Andrea Kroetz
Primary Production and Food Web Dynamics of the Southern Ocean
Polar marine ecosystems, such as the Southern Ocean, are extremely important to global
oceanic systems (Smetacek and Nicol 2005; Murphy et al. 2012). Sequestration of atmospheric
carbon in the deep waters of the Southern Ocean may have potential impacts on future
atmospheric carbon concentrations (Le Quere et al. 2007). The Southern Ocean also has strong
influences on nutrient distribution and production dynamics across the global oceans and
supports a large biomass of finfish and zooplankton fisheries (Constable 2002; Gruber et al.
2009). Ecologically important invertebrates and a wide range of air-breathing predators such as
whales, seals, and seabirds are all inhabitants of this Antarctic Ocean (Smetacek and Nicol
2005). The Southern Ocean is a very dynamic and unique ecosystem in which all of the
constituents are intricately linked (Murphy et al. 2012). The roles of primary production,
upwelling, sea ice, and large predators fill specific niches that help shape the Antarctic
ecosystem.
The Southern Ocean is an important marine system that surrounds Antarctica in its
entirety. The Southern Ocean is driven by the Antarctic Circumpolar Current (ACC) which is
driven by strong westerly winds (Hill et al. 2006). The ACC circulates the Antarctic continent
and connects with all of the ocean basins (Rintoul et al. 2001). The ACC is a strong, cold,
nutrient-rich current that circulates and isolates Antarctica from warmer waters of lower latitudes
allowing for large ice sheets to exist (Rintoul et al. 2001). The ACC brings in nutrient rich waters
from the Pacific to the Southern Ocean, yet this region is characterized as high nutrient and low
chlorophyll (HNLC) area (Moore et al. 2000; Lancelot et al. 2000). HNLC areas are those in
which there are high concentrations of available nutrients, such as nitrogen and phosphorus, but
chlorophyll in the open ocean is very low (Lancelot et al. 2000). The Southern Ocean suffers
from iron limitation (Martin et al., 1990) yet highly productive in terms of biomass (Smetacek
and Nicol 2005).
Phytoplankton play a significant role in pelagic and benthic food webs, on the transfer of
organic matter to the sea floor and on ocean biogeochemistry (Falkowski et al. 1998). The
continental margin of Antarctica is a productive region of the Southern Ocean and phytoplankton
blooms occur on this margins seasonally and can extend offshore where water is enriched with
iron from upwelling or land inputs (Smetacek and Nicol 2005). Upwelling is an important
process that brings cold, nutrient-rich waters from depth to the surface where biological
productivity is stimulated. Upwelling regions in the Southern Ocean are important because they
support high concentrations of phytoplankton, krill, and warm-blooded predators (Atkinson et al.
2004; Nicol 2006). During phytoplankton blooms, it has been estimated that primary production
in the Southern Ocean produces approximately 3.5 gigatons of carbon every year (Huntley et al.
1991). Large amounts of primary production is transferred to mammal predators and transfer in
polar regions may be greater than those at lower latitudes (Hart 1942).
Sea ice plays an integral role in primary production in the Southern Ocean. The seasonal
advance and retreat of sea ice is tightly coupled with phytoplankton blooms. In the winter, sea
ice advances out into the open waters and light penetration is greatly reduced by the thick mats
of ice (Smetacek and Nicol 2005). Reduced light availability decreases the amount of primary
production in the underlying waters which in turn decreases overall biomass production
Andrea Kroetz
(Smetacek and Nicol 2005). In the spring, the sea ice begins to retreat by melting, which supplies
microorganisms, algae, and iron back into the water (Smetacek and Nicol 2005). A short-lived
spring phytoplankton bloom in shallow surface waters is stimulated by the retreating sea ice
particularly due to the addition of iron into the iron-deficient waters (Brierley and Thomas 2002).
These blooms are equated to temperate phytoplankton blooms and can divert potential food from
the pelagic to benthic systems (Smetacek 1985). Spring blooms are crucial because they provide
nutrients for many macroorganisms, including the ecologically important Antarctic krill
(Euphausia superba) which is at the center of the Antarctic food web (Huntley et al. 1991).
The marine Antarctic food web is generally thought of as a relatively simplistic one with
Antarctic krill as the key component (Murphy 1995); however, recent research has shown that
this food web may be more complex (Murphy et al. 2012). The Antarctic food web is made up of
principal components such as phytoplankton, zooplankton such as krill, fishes, birds, mammals,
and the microbial loop (Huntley et al. 1991). In this food web, krill are the principal component
and provide the connection between microorganisms and top predators (Smetacek and Nicol
2005). Krill are efficient in transferring energy to higher trophic levels: krill feed on
phytoplankton, fish feed on krill, and large predators such as seals and whales feed on fish or
directly on krill (Murphy et al. 2012). There are several potential alternate pathways of energy
flow to higher trophic levels through other zooplankton (copepods, salps, etc.), but krill receive
the most attention because of their abundance in the surrounding waters of Antarctica (Murphy et
al. 2012). Chemical and physical processes greatly influence food web productivity and
consequently the Southern Ocean food web is highly variable spatially and temporally (Murphy
et al. 2012). Current Southern Ocean food webs lack a mechanistic view of functional
relationships between habitat structure and biogeochemical cycling; thus, a more integrated
ecosystem analysis is required to properly outline food web dynamics in this region (Murphy et
al. 2012).
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