The Pan-Arctic Continental Slope: Sharp Gradients of Physical Processes Affect Pelagic and Benthic Ecosystems

Abstract

Continental slopes – steep regions between the shelf break and abyssal ocean – play key roles in the climatology and ecology of the Arctic Ocean. Here, through review and synthesis, we find that the narrow slope regions contribute to ecosystem functioning disproportionately to the size of the habitat area (∼6% of total Arctic Ocean area). Driven by inflows of sub-Arctic waters and steered by topography, boundary currents transport boreal properties and particle loads from the Atlantic and Pacific Oceans along-slope, thus creating both along and cross-slope connectivity gradients in water mass properties and biomass. Drainage of dense, saline shelf water and material within these, and contributions of river and meltwater also shape the characteristics of the slope domain. These and other properties led us to distinguish upper and lower slope domains; the upper slope (shelf break to ∼800 m) is characterized by stronger currents, warmer sub-surface temperatures, and higher biomass across several trophic levels (especially near inflow areas). In contrast, the lower slope has slower-moving currents, is cooler, and exhibits lower vertical carbon flux and biomass. Distinct zonation of zooplankton, benthic and fish communities result from these differences. Slopes display varying levels of system connectivity: (1) along-slope through property and material transport in boundary currents, (2) cross-slope through upwelling of warm and nutrient rich water and down-welling of dense water and organic rich matter, and (3) vertically through shear and mixing. Slope dynamics also generate separating functions through (1) along-slope and across-slope fronts concentrating biological activity, and (2) vertical gradients in the water column and at the seafloor that maintain distinct physical structure and community turnover. At the upper slope, climatic change is manifested in sea-ice retreat, increased heat and mass transport by sub-Arctic inflows, surface warming, and altered vertical stratification, while the lower slope has yet to display evidence of change. Model projections suggest that ongoing physical changes will enhance primary production at the upper slope, with suspected enhancing effects for consumers. We recommend Pan-Arctic monitoring efforts of slopes given that many signals of climate change appear there first and are then transmitted along the slope domain.

Sea ice decline drives biogeographical shifts of key Calanus species in the central Arctic Ocean

“…Calanus glacialis forms a distinct band of elevated biomass around the outer shelf and slope of the Arctic Ocean (Kosobokova, 2012; Kosobokova & Hirche, 2009; Wassmann et al, 2015). The shelf break, which was postulated to be a boundary for the horizontal distribution of this species, has historically coincided with the marginal ice zone, but this is now rapidly changing (Bluhm et al, 2020). Thus, it becomes important to distinguish that C. glacialis may not, in fact, be a shelf species within the Arctic, but rather a seasonal ice zone (SIZ)‐associated species.…”

On the Along‐Slope Heat Loss of the Boundary Current in the Eastern Arctic Ocean

“…Further, the effect of topographic irregularities such as sills on the heat budget requires detailed studies, as these are often too small to be resolved in bathymetric data products and ocean models. Arctic continental slopes generally feature productive ecosystems (Bluhm et al., 2020), which are supported and maintained by complex ocean dynamics including boundary layer mixing and enhanced vertical nutrient fluxes (Randelhoff et al., 2020). The episodic nature of turbulence is a major source of uncertainty for heat budgets as well as for nutrient fluxes, and therefore requires enhanced efforts to develop and improve mooring‐based methods to measure turbulent mixing year‐round.…”

Vertical zonation of the Siberian Arctic benthos: bathymetric boundaries from coastal shoals to deep-sea Central Arctic

“…Just two rivers, the Ob and Yenisei, discharge about 30% of the total annual river run-off into the Arctic Ocean through the Kara Sea ( Fütterer & Galimov, 2003 ; Stein, 2000 ). From the ocean side, the upper slope of the Siberian seas is bathed by the warm Atlantic waters ( Aagaard, 1989 ; Rudels et al, 1994 ; Bluhm et al, 2020 ), the factor that potentially can influence trophic conditions by enriching the upper slope with organic carbon ( Dunton, 1992 ; Wassmann, Slagstad & Ellingsen, 2019 ; Bluhm et al, 2020 ). The lower boundary of the Atlantic layer in the Siberian Arctic represents a rather sharp gradient from 2–3 °C and ~35 psu to ~−1 °C and ~32–33 psu at depths of 500–700 m ( Bluhm et al, 2020 ).…”

“…From the ocean side, the upper slope of the Siberian seas is bathed by the warm Atlantic waters ( Aagaard, 1989 ; Rudels et al, 1994 ; Bluhm et al, 2020 ), the factor that potentially can influence trophic conditions by enriching the upper slope with organic carbon ( Dunton, 1992 ; Wassmann, Slagstad & Ellingsen, 2019 ; Bluhm et al, 2020 ). The lower boundary of the Atlantic layer in the Siberian Arctic represents a rather sharp gradient from 2–3 °C and ~35 psu to ~−1 °C and ~32–33 psu at depths of 500–700 m ( Bluhm et al, 2020 ). Faunal changes coinciding with the inflow of Atlantic waters were reported for annelids, isopods, gastropods and fishes in the Norwegian Sea, where the gradient is particularly high ( Svavarsson, Brattegard & Strömberg, 1990 ; Bergstad, Bjelland & Gordon, 1999 ; Høisæter, 2010 ; Oug et al, 2017 ).…”

“…This zone contours all continents and it was marked by Sokolova (2000) as the “Circumcontinental eutrophic zone”. In the Arctic Ocean the decrease of the slope inclination at the transition from the continental shelf to the continental rise was shown for some areas north of the Kara, Laptev and East-Siberian seas at ~2,000 m ( Jakobsson et al, 2020 ; Bluhm et al, 2020 ). This is shallower than in the scheme of Sokolova (2000) and this is apparently a regional peculiarity of the basin.…”