Iron plays an important role in the biogeochemical cycle of the major nutrients (N, P, Si) in the ocean and is considered to limit biological activity in certain ocean regions that have a replete complement of major nutrients but low production of phytoplankton (HNLP) due to low ambient iron concentrations. The controversy over the role of iron in ocean productivity has stimulated the development of new methodologies for rapidly analyzing low-level iron concentrations in seawater. The iron content in the water body also indicates the state of hydrological pollution in the coastal system. Its chemical speciation is very important to consider the iron bioavailability in the primary production context. However, it remains as a major challenge, although much progress has been made in order to rationalize its biogeochemical cycling in different scenarios on the Brazilian coast.Ducatti et al. (2003), in a set of measurements collected at the Santos Estuarine system and in the Santos Bay, considering Fe(II) and Fe(III) speciation and taking into account the Spring and Neap tides in the winter and summer, found for Fe(II) in the winter (2002), values ranging from 0.05 to 3.58 µmol/L, in the Spring tide, and from 0.09 to 1.93 µmol/L in the Neap tide. In the summer (2002) those values ranged from 0.02 to 1.39 µmol/L in the Spring tide and from 0.05 to 2.52 µmol/L, in the Neap tide. For Fe(III), the author op. cit. reported the following range of values for the winter (2002): from 0.07 to 3.82 µmol/L in the Spring tide and from 0.15 to 9.04 µmol/L in the Neap tide; for the summer (2002) those values ranged from 0.02 to 42.64 µmol/L in the Spring tide and from 0.09 to 17.20 µmol/L in the Neap tide. Considering the total iron content in water, the values ranged from 0.47 to 5.14 µmol/L in the Spring tide and from 0.31 to 9.61 µmol/L in the Neap tide for the winter (2002). For the summer (2002), those values ranged from 0.17 to 42.76 µmol/L in the Spring tide and from 0.24 to 19.27 µmol/L in the Neap tide.This work aims to show the seasonal (winter and summer) and physical influences on the estuarine and coastal distribution of the total iron and its chemical speciation (Fe(II) and Fe(III)) at the Santos/São Vicente Estuarine System and coastal region around it (SP) and at the Coastal region of Recife (PE), both under important populous areas that influences the nutrient distributions. A more remote region nearby Fernando de Noronha will also be considered as a preserved scenario. The knowledge of the differences on the chemical species dynamic of iron in various scenarios can contribute to the policies and programs to using and preserving the coastal areas, the most important productive areas of the world. The iron bioavailability and total concentration is essential for elucidating the current and future effects of both natural and anthropogenically induced changes.
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