Distributions of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) in coastal waters of the northern Tyrrhenian Sea (Italy)

An investigation on the distribution of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) in coastal waters of the northern Tyrrhenian Sea, affected by the Arno River discharge, is reported and discussed. Data refer to a survey carried out aboard of the R/V Urania in January 2000. The study period was characterized by a rather low river discharge due to the particularly calm meteorological conditions. The river plume region extended to north of the Arno delta and it was confined to the inner shelf with depths less than 40 m and salinities !38.1. Surface DOC concentrations ranged in a narrow interval of values (56e76 mM). Plume waters exhibited the highest levels of DOC (O70 mM). These values were slightly higher than those found in the outer shelf (56e66 mM) which in turn, were generally comparable to those reported in the literature for open sea waters of different regions of the Mediterranean Sea. The distribution of CDOM was described by the two components of fluorescent material, i.e., the ‘‘protein-like’’ (Fn(280)) and the ‘‘humic-like’’ (Fn(355)), respectively, characterized by different spectral regions for both excitation and emission. The Fn(355) signature decreased towards the north and west generally mirroring the spatial pattern of salinity, temperature and DOC. On the contrary, the distribution of Fn(280) differed from that of Fn(355) without any apparent relationship with the parameters above cited. This behavior supports the hypothesis that the ‘‘protein-like’’ fluorescent material could be released to the coastal waters by the local activity of planktonic organisms. Conversely, DOC was conservatively exported through the plume to the coastal waters and, similarly, Fn(355) appeared largely controlled by a conservative mixing. The correlation between DOC and Fn(355) (r2 ¼ 0:78) represents a further confirmation of the terrestrial origin of DOC in the plume region. Vertical profiles of DOC, Fn(280) and Fn(355) and those of the specific fluorescence, Fs(280), and Fs(355), in stations located off the shelf, showed differences between northern and southern stations, suggesting a different composition of DOC in the two regions. Finally, although these data refer to a single survey, they represent the first simultaneous DOC and CDOM view on the influence of the Arno River to the carbon cycle in an estuarine Mediterranean region.