Contrasting behaviours of dissolved organic- iron, aluminium and phosphorus fractions in the Amazon Estuary

Estuaries are critical transition zones that ultimately determine the magnitude of riverine fluxes of trace elements to the Ocean. Here we utilised high performance liquid chromatography with parallel detection by elemental (ICP) and organic (HR-ESI) mass spectrometry (MS) to determine changes in the concentration of dissolved organic (<0.2 µm) fractions of iron (OFe) and aluminium (OAl) in the Amazon estuary during the GEOTRACES process cruise GApr11. For the first time we show that aluminium binds with organic matter in the marine environment and is associated with the same types of organic matter as iron. We observed a strong riverine source for the major OFe and OAl fractions. Furthermore, we found that OAl and OFe exhibited similar non-conservative behaviour in the estuary, which was consistent with the total dissolved geochemistry of these two elements. We compared our results for OAl and OFe to those of dissolved organic-phosphorus (OP). In contrast to the trace metals, OP did not have a strong riverine source, but showed a marked increase at high salinities, which likely related to increased phytoplankton biomass on the Brazilian Shelf. We used HR-ESI-MS data to explore the character of different organic fractions of the three elements and found contrasting distributions for specific Fe compounds (siderophores) in comparison to the bulk fractions. Our results suggest that the organic fractions of aluminium, iron and phosphorus detected in this study are dynamic in both abundance and character and are consistently influenced by different localised sources.