**7. Trace metals**

The concentrations of vanadium increased significantly over time in both the continental shelf and the continental slope (from 121.74 ± 14.44 μg/g in M-I to 144.86 ± 28.51 μg/g in M-III), showing a recent input. The values observed in this study in some deep regions of the northwestern Gulf may constitute evidence of the influence of the 1500 m depth plume oil derived from the accidental DWH oil spill. The concentrations of certain trace metals increase as the oil weathering increases [45]. Similarly, the concentration of nickel increased gradually and significantly over time (from 31 ± 4.87 μg/g in M-I to 42.16 ± 8.52 μg/g in M-III) exceeding the sediment quality criteria LRE in most of the stations throughout the study, particularly in sites deeper than 1500 m. The detection of values higher than the MRE during M-III indicated the potential damage to the benthic fauna. The concentration of this metal in deep sites may be linked to processes of sediment transport from the northern Gulf, which includes degraded petroleum products. The concentration of cobalt also increased slightly over time (from 12.51 ± 1.6 μg/g in M-I to 16.08 ± 2.61 μg/g in M-III). This trace element was mostly concentrated along the outer continental shelf and upper slope of the area of study, showing a similar pattern to that of vanadium and nickel. The chrome maintained dissimilar concentrations during the 3-year monitoring period of observation. The highest concentrations of Cr were detected in coastal areas exposed to the intensive river runoff from the Bravo River and Pánuco River.
