ReviewAdvection in polar and sub-polar environments: Impacts on high latitude marine ecosystems
Introduction
Advective processes, in both the atmosphere and ocean, are critical for connecting polar marine ecosystems to those at lower latitudes. Atmospheric circulation carries heat from lower latitudes to polar regions, and helps drive the major high-latitude surface ocean currents. These currents transport nutrients and particulate carbon, in the form of detritus, phytoplankton, and zooplankton, between ecosystems. Spatial and temporal variability in these transports influence organisms at all trophic levels, from phytoplankton to marine birds and mammals. In this review paper, we seek to highlight the different ways in which advective processes in the Northern and Southern Hemispheres affect their high latitude marine ecosystems, and how climate change may alter these systems.
Much can be learned from comparisons of aquatic biomes, especially since direct experimentation on large marine ecosystems is impossible. The comparative approach can provide insights into fundamental ecosystem processes and what might be unique to a particular ecosystem (Murawski et al., 2010). Such mechanistic information can also be used to understand and model other marine ecosystems where such comparisons have not been completed. Comparative studies have been successfully applied and have helped to understand the impacts of climate on marine ecosystems (e.g., Moline et al., 2008, Megrey et al., 2009, Barange et al., 2010, Mueter et al., 2012, Hunt et al., 2013). Many studies comparing the Arctic and Antarctic have been undertaken in the past, focusing on such topics as: climate change (Turner and Overland, 2009, Marshall et al., 2014), microbes (Bano et al., 2004), ice bacteria (Brinkmeyer et al., 2003), sea-ice algae and phytoplankton (Tremblay and Smith, 2007, Arrigo et al., 2010), foraminifera (Darling et al., 2000), fungi (Robinson, 2001), benthos (George, 1977, Starmans and Gutt, 2002, Zacher et al., 2009), zooplankton (Falk-Petersen et al., 2000, Deibel and Daly, 2007, Walkusz et al., 2004, McBride et al., 2014), fishes (Eastman, 1997, McBride et al., 2014) and seabirds (Hunt and Nettleship, 1988, Joiris, 2000). However, no comparative studies have focused on the role of advection, which can play a defining role in the structure and functioning of polar marine ecosystems. By looking at both polar systems, it was our intent that this paper would be useful as a means of increasing our understanding of connections within the systems by contrasting two superficially similar but very different systems.
Our geographic focus in the Arctic is the waters in the deep basins, which are separated by a series of ridge systems, and the surrounding continental shelves. We also address processes in the Bering Sea that modify Pacific Water flowing into the Arctic, and processes in the Canadian Archipelago, as large quantities of water from the Western Arctic Ocean flow southward through the straits of the Archipelago (Fig. 1). The Arctic Ocean is mostly surrounded by land; the only seawater exchange with the Pacific Ocean occurs through Bering Strait (Fig. 2), while exchanges with the Atlantic Ocean occur through Fram Strait, the Barents Sea, and the Canadian Archipelago (Fig. 1, Fig. 3). The Arctic is greatly influenced by its broad, shallow (50–200 m) continental shelves that comprise approximately 50% of the Arctic Ocean area.
The Southern Ocean physically encompasses the region from the Antarctic continent north to the Subtropical Front, but here we mainly focus on the regions south of the Polar Front (Orsi et al., 1995). Bottom depths are typically 4000–5000 m, with relatively narrow but deep (50–800 m, mean 500 m) continental shelf regions (Fig. 4). There are also deep shelves around the sub-Antarctic islands, such as at South Georgia and at the Kerguelen Plateau. The open Southern Ocean encircles the continent of Antarctica with waters of the Antarctic Circumpolar Current that mainly flow clockwise in latitudinal bands separated by frontal zones at which current speeds are elevated (Fig. 4). This current and its associated fronts limit lateral exchange between the Antarctic and sub-Antarctic surface waters, thereby tending to isolate the upper layers in Southern Ocean. South of the Antarctic Circumpolar Current, marked by the Southern Boundary of the Antarctic Circumpolar Current, the Antarctic Coastal Current, flows counterclockwise around the coastal region. Between the Southern Boundary of the Antarctic Circumpolar Current and coastal regions, there are major clockwise-flowing gyres in the embayment areas of the Ross and Weddell Seas. Dense waters, which form on the continental shelves through cooling and brine rejection, sink and flow away from the Southern Ocean at great depths, thereby playing a major role in the global thermohaline circulation (Sloyan and Rintoul, 2001). Wind-driven upwelling in the Southern Ocean is equally important to this global circulation as a return path from the interior ocean to the surface (Marshall and Speer, 2012).
Here we synthesize knowledge of the major polar ocean currents, and how these influence the marine ecosystems in both northern and southern regions. We begin with the major atmospheric circulation patterns (Section 2), examining how they impact the ocean circulation patterns at high latitudes and how these currents serve to influence polar oceans and their ecosystems (Section 3). We explore the meridional fluxes of heat, nutrients and plankton in the Northern Hemisphere, and how these help to support local primary and secondary production, as well as benthic biota, fish, and marine birds and mammals. For the Southern Hemisphere, we examine how the annular currents isolate the Southern Ocean from regions to the north and disperse organisms. Sections 4 Sea ice and its biota, 5 Nutrients and primary production, 6 Benthos, 7 Zooplankton, 8 Fish, 9 Impacts of advection on seabirds and marine mammals, respectively, focus on major ecosystem functional groups: sea ice and its associated biota, nutrients and phytoplankton, benthos, zooplankton, fish, and seabirds and marine mammals. In Section 10, we provide a discussion of how a warming climate may affect these advection patterns and consequently the marine ecosystems of the high latitudes. The final section (11) summarizes our findings.
Section snippets
Atmospheric conditions and circulation
The mean atmospheric conditions in the Arctic and Antarctic have some similarities: cold air temperatures, strongly seasonal light and heat input, relatively low precipitation, high sea-level pressure, and characteristic cyclonic wind patterns (counter-clockwise in the Arctic, clockwise in the Antarctic). However, there are marked differences in the mean sea level pressure fields (Fig. 5), and therefore in the near-surface winds between the Arctic and Antarctic, which are a result of the
Physical conditions and circulation in the Arctic
The Arctic Ocean consists of four deep basins (Canada, Makarov, Nansen, and Amundsen) (maximum depth ∼3000–4000 m) separated by seafloor ridges (Lomonosov, Gakkel, Alpha, and Mendeleev) that are surrounded by broad, shallow (typically <150 m) continental shelves (Chukchi, Beaufort, Barents, Kara, Laptev and East Siberian seas; Fig. 1). Flow from the Pacific through the shallow (50 m) and narrow (85 km) Bering Strait (Fig. 2) from 1990 to 2004 resulted in an annual mean northward transport of ∼0.8 ±
Sea ice and its biota
In both Polar Regions, sea ice is impacted by heat advected in the atmosphere and the ocean, and is itself advected by currents and wind. Because sea-ice movement is a function of wind on short time-scales, as well as ocean currents and internal stress, sea ice moves in a different manner than the underlying ocean. On shallow shelves, immobile land-fast ice that is anchored to the seafloor can isolate the underlying ocean waters from wind forcing. Drifting ice is classified as first-year ice
Nutrients and primary production
Primary production has been estimated in the Arctic (>66°N, i.e., north of the Arctic Circle) at ∼1.5 Gt C yr−1 (Arrigo and van Dijken, 2011, Tremblay et al., 2015). Given that the area of the Arctic north of 66°N is ∼14 × 106 km2, production there, on average, is ∼107 g C m−2 yr−1. Primary production in the Southern Ocean south of 50°S is between 2 and 4 Gt C yr−1 (Moore and Abbott, 2000, Arrigo et al., 2008). The area of the Southern Ocean south of 50°S is ∼45 × 106 km2, and thus the average rate of primary
Benthos
Benthic organisms are strongly affected by advective processes. Their habitats and food supplies are determined by wind and surface currents advecting sea ice, open-ocean currents redistributing organic material in the pelagic zone, and near-bottom currents interacting with bottom topography. The deep-sea floor is thought to be food-limited in many areas. Advective processes impact the distribution/redistribution of organic carbon fixed within the sea ice or surface waters, and therefore,
Zooplankton
Current flow and direction leads to dispersal or retention of zooplankton, which impacts scales of connectivity and, therefore, zooplankton dynamics, genetic diversity, biogeography, and resilience to human exploitation (Huntley and Niiler, 1995). Knowledge of the scales and links between physical and biological processes impacting zooplankton in both polar ecosystems is critical, as the dominant species are relatively long lived (copepods 1–4 years, Arctic euphausiid species 3–4 years, Antarctic
Fish
The contrasting oceanographic regimes of the Arctic and Antarctic Oceans are major determinants of their fish communities. The Arctic has a more recent evolutionary history than the Antarctic, and the current diversity of fish in the Arctic developed as a result of direct connections to lower latitude sub-Arctic regions, as well as the relative complexity of the Arctic Ocean habitats. The intensification of Northern Hemisphere glaciation that began about 2.9 million years ago reduced sea surface
Impacts of advection on seabirds and marine mammals
There are three principal ways in which oceanic advection influences seabirds and marine mammals: by transporting prey to the vicinity of colonies located far from where prey are being recruited, by concentrating planktonic prey to a sufficiently high density to be harvested efficiently, and through the effects of advection on the availability and distribution of sea ice, and thus access to prey. The direction of basin-scale ocean currents influences the distributions of seabird colonies. In
Climate warming, advection, and responses of polar marine ecosystems
In both the Arctic and the Antarctic, ongoing climate change is altering wind strength and patterns (Marshall et al., 2006, Bracegirdle et al., 2008, Bracegirdle et al., 2013) and ocean currents. Also, atmospheric and ocean warming are altering the onset, extent, and duration of seasonal sea ice (Smith et al., 2008, Smith et al., 2014b, Stammerjohn et al., 2008a, Stammerjohn et al., 2008b, Markus et al., 2009, Turner and Overland, 2009, Massom and Stammerjohn, 2010, Stroeve et al., 2012, Frey
Conclusions
The differences between the Arctic Seas and the Southern Ocean are profound. The Arctic is an ocean surrounded by land, while the Antarctic is a land mass surrounded by ocean, which influences both the atmospheric and oceanic circulation patterns, as well as the living marine resources in both of these polar seas.
General circulation patterns
The Arctic Ocean has limited but strong connections to lower-latitude oceans, whereas in the Southern Ocean, despite its apparent broad connections to the
Geographic locators
Arctic, sub-Arctic, North Pacific Ocean, Bering Sea, Chukchi Sea, Beaufort Sea, Arctic Ocean, North Atlantic Ocean, Barents Sea, Baffin Bay, Southern Ocean, sub-Antarctic, Antarctic, Ross Sea, Weddell Sea, West Antarctic Peninsula region, South Georgia.
Acknowledgments
The seeds for this paper were sown in May 2012, when the IMBER regional programs, ESSAS (Ecosystem Studies of Sub-Arctic and Arctic Seas) and ICED (Integrating Climate Ecosystem Dynamics), which focuses on the Antarctic, held a workshop on “Polar Comparisons: the effects of climate change on advective fluxes in high latitude regions” in Yeosu, Korea. The authors thank both PICES and ICES for support of the workshop. GLH’s travel was supported by ESSAS, and EJM’s, DWG’s and WOS’s by ICED.
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