Fluorescence properties of dissolved organic matter in coastal Mediterranean waters influenced by a municipal sewage effluent (Bay of Marseilles, France)

Abstract

Environmental contextMarine dissolved organic matter plays a key role in the global carbon cycle. Questions remain, however, as to the influence of anthropogenic activities on its composition and distribution in coastal waters. It was found that dissolved organic matter in the vicinity of a municipal sewage effluent (Marseilles City, France) contained a high proportion of protein-like material, thereby demonstrating the influence of human activities on coastal dissolved organic matter. AbstractFluorescent dissolved organic matter (FDOM) in coastal marine waters influenced by the municipal sewage effluent (SE) from Marseilles City (France, north-western Mediterranean Sea) has been characterised. Samples were collected eleven times from September 2008 to June 2010 in the Bay of Marseilles along a coast–open sea transect from the SE outlet in the South Bay and at the Mediterranean Institute Observation site in the central Bay. Fluorescence excitation–emission matrices combined with parallel factor analysis (PARAFAC) allowed the identification of two protein-like (tyrosine C1, with excitation maxima (λEx) and an emission maximum (λEm) of <230, 275/306 nm; tryptophan C2, λEx/λEm <230, 270/346 nm) and three humic-like components (marine humic C3, λEx/λEm 280/386 nm; C4, λEx/λEm 235, 340/410 nm; C5, λEx/λEm 255, 365/474 nm). From the SE outlet to the central Bay, a gradient appeared, with decreasing FDOM intensities, decreasing dissolved organic carbon, particulate carbon, nutrients and faecal bacteria concentrations and increasing salinity values. This gradient was associated with decreasing abundances in protein-like fluorophores and rising abundances in humic-like (C3 and C5) materials. This shift in FDOM composition illustrated the decrease in wastewater inputs and the increase in marine sources of DOM along the transect. FDOM data showed that the Marseilles SE spread up to 1500 m off the outlet, but it did not reach the central Bay. Tryptophan-like material was the dominant fluorophore in the SE and displayed the highest correlations with biogeochemical parameters (organic carbon, phosphates, faecal bacteria). Therefore, it is proposed to use its fluorescence intensity to detect and track SE inputs in the Marseilles coastal marine waters.