Spatial distribution of the iron supply to phytoplankton in the Southern Ocean: a model study

Abstract

Abstract. An upgraded version of the biogeochemical model SWAMCO is coupled to the ocean-sea-ice model NEMO-LIM to explore processes governing the spatial distribution of the iron supply to phytoplankton in the Southern Ocean. The 3-D NEMO-LIM-SWAMCO model is implemented in the ocean domain south of latitude 30° S and runs are performed over September 1989–December 2000. Model scenarios include potential iron sources (atmospheric deposition, iceberg calving/melting and continental sediments) as well as iron storage within sea ice, all formulated based on a literature review. When all these processes are included, the simulated iron profiles and phytoplankton bloom distributions show satisfactory agreement with observations. Analyses of simulations and sensitivity tests point to the key role played by continental sediments as a primary source for iron. Iceberg calving and melting contribute by up to 25% of Chl-a simulated in areas influenced by icebergs while atmospheric deposition has little effect at high latitudes. Activating sea ice-ocean iron exchanges redistribute iron geographically. Stored in the ice during winter formation, iron is then transported due to ice motion and is released and made available to phytoplankton during summer melt, in the vicinity of the marginal ice zones. Transient iron storage and transport associated with sea ice dynamics stimulate summer phytoplankton blooming (up to 3 mg Chl-a m-3 in the Weddell Sea and off East Antarctica but not in the Ross, Bellingshausen and Amundsen Seas. This contrasted feature results from the simulated variable content of iron in sea ice and release of melting ice showing higher ice-ocean iron fluxes in the continental shelves of the Weddell and Ross Seas than in the Eastern Weddell Sea and the Bellingshausen-Amundsen Seas. This study confirms that iron sources and transport in the Southern Ocean likely provide important mechanisms in the geographical development of phytoplankton blooms and associated ecosystems.

PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies

“…Thus, iron is taken up from sea water when ice forms and is released back to the ocean when it melts. Lancelot et al (2009) have studied the impact of this source in the Southern Ocean and shown that it is of primary importance in the seasonal ice zone. Their approach relies on the modeling of iron concentration within sea ice.…”

The iron budget in ocean surface waters in the 20th and 21st centuries: projections by the Community Earth System Model version 1

“…Because sea ice and icebergs play an important role in offshore iron transport in the Southern Ocean (Smith et al, 2007;Lannuzel et al, 2008;Lancelot et al, 2009), a reduction of their production rates will change the spatial pattern of the iron supply. Recent studies have suggested that ice sheets and melt water play a role in supplying iron to the ocean (Raiswell et al 2008;Wadham et al 2013;Bhatia et al, 2013); the inclusion of such sources in numerical models (e.g., Death et al, 2013) is potentially important because the high latitudes are experiencing rapid warming.…”

“…Physical processes that may contribute to the supply of new iron include upwelling (de Baar et al, 1995), vertical diffusive flux , horizontal advection (Ellwood et al, 2008;Sedwick et al, 2008;Bowie et al, 2009), the interaction between bathymetry and currents (Blain et al, 2007;Sokolov et al, 2007), and mesoscale eddies and cross-frontal mixing (Kahru et al, 2007). In addition, the melting of sea ice and icebergs also can supply or redistribute iron in surface waters (Smith et al, 2007;Lannuzel et al, 2008;Lancelot et al, 2009). …”

The multiphase physics of sea ice: a review for model developers

“…Understanding why iron accumulates in sea ice depends on a better understanding of sea ice multiphase physical processes and how they interact with biogeochemical processes. The storage and transport of iron associated with sea ice dynamics is important because it may stimulate summer phytoplankton blooms in the Southern Ocean, as shown by a preliminary model investigation of the Southern Ocean (Lancelot et al, 2009). Overall, the sea-ice habitat appears to be highly productive and is thought to make a significant contribution to the carbon, sulfur and iron cycles, which needs to be carefully assessed.…”