Fluxes of dissolved aluminum and manganese to the Weddell Sea and indications for manganese co-limitation

Rob Middag, Hein J. W. de Baar, Maarten B. Klunder and Patrick Laan

Limnol. Oceanogr., 58(1), 2013, 287-300 | DOI: 10.4319/lo.2013.58.1.0287

ABSTRACT: The trace metals aluminum (Al) and manganese (Mn) were studied in the Weddell Sea in March 2008. Concentrations of dissolved Al ([Al]) were slightly elevated (0.23–0.35 nmol L−1) in the surface layer compared to the subsurface minimum (0.07–0.21 nmol L−1) observed in the winter water. Atmospheric deposition is the main source of Al to the central Weddell Sea (22 µmol m−2 yr−1), and the residence time of dissolved Al in the upper mixed layer is ∼ 0.8 yr. The flux from the shelf and slope regions equals about 50% of the atmospheric input of Al to the western Weddell Sea. The highest [Al] in the Weddell Sea bottom water (WSBW) is related to the formation of deep water, and the associated downward flux is in the range of 3–10 µmol Al m−2 yr−1. The concentrations of dissolved Mn ([Mn]) were depleted in the surface layer, likely as a result of biological uptake, as indicated by the correlation among Mn, major nutrients, and fluorescence. The significant negative relation between the ΔMn : ΔP ratio and the ambient concentration of dissolved iron indicates iron-Mn co-limitation. The flux of Mn from the continental margin is about 2.2 times greater than atmospheric input (1.1 µmol m−2 yr−1). The flux of both Al and Mn from the continental margin indicates melting of continental ice (icebergs) or direct continental runoff. The slightly elevated [Mn] in the WSBW is due to a relatively small flux of 1 µmol Mn m−2 yr−1 associated with WSBW formation.

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