Seasonal cycling of zinc and cobalt in the Southeast Atlantic along the 4 GEOTRACES GA10 section. 5

21 We report the distributions and stoichiometry of dissolved zinc (dZn) and cobalt (dCo) in sub- 22 tropical and sub-Antarctic waters of the Southeast Atlantic Ocean during austral spring 2010 23 and summer 2011/12. In sub-tropical surface waters, mixed-layer dZn and dCo concentrations 24 during early spring were 1.60 ± 2.58 nM and 30 ± 11 pM, respectively, compared with summer 25 values of 0.14 ± 0.08 nM and 24 ± 6 pM. The elevated spring dZn concentrations resulted from an apparent offshore transport of elevated dZn at depths between 20 – 55 m, derived from from 27 the Agulhas Bank. In contrast, open-ocean sub-Antarctic surface waters displayed largely 28 consistent inter-seasonal mixed-layer dZn and dCo concentrations of 0.10 ± 0.07 nM and 11 ± 29 5 pM, respectively. Trace metal stoichiometry, calculated from concentration inventories, 30 suggest a greater overall removal for dZn relative to dCo in the upper water column of the 31 Southeast Atlantic with an inter-seasonally decreasing dZn/dCo inventory ratios of 19 to 5 mol 32 mol -1 and 13 to 7 mol mol -1 for sub-tropical surface water and sub-Antarctic surface water, 33 respectively. In this paper, we investigate how the seasonal influences of external input and 34 phytoplankton succession may relate to the distribution of dZn and dCo, and variation in 35 dZn/dCo stoichiometry, across these two distinct ecological regimes in the Southeast Atlantic.


38
The trace metal micronutrients zinc (Zn) and cobalt (Co) play an important role in the 39 productivity of the oceans as key requirements in marine phytoplankton metabolism (Morel,   Briefly, dCo was determined in UV-irradiated samples using the reaction between pyrogallol

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and PO4 3were at least 2.1-fold higher than for STSW during the study, whilst the Si   (Table 2).    (Table 2), and would not be 299 sufficient to generate distinct mixed-layer maxima. It is likely, therefore, that the dZn and dCo

359
Distinct temporal trends in the stoichiometric relationship with PO4 3were evident for both dZn 360 and dCo (Fig. 4). Within STSW, the dZn/PO4 3inventory ratio ranged from 699 to 1876 µmol summer was lower than for STSW, but was greater for PO4 3-, likely reflecting the increased 367 availability of PO4 3in these Southern Ocean derived waters (Table 2)   In contrast to dZn, the spatiotemporal variation observed for STSW dCo/PO4 3 was small with 374 ratios ranging from 82 to 129 µmol mol -1 ( during late spring (Fig. 2) and subsequent mixing with SASW depleted in dCo relative to PO4 3-379 (Fig. 3). This dilution is likely also true of dZn and Si, yet their STSW concentration inventories 380 may be sufficiently high as to mask this effect. Unfortunately, an insufficient quantity of late 381 spring SASW data are available with which to affirm this postulation. The highest dCo/PO4 3-382 ratio was observed during summer due to the preferential biological removal of PO4 3relative 383 to dCo.

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In SASW, dCo/PO4 3was consistently low with ratios of 23 and 26 µmol mol -1 for early spring 385 and summer, respectively. Much higher inventory ratios of ~580 µmol mol -1 can be calculated  having at least 4-fold higher cellular Zn/P ratios than co-occurring cell types (Twining and 422 Baines, 2013).

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The fact that both Phaeocystis and diatomaceous nanophytoplankton maintain a contribution 424 to the summer STSW chlorophyll-a complement, when dZn availability is low, is intriguing.  respectively) relative to early spring abundance (Fig. 5b). This pattern suggests an inter- Co 2+ ), further suggests that Phaeocystis species may more effectively occupy low dZn and dCo 501 environments (Saito and Goepfert, 2008), such as SASW of the South Atlantic.

502
Conversely, the absence of a significant diatom contribution to summer SASW chlorophyll-a 503 (Fig. 5a), relative to early spring, is surprising as the summer dZn/PO4 3inventory ratio is in 504 excess of the cellular Zn/P requirements of typical oceanic diatoms such as T. oceanica (Fig.   505 6). Furthermore, whilst the dCo/PO4 3ratio of summer SASW is in deficit of the cellular Co/P 506 below which growth limitation of T. oceanica may occur, this species has been shown to grow 507 effectively at Co 2+ <0.1 pM in culture in the presence of Zn (Sunda and Huntsman, 1995 the 4 mol mol -1 shown to limit diatom growth in culture (Gilpin et al., 2004), and in contrast to 513 the 2.9 mol mol -1 calculated for early spring. phytoplankton species estimated to be present in these waters by diagnostic pigment analyses. 532 We therefore suggest cambialistic metabolic substitution between Zn and Co, and potentially 533 Cd, is an important factor regulating the growth, distribution and diversity of phytoplankton in 534 the Southeast Atlantic.