**6. Polycyclic aromatic hydrocarbons (PAHs)**

 The PAH included a range of 16 individual compounds that are commonly analyzed [38] in monitoring programs. The concentrations of total PAH in subsurface waters remained below the analytical detection limits (<0.003–0.03 μg/L) in the three surveys. Similarly, the ∑ individual PAH ranged between 0.1 and 0.02 μg/L. In M-I, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, and indeno(1,2,3-cd)pyrene were recorded in 14.3% of the sites [39]. In M-II, only low concentrations of benzo(b)fluoranthene were recorded in seven sites which is equivalent to 28%. In W-III, the individual PAH identified were indeno(1,2,3-cd) pyrene, benzo(b)fluoranthene, benzo(a)pyrene, and pyrene. These compounds were detected in seven sites, representing 21.2%.

 The concentrations of PAH contained in sediments fluctuated significantly among the three oceanographic campaigns. The values were similar to those previously recorded by [40] in the Tamaulipas continental shelf and by [41] in the Campeche and Tabasco continental shelves, regions widely exposed to intense oil activities. In M-I, the concentrations oscillated among 0.01 and 0.70 (0.29 ± 0.17 μg/g). These values decreased in 90% of the sampling stations in M-II, presumably as a consequence of the Hurricane Alex and associated rains. In this period, concentrations of 0.03–0.51 (0.16 ± 0.12 μg/g) were recorded. Throughout the following winter (M-III), PAH increased, presenting values of 0.05–1.54 (0.44 ± 0.03 μg/g). This increase indicated a recent deposit of hydrocarbons, considering that the sedimentation rate is very low in the deep GoM [42].

The total PAH recorded in sediments exhibited a heterogeneous spatial distribution, but high concentrations were frequently recorded in the northern transect of the study area. In M-I, the highest concentrations were observed at 100 and 500 m depth, in M-II at 500 and 1500 m depth and in M-III at >2000 m depth. The presence of high concentrations of PAH in deep sediments (>500 m) is not likely related to Rio Bravo runoff but rather to a far-field transport of hydrocarbons from other than local sources. The observed interannual heterogeneity (temporal/spatial) in the PAH concentrations in the NW Gulf can find an explanation in the geochemical processes acting upon different sources of hydrocarbon compounds. Indeed, one of such processes is the biodegradation of fossil fuels oil by oil-degrading bacteria [3, 43] that takes place when a massive oil spill occurs.

 The distribution of individual PAH was heterogeneous throughout the study, but among the predominant were the chrysene in M-I (0.06 ± 0.02 μg/g), the fluorene in M-II (0.06 ± 0.01 μg/g), and the benzo(a)anthracene in M-III (0.08 ± 0.07 μg/g). These are low molecular weight compounds generated from the burning of fossil fuels and are abundant in crude oil, so they are indicators of a recent input of anthropogenic hydrocarbons [44]. The benzo(a)anthracene and chrysene have acute toxicity and represent an environmental risk.

The primary origin of PAH in the area of study was pyrolytic. In some stations, the combustion of fossil fuels is predominated, while in others, the combustion of organic carbon, plants, wood, and other vegetal compounds prevailed. A mixture of PAH of pyrolytic and petrogenic sources was restricted to few stations throughout the study. Among the petrogenic sources, crude oil was remarkable. This indicates an anthropogenic input to the area of study because of the fossil fuel burning.

Total PAH concentrations were below sedimentary quality criteria (LRE and MRE), whose exceeded limits indicate a potential adverse effect on benthic biota. However, among individual PAH, acenaphthylene, fluorene, benzo(a)anthracene, and dibenzo(a,h)anthracene exceeded the low-range effect (LRE) criterion in four stations in M-I. In M-III, acenaphthene, fluorene, dibenzo(a,h)anthracene, and

anthracene exceeded the LRE in more than 23 stations. These results showed that the toxicity of the sediments caused by the presence of hydrocarbons increased throughout the study and hence the potential risk to the benthic fauna.
