Sources and spatial distribution of dissolved aliphatic and polycyclic aromatic hydrocarbons in surface coastal waters of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea)

The goal of this work is to investigate the spatial distribution and sources of dissolved aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs), as well as their relationships with a series of biogeochemical parameters (total chlorophyll a, phaeopigment a, suspended particulate matter, particulate and dissolved organic carbon, and nutrients), in surface coastal waters of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea). Samples were collected off the Northern and Southern coasts of Sfax city, as well as in the Gabès-Ghannouch area, from October-November 2014 and in March 2015. Total dissolved AH and PAH concentrations ranged from 0.02 to 6.3 μg L−1 and from 8.9 to 197.8 ng L−1, respectively. These data suggest that this area has been moderate-to-highly impacted by hydrocarbons compared to other Mediterranean coastal environments. The highest AH concentrations were measured off of the Sfax Northern coast (near Ezzit wadi) and in the Gabès-Ghannouch area, whereas the highest PAH concentrations were recorded in the Sfax Southern coast (near commercial and fishing harbours and the Sidi Salem channel). Analysis of the AH molecular patterns reveals that low molecular weight (LMW) compounds in these samples are dominated by n-C16, n-C17 and n-C18, which suggests that these samples feature high biogenic activity (in the form of bacteria or algae), and that high molecular weight (HMW) compounds display monomodal distributions centred on n-C25 and n-C26, which can be attributed to petrogenic inputs. The PAH molecular patterns are characterized by the dominance of LMW (2–3 rings) compounds and their alkylated homologues, which account for 90 ± 7% of total PAHs. The observed PAH distribution and isomer ratios thus indicate the presence of major petrogenic contaminations. The concentrations of AHs, PAHs and dissolved organic carbon (DOC) do not correlate with each other, thus demonstrating that these three pools likely originated from different sources and subsequently underwent different processes in the Gulf of Gabès.