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

C Lancelot,M Vancoppenolle,A De Montety,S Becquevort,B Pasquer,H Goosse,V Schoemann

doi:10.5194/bg-6-2861-2009

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.

Highlights

The HNLC (High Nutrient Low Chlorophyll) nature of the modern Southern Ocean is attributed to the low availability of dissolved iron, an essential micronutrient for phytoplankton growth
In a second series of model tests we further investigate the sensitivity of the simulated iron distribution fields in winter to the numbers chosen for the different iron sources and the maximal DFe concentration in sea-ice
The availability of dissolved iron in combination with light shapes the geographical distribution of phytoplankton growth in the Southern Ocean

Summary

Introduction

The HNLC (High Nutrient Low Chlorophyll) nature of the modern Southern Ocean is attributed to the low availability of dissolved iron, an essential micronutrient for phytoplankton growth. In a review of in situ iron-enriched experiments, de Baar et al (2005) conclude that iron is necessary – but not sufficient – for phytoplankton blooms to occur. Another important factor is light availability within the upper mixed layer, which controls the onset and maintenance of the bloom, as already suggested in earlier studies (Smith and Nelson, 1985; Mitchell et al, 1991). Transient spots with relatively much higher concentrations (∼2–20 mg Chl-a m−3) are commonly

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