Hydrocarbons in size-fractionated plankton of the Mediterranean Sea (MERITE-HIPPOCAMPE campaign)

Aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs, respectively) were analyzed in the dissolved fraction (<0.7 mu m) of surface water and in various particulate/planktonic size fractions (0.7-60, 60-200, 200-500 and 500-1000 mu m) collected at the deep chlorophyll maximum, along a North-South transect in the Mediterranean Sea in spring 2019 (MERITE-HIPPOCAMPE campaign). Suspended particulate matter, biomass, total chlorophyll a, particulate organic carbon, C and N isotopic ratios, and lipid biomarkers were also determined to help characterizing the size-fractionated plankton and highlight the potential link with the content in AHs and PAHs in these size fractions. Sigma(28)AH concentrations ranged 18-489 ng L-1 for water, 3.9-72 mu g g(-1) dry weight (dw) for the size fraction 0.7-60 mu m, and 3.4-55 mu g g(-1) dw for the fractions 60-200, 200-500 and 500-1000 mu m. AH molecular profiles revealed that they were mainly of biogenic origin. Sigma(14)PAH concentrations were 0.9-16 ng L-1 for water, and Sigma(27)PAH concentrations were 53-220 ng g(-1) dw for the fraction 0.7-60 mu m and 35-255 ng g(-1) dw for the three higher fractions, phenanthrene being the most abundant compound in planktonic compartment. Two processes were evidenced concerning the PAH patterns, the bioreduction, i.e., the decrease in concentrations from the small size fractions (0.7-60 and 60-200 mu m) to the higher ones (200-500 mu m and 500-1000 mu m), and the biodilution, i.e., the decrease in concentrations in plankton at higher suspended matter or biomass, especially for the 0.7-60 and 60-200-mu m size fractions. We estimated the biological pump fluxes of Sigma(27)PAHs below 100-m depth in the Western Mediterranean Sea at 15 +/- 10 ng m(-2) day(-1), which is comparable to those previously reported in the South Pacific and Indian Ocean.

Automated summary

This study analyzed the distribution of aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs) in the Mediterranean Sea and investigated their relationship with plankton. The results showed that AHs were mainly of biogenic origin, while the concentrations of PAHs varied among different size fractions of plankton and were influenced by biomass.