| Summary: | Dissolved organic matter present in natural aquatic environments is a heterogeneous mixture of allochthonous and autochthonous materials. In coastal areas vulnerable to sewage waste, its biologically active component, the chromophoric dissolved organic matter (CDOM), is expected to change its composition and distribution in relation to anthropogenic activities, suggesting the possible use of CDOM as a proxy of fecal contamination. This study aimed at testing such hypothesis by investigating and relating the optical properties of CDOM with <i>Escherichia coli</i> abundance, physiological state, and enzymatic activities in a bathing area of the Northern Tyrrhenian Sea (Latium, Italy) affected by urban wastewaters. The parallel factor analysis (PARAFAC) applied to the excitation–emission matrices (EEMs) of CDOM allowed us to distinguish three main components: C1 (λ<sub>Ex</sub>/λ<sub>Em</sub> = 342 nm/435 nm), C2 (λ<sub>Ex</sub>/λ<sub>Em</sub> = 281–373 nm/460 nm), and C3 (λ<sub>Ex</sub>/λ<sub>Em</sub> = 286 nm/360 nm). C1 and C2 corresponded to humic acids of terrestrial origin, while C3 to tryptophan, whose fluorescence peak was detected close to sewage sites, strongly related to active <i>E. coli</i> cells. The comparison between spectral and microbiological methods is suggested as a suitable approach to monitor bathing water quality for the implementation of coastal observing system capability.
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