphotic layer. These conditions would likely be met in the Eurasian interior shelves of the Arctic Ocean (i.e. the Kara, Laptev, and Siberian seas) where surface waters are warming and freshening. If the increase in advection along the shelf slope continues in the future, Oziel et al. expect Emiliania huxleyi to become a summer resident of the newly 'Atlantified; Eurasian interior shelves, as previously revealed during the last interglacial.
By driving such a poleward expansion, advective processes could affect the entire marine ecosystems by shifting species distribution and modifying interactions at higher trophic levels. The concomitant decline of silicate concentrations in North Atlantic Waters may also contribute to the success of non-silicifying and small phytoplankton such as Emiliania huxleyi. n addition, a change toward temperate pelagic species could have an impact on energy transfer to higher trophic levels, including Marine Mammals and commercial Fish stocks. Considering the role of 'bio-advection' in ecological models (i.e., trait-based and niche-based approaches) must improve predictions of community shifts. The comparable increase in poleward advection of Pacific waters occurring in the Bering Strait suggests that the shrinking polar domain of the Arctic Ocean may be prone to intrusions of temperate species at a pan-Arctic scale.
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