Winter-Spring Development of the Zooplankton Community Below Sea Ice in the Arctic Ocean

The impact of the rapidly changing Arctic on zooplankton community structure and seasonal behaviour is not yet understood. Here we examine 6 months of under-ice zooplankton observations from the N-ICE2015 expedition (January to June 2015) in the Nansen Basin and on the Yermak Plateau north of Svalbard. Stratified sampling in the water column was done with MultiNet during the entire expedition, and sampling in the upper 5 m below sea ice was performed during April-May by divers using a hand-held net. Hydrographic conditions were dominated by northward-flowing warm and saline Atlantic Water at intermediate depth, and southward-flowing cold Polar Surface Water in the upper 100 m. The mesozooplankton was dominated by copepods. Most numerous were the small ubiquitous Oithona similis in the upper 200 m, with Microcalanus spp. and Triconia borealis further down the water column. Calanus finmarchicus dominated among the Calanus species while Metridia longa was also numerous. The most abundant deep-water copepods were Paraeuchaeta spp. and Spinocalanus spp. Arrow worms (Chaetognatha) and comb jellies (Ctenophora) were the most numerous non-copepods. The mesozooplankton community was more dependent on surrounding water mass characteristics, such as salinity and depth, than geographical location. Algal food availability, which was closely linked to seasonality, explained the community changes seen in surface waters in May and June due to seasonal ascent and recruitment. Seasonal changes from winter to spring mostly involved an increase in the herbivorous C. finmarchicus and its nauplii in the upper 200 m of the water column coinciding with the peak of the phytoplankton bloom in late May. The Yermak Plateau and adjacent Nansen Basin were characterised by oceanic North Atlantic and Arctic species, many of which are deep water specialists. Despite the late onset of the spring bloom due to consolidated sea ice, both North Atlantic and Arctic species successfully reproduced in the study area. This explains the species-rich mesozooplankton community in this region as opposed to the less productive central Arctic Ocean. Future prospects of less sea ice and earlier onset of the bloom will likely be positive for the overall secondary production by both Arctic and boreal zooplankton in this region.

Nonlinear internal wave generation over the Yermak Plateau

<p>North of the critical latitude (78.4), internal waves of the M<sub>2</sub> tidal frequency can no longer freely propagate, and the energy conversion from the barotropic to the internal tides vanishes. Near the continental slopes around the Arctic Ocean, internal wave energy is enhanced and comparable to values at mid-latitudes (Rippeth et al. 2015, Levine et al. 1985). Observations on the northern flank of the Yermak Plateau (YP) has characterized the region as one of enhanced internal wave activity and nonlinear internal waves have been observed (Czipott et al. 1991, Padman and Dillon 1991).</p><p>The YP is a bathymetry feature stretching out into the Fram Strait north-west of Svalbard. The YP plays a prominent role in the Arctic’s heat balance due to its interaction with the West-Spitsbergen current which is a main contributor to the heat transport into the Arctic Ocean. Nonlinear waves generated over the YP are a significant energy source for mixing and can therefore modulate and force exchange processes.</p><p>To study the nonlinear internal wave generation mechanisms over the YP, we used a high resolution, nonlinear, non-hydrostatic model. We found that nonlinear internal waves are forced not by the M<sub>2</sub> but the K<sub>1</sub> tide which has been observed to have significant variability over the YP (Padman et al. 1992). Barotropic, diurnal shelf waves generated on the eastern side of the YP propagates counter-clockwise, amplifying the cross-slope currents. This amplification is the necessary condition for nonlinear internal wave generation over the YP.</p>

Atlantic Water Modification North of Svalbard in the Mercator Physical System From 2007 to 2020

<div> <div> <div> <p>The Atlantic Water (AW) inflow through Fram Strait, largest oceanic heat source to the Arctic Ocean, undergoes substantial modifications in the Western Nansen Basin (WNB). Evaluation of the Mercator system in the WNB, using 1,500 independent temperature‐salinity profiles and five years of mooring data, highlighted its performance in representing realistic AW inflow and hydrographic properties. In particular, favorable comparisons with mooring time‐series documenting deep winter mixed layers and changes in AW properties led us to examine winter conditions in the WNB over the 2007–2020 period. The model helped describe the interannual variations of winter mixed layers and documented several processes at stake in modifying AW beyond winter convection: trough outflows and lateral exchange through vigorous eddies. Recently modified AW, either via local convection or trough outflows, were identified as homogeneous layers of low buoyancy frequency. Over the 2007–2020 period, two winters stood out with extreme deep mixed layers in areas that used to be ice‐covered: 2017/18 over the northern Yermak Plateau‐Sofia Deep; 2012/13 on the continental slope northeast of Svalbard with the coldest and freshest modified AW of the 12‐year time series. The northern Yermak Plateau‐Sofia Deep and continental slope areas became “Marginal Convection Zones” in 2011 with, from then on, occasionally ice‐free conditions, 50‐m‐ocean temperatures always above 0 °C and highly variable mixed layer depths and ocean‐to‐atmosphere heat fluxes. In the WNB where observations require considerable efforts and resources, the Mercator system proved to be a good tool to assess Atlantic Water modifications in winter.</p> </div> </div> </div>

Changes in Atlantic Water circulation patterns and volume transports North of Svalbard over the last 12 years (2008-2020)

<p>Atlantic Water (AW) enters the Arctic through Fram Strait as the West Spitsbergen Current (WSC). When reaching the south of Yermak Plateau, the WSC splits into the Svalbard, Yermak Pass and Yermak Branches. Downstream of Yermak Plateau, AW pathways remain unclear and uncertainties persist on how AW branches eventually merge and contribute to the boundary current along the continental slope. We took advantage of the good performance of the 1/12° Mercator Ocean model in the Western Nansen Basin (WNB) to examine the AW circulation and volume transports in the area. The model showed that the circulation changed in 2008-2020. The Yermak Branch strengthened over the northern Yermak Plateau, feeding the Return Yermak Branch along the eastern flank of the Plateau. West of Yermak Plateau, the Transpolar Drift likely shifted westward while AW recirculations progressed further north. Downstream of the Yermak Plateau, an offshore current developed above the 3800 m isobath, fed by waters from the Yermak Plateau tip. East of 18°E, enhanced mesoscale activity from the boundary current injected additional AW basin-ward, further contributing to the offshore circulation. A recurrent anticyclonic circulation in Sofia Deep developed, which also occasionally fed the western part of the offshore flow. The intensification of the circulation coincided with an overall warming in the upper WNB (0-1000 m), consistent with the progression of AW. This regional description of the changing circulation provides a background for the interpretation of upcoming observations.</p>

Benthic phosphorus cycling within the Eurasian marginal sea ice zone

The Arctic Ocean region is currently undergoing dramatic changes, which will likely alter the nutrient cycles that underpin Arctic marine ecosystems. Phosphate is a key limiting nutrient for marine life but gaps in our understanding of the Arctic phosphorus (P) cycle persist. In this study, we investigate the benthic burial and recycling of phosphorus using sediments and pore waters from the Eurasian Arctic margin, including the Barents Sea slope and the Yermak Plateau. Our results highlight that P is generally lost from sediments with depth during organic matter respiration. On the Yermak Plateau, remobilization of P results in a diffusive flux of P to the seafloor of between 96 and 261 µmol m −2 yr −1 . On the Barents Sea slope, diffusive fluxes of P are much larger (1736–2449 µmol m −2 yr −1 ), but these fluxes are into near-surface sediments rather than to the bottom waters. The difference in cycling on the Barents Sea slope is controlled by higher fluxes of fresh organic matter and active iron cycling. As changes in primary productivity, ocean circulation and glacial melt continue, benthic P cycling is likely to be altered with implications for P imported into the Arctic Ocean Basin. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Atlantic- and Arctic Water transport across the Yermak Plateau

<p><span>The pathway by which Atlantic Water ultimately inflows to the Arctic Ocean via the Yermak Plateau are of great interest for improving the current understanding of the evolving state of the European Arctic. The Arctic branches of the West Spitsbergen Current (WSC), i.e. the Svalbard Branch (SB), the Yermak Pass Branch (YPB) and the Yermak Branch (YB), are the primary routes through which warm AW enters the Arctic Ocean (AO). These branches either flow around (YB) or passes (SB, YPB) over the Yermak Plateau, the Arctic Sill, which is a topographic obstacle for warm water intrusion to the Arctic and possible melting of sea ice. In addition, The Spitsbergen Polar Current (SPC), carrying fresh costal and Arctic type water from the Barents Sea has to cross the Yermak Platea along the northwestern corner of the Spitsbergen coastline. In order to reveal the dynamics across the YP and the roles of the different AW branches in heat flux variability across this arctic sill, a set of in situ ocean data, ocean climatology (UNIS HD), reanalyzed atmospheric data (NORA10) and altimetry data products from Ssalto/Duacs (CMEMS), where synthesized in order to study the seasonal and year-to-year variability in ocean currents across the YP. In situ data from the <em>Remote Sensing of Ocean Circulation and Environmental Mass Changes (REOCIRC)</em> project consist of water time series of temperature, salinity, ocean current and Ocean Bottom Pressure (OBP), which covered the SB and the SPC. Air-ocean interaction mechanisms for controlling volume transport and heat fluxes in the SB and SPC are presented, and further linked to the variability of the other primary AW routes towards the AO. Moreover, surface geostrophic currents from Absolute Dynamic Topography (ADT) are calibrated against the geostrophic bottom current calculated from in situ OBP recorders. Estimates of winter volume- and heat transports across the YP for the time period 1993-2019 are presented, and interannual variability in the SB linked to the WSC and other AW branches are discussed together with consequences for sea ice melting north of Svalbard.</span></p>

Assessing the Atlantic Water Pathways To The Arctic In a High-Resolution NEMO Model

<p><span>Most Atlantic Water (AW) enters the Arctic Ocean through the West Spitzbergen Current, passing north </span>of Svalbard, either moving along the upper slope or passing over and around the Yermak Plateau. Recent model studies (Koenig et al. 2017 and Crews et al. 2019) have improved our understanding of these pathways but were limited to periods of 1-5 years. This is insufficient for examining the contributions of AW inflow to climate-scale problems such as the ‘Atlantification’ of the Arctic.</p><p>In this study we use 23 years (1995-2018) of high resolution (~1/24°) velocity fields from a NEMO 3.6 model (DOI: 10.5281/zenodo.2682406) allowing us to examine the geographic distributions and strengths of AW inflow pathways using a Lagrangian particle tracking approach. Virtual particles were released on a section at 30° E and tracked backwards in time using the PARCELS 2.0 particle tracking system (Delandmeter and van Sebille 2019).</p><p>For the present analysis, we focus on trajectories of particles which are contained in AW layer at the release line (SA>34.9 and CT>2°C) and could be tracked backwards to the Nowegian Sea (here taken as south of 75° N). A control line was selected across the Yermak Plateau to allow us to separate particles passing through the Svalbard and Yermak branches. Using these particle trajectories, we created a time-series of transport of AW reaching the southern rim of the western Nansen Basin. The transport was found to vary between 0.5 Sv and 3.75 Sv, comparable to previous studies (e.g. Beszczynska-Möller et al. 2012), and to be dominated, on average, by the Yermak Branch.</p>

Amino acid racemization in Quaternary foraminifera from the Yermak Plateau, Arctic Ocean

Amino acid racemization (AAR) geochronology is a powerful tool for dating Quaternary marine sediments across the globe, yet its application to Arctic Ocean sediments has been limited. Anomalous rates of AAR in foraminifera from the central Arctic were reported in previously published studies, indicating that either the rate of racemization is higher in this area, or inaccurate age models were used to constrain the sediment ages. This study investigates racemization rates in foraminifera from three well-dated sediment cores taken from the Yermak Plateau during the 2015 TRANSSIZ (TRansitions in the Arctic Seasonal Sea Ice Zone) expedition on RV Polarstern. D and L isomers of the amino acids aspartic acid (Asp) and glutamic acid (Glu) were separated in samples of the planktic foraminifer Neogloboquadrina pachyderma and the benthic species Cassidulina neoteretis to quantify the extent of racemization. In total, 241 subsamples were analysed, extending back to marine oxygen isotope stage (MIS) 7. Two previously published power functions, which relate the extent of racemization of Asp and Glu in foraminifera to sample age are revisited, and a comparison is made between the ages predicted by these calibrated age equations and independent geochronological constraints available for the cores. Our analyses reveal an excellent match between ages predicted by a global compilation of racemization rates for N. pachyderma and confirm that a proposed Arctic-specific calibration curve is not applicable at the Yermak Plateau. These results generally support the rates of AAR determined for other cold bottom water sites and further highlight the anomalous nature of the purportedly high rate of racemization indicated by previous analyses of central Arctic sediments.