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1000 µatm by the end of this century (RCP6.0 scenario; IPCC 2014). The dissolution of CO 2 in seawater alters its chemistry by increasing the dissolved CO 2 concentration and lowering pH (called 'ocean acidifi-cation', OA). Since the beginning of the industrial revolution, the ocean has absorbed about a third of the anthropogenic CO 2 emissions. Among the world oceans, the Southern Ocean (SO) sequesters a disproportionally large share of anthropogenic CO 2 , accounting for about 40% of the global oceanic uptake of anthropogenic CO 2 (Sabine et al. 2004, Landschützer et al. 2015). However, in this region the biological sequestration potential is constrained by iron (Fe) input (Martin et al. 1990, Boyd et al. 2007,

The Antarctic Circumpolar Current has a high potential for primary production and carbon sequestration through the biological pump. In the current study, two large-scale blooms observed in 2012 during a cruise with R.V. Polarstern were investigated with respect to phytoplankton standing stocks, primary productivity and nutrient budgets. While net primary productivity was similar in both blooms, chlorophyll a –specific photosynthesis was more efficient in the bloom closer to the island of South Georgia (39 °W, 50 °S) compared to the open ocean bloom further east (12 °W, 51 °S). We did not find evidence for light being the driver of bloom dynamics as chlorophyll standing stocks up to 165 mg m−2 developed despite mixed layers as deep as 90 m. Since the two bloom regions differ in their distance to shelf areas, potential sources of iron vary. Nutrient (nitrate, phosphate, silicate) deficits were similar in both areas despite different bloom ages, but their ratios indicated more pronounced iron limitation at 12 °W compared to 39 °W. While primarily the supply of iron and not the availability of light seemed to control onset and duration of the blooms, higher grazing pressure could have exerted a stronger control toward the declining phase of the blooms.

The concentrations of Pt in sediments, water and suspended particulate matter (SPM) were determined in the Lérez river estuary (Pontevedra Ria, NW Iberian Peninsula) on two sampling dates in 2008 and 2011, by means of catalytic adsorptive stripping voltammetry. Average concentrations in sediments (1.2 ± 0.5 ng g -1 ; n=13) corresponded to an enrichment factor of 2-4 compared to background values, and were in the range of those found in the SPM during the 2011 sampling (2.1 ± 2.4 ng g -1 ; n=15), but significantly lower than in 2008 SPM (8.0 ± 4.4 ng g -1 ; n=15). Higher dissolved Pt concentrations were also found in 2008 (0.21 and 0.62 pM in the freshwater and seawater end-member) compared to the 2011 campaign (0.03 -freshwater -and 0.40 pM -seawater endmember). Concentrations in the seawater end-members exceed those of typical North Atlantic waters, suggesting inputs of this element within the estuary. The extremely low value for the freshwater end-member in 2011 appears to be among the lowest Pt concentrations ever reported in the literature. Non-conservative behaviour of Pt during estuarine mixing was observed on both sampling dates. Particle-water distribution coefficients (KD) show a decrease with salinity, in agreement with speciation calculations, which predict a transfer from neutral Pt(II) (as Pt(OH)2) in freshwater to negatively charged Pt(IV) (as PtCl5(OH) 2-) species in seawater. Such behaviour during estuarine mixing can play an important role in Pt mobilization from contaminated particles discharged into estuaries and coasts.

A new analytical protocol for the challenging analysis of total dissolved iron at the low picomolar level in oceanic waters suitable for onboard analysis is presented. The method is based on the revision of the adsorptive properties of the iron/2,3-dihydroxynaphthalene (Fe/DHN) complexes on the hanging mercury drop electrode with catalytic enhancement by bromate ions. Although it was based on a previously proposed reagent combination, we show here that the addition of an acidification/alkalinization step is essential in order to cancel any organic complexation, and that an extra increment of the pH to 8.6-8.8 leads to the definition of a preconcentration-free procedure with the lowest detection limit described up to now. For total dissolved iron analysis, samples were acidified to pH 2.0 in the presence of 30 μM DHN and left to equilibrate overnight. A 10 mL sample was subsequently buffered to a pH of ∼8.7 in the presence of 20 mM bromate: a 60 s deposition at 0 V led to a sensitivity of 34 nA nM(-1) min(-1), a 4-fold improvement over previous methods, that translated in a limit of detection of 5 pM (2-20 fold improvement). Several tests proved that a nonreversible reaction in the time scale of the analysis, triggered by the acidification/alkalinization step, was behind the signal magnification. The new method was validated onboard via the analysis of reference material and via intercalibration against flow injection analysis-chemiluminescence on Southern Ocean surface samples.

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The determination of trace elements using stripping voltammetry may be seriously affected by the presence of intensive matrix background or interfering peaks, leading to poorer detection limits and/or inaccurate quantitative results. In this work, we have tested the use of signal transformation (e.g., second derivative) in the analysis of platinum in seawater and sediment digests by means of catalytic adsorptive stripping voltammetry. In natural waters, the limit of detection of Pt is affected by a broad background wave due to the formazone complex used in the sample matrix for its determination, while in sediment digests, the Pt peak may be interfered with due to the presence of elevated concentrations of Zn, affecting the accuracy of the determination. Results applying second derivative signal transformation revealed a significant improvement (2-3-fold) of the detection limit in water due to the minimization of background effects, therefore allowing shorter accumulation times and faster determinations. In the presence of interfering peaks, the inaccuracy resulting from erroneous baseline selection in the original signal is eliminated when the second derivative is used. Signal processing should be considered as a useful tool for other voltammetric methodologies where more accurate or faster determinations are needed.

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