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.

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.

Dissolved and particulate platinum (Pt D and Pt P , respectively) have been measured along the salinity gradient in surface water of the Gironde Estuary during two contrasting hydrological regimes; namely, moderate discharge (November 2012) and high discharge (March 2013). During both sets of conditions, Pt D concentrations displayed maxima but at different locations along the salinity gradient. These observations are attributed to the addition of Pt from suspended particles traversing the estuary, an effect that is kinetically constrained and sensitive to discharge conditions. A minimum in Pt D concentration in the low salinity range observed under high discharge conditions also suggests that Pt removal may occur in the maximum turbidity zone (MTZ). Particulate Pt also exhibited mid-estuarine maxima, an effect attributed to the presence of anthropogenic Pt in this region, but no clear trend for Pt P -Pt D distribution coefficients was evident during either sampling.Estimated daily gross dissolved (Pt D ) and particulate Pt (Pt P ) fluxes at La Réole, the main fluvial entry to the Gironde Estuary, were 0.018-0.11 mole day -1 (3.6-22 g day -1 ) and 0.001-0.18 mole day -1 (0.21-35 g day -1 ) in November and March, respectively.Using Boyle's method, the daily net Pt D fluxes from the Gironde Estuary to the coastal ocean were estimated at 0.03 mole day -1 (6.5 g day -1 ) for moderate discharge and 0.17 mole day -1 (34 g day -1 ) during high discharge, reflecting the regime-dependent magnitude of Pt remobilisation along the salinity gradient. The findings of the study suggest that estuarine reactivity is of major importance for both the distributions of Pt in estuaries and its export to the global ocean.

The cycling of nutrients (silicate, nitrate, ammonium and phosphate) in a quasi-pristine barrierlagoon complex (Louro Lagoon, NW Iberian Peninsula) was studied both in a diel and temporal timescale covering different seasonal conditions. Hydrographical and meteorological conditions exert a major control on the nutrient cycling in the lagoon. From late autumn to early spring the lagoon is characterized by oxic conditions due to significant freshwater inputs and sporadic communication with the sea. In summer, elevated primary production coupled with water stagnancy leads to bottom waters hypoxia and marked diel variation of pH, oxygen and redox potentials. Diel variation of nutrients was only evident when mixing of surface and bottom waters during the night was observed.Temporal changes in nutrients concentrations and their relative abundance were found to induce a clear shift in the phytoplankton composition. In winter, during silicate and nitrate abundance and low phosphate, diatoms predominate. In summer, under hypoxic conditions, sediments take control on the supply of nutrients leading to increased phosphate availability relative to nitrate/ammonium or silicate. Diatoms are then replaced during the summer blooms -linked to low atomic N:P ratios (typically <2) -by flagellates and green algae with minor contributions of cyanophytes and dinoflagellates.

The water column profiles of dissolved Pt were determined in samples taken during the GEOTRACES GA02 section at three stations located in the North, Central and South West Atlantic Ocean. Average concentrations obtained in this study for dissolved Pt (0.22 ± 0.04 pM; n=59) are in agreement with the only previous dataset for the Atlantic Ocean reported in the early nineties. However, results presented here do not show invariant concentrations with depth as previously indicated for the Atlantic and recently reported for the Pacific Ocean.Instead, these new data suggests that (i) atmospheric inputs can account for the observed Pt enrichment in surface waters of the North West Atlantic; (ii) conservative mixing control the Pt distribution in deep (>1000 m) waters; and (iii) oxygen may play a major role on the Pt distribution in the oceans, since similar trends of Pt with dissolved oxygen were observed with Pt depletion at water depths of maximum oxygen utilization. There is still, however, important open questions on the (bio)geochemistry of Pt in natural waters including the redox behavior of the Pt(II)/Pt(IV) couple and its impact on particle (inorganic and/or biogenic) reactivity, which impedes a better characterization of the observed distribution in this study. Also, potential analytical artifacts that could be behind the differences in the Pt distributions observed for the different ocean basins must be further explored.