Absorbance and induced fluorescence of rain water in a temperate coastal embayment (Ría de Vigo, NW Spain)

Poster.-- International DOM Spectroscopy Workshop, 19-21 May 2010, Granada, Spain

The dynamics of the Ría de Vigo, a large embayment in the NW Iberian coast, is primarily controlled by remote winds and secondarily by continental runoff. It is characterised by a 2–layered residual circulation pattern, positive under upwelling–favourable northerly winds and negative under downwelling–favourable southerly winds. The spring and autumn phytoplankton blooms occur during the seasonal transitions from downwelling to upwelling conditions and vice versa, respectively. Copious rainfall during those transitional periods could be a significant source of organic and inorganic nutrients to the surface layer of the ría.

The chemical composition of rain water (total deposition) in contrasting rural, urban and marine environments of the Ría de Vigo was monitored on a daily basis during 2 years. Dissolved organic carbon (DOC), absorbance, and induced fluorescence spectra are used to quantify and characterise the organic loads of rain water. The concentration of dissolved organic carbon (DOC) correlated inversely (R2 > 0.42; p < 0.001) with the daily precipitation rate (P) following the equation DOC = a • Pb, where a is the DOC concentration at P = 1 mm/d and b is the dilution factor. The coefficient a was around 1050 ± 200 μmol/L in the rural and urban stations but it was significantly lower (p < 0.05), 650 ± 100 μmol/L, in the marine station. The coefficient b was –0.8 ± 0.1 at the three sites. Linear regressions through the origin (R2 > 0.90, p < 0.001) were obtained between DOC and the absorption coefficient at 254 nm, a(254), but with different slopes: the absorptivity at 254 nm, a*(254), was significantly higher (p < 0.005) at the marine station (29.2 ± 0.8 m2/mol C) than at the rural (23.6 ± 0.6 m2/mol C) and urban (22.3 ± 0.5 m2/mol C) sites. Rain water at the marine station also presented significantly lower (p < 0.001) values of the a(254)/a(365) ratio, 9.0 compared to >13.2, and the fluorescence quantum yield at 340 nm, Φ(340), 1.6% compared to 2.7%. Conversely, the ratio of the induced fluorescence at peak–T (Ex/Em = 280nm/350nm) and peak–A (Ex/Em = 250/450), characteristic of protein– versus humic–like fluorophores, was significantly higher (p < 0.001) at the marine (11.3) than at the urban and rural sites (<7.6). Concomitant collection of wet deposition samples at the urban site yielded that a = 95 ± 10 μmol/L, b = -0.57 ± 0.05, a*(254) = 30.6 ± 0.9 m2/mol C, a(254)/a(365) = 13.5, Φ(340) = 1.4%, and peak–T/peak–A = 11.8. These optical indices suggest that: (i) 100% of the DOC in rainwater is coloured; (ii) the aromaticity of rain water in the marine site is significantly higher than in the urban and rural sites; and (iii) aromatic amino acids are proportionally more abundant than aromatic humic substances in the marine site

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