Study Demonstrates Sinking Marine Particles Help Remove PAHs from Water Column by Puspa L. Adhikari, Kanchan Maiti, and Edward B. Overton.
Researchers measured polycyclic aromatic hydrocarbon (PAH) concentrations in water collected near the Deepwater Horizon site to understand how sinking particles, such as marine snow, influence the residence time of PAHs in the upper ocean. They observed that dissolved PAH concentrations had decreased by over 1,000 times compared with concentrations reported during and immediately after the oil spill. They found that sinking particulate matter can remove up to 7% of particle-bound PAHs daily in the Gulf of Mexico. The researchers published their findings in Marine Chemistry: Vertical fluxes of polycyclic aromatic hydrocarbons in the northern Gulf of Mexico.
Crude oils are composed of thousands of chemical compounds, and PAHs are a class of these compounds that cause health concerns in an oil spill. PAHs enter the Gulf of Mexico marine environment year round via oil seeps and spills, land runoff, river discharges, coastal erosion, and atmospheric deposition. These hydrophobic compounds, which do not dissolve quickly in water, attach to marine particles and are deposited to sediments. While studies have investigated the fate and transport of Deepwater Horizon oil in the northern Gulf of Mexico, this study is the first to address the residence times of PAHs in the water column.
Researchers collected water samples (for dissolved PAHs), suspended particles (for particulate PAHs) and settling particles (for sinking PAHs). The team used sediment traps to collect sinking particles and pumps to filter water for suspended particles. They characterized PAH distribution, the removal mechanisms, and residence times.
Vertical profile samples exhibited a general pattern of low PAH concentrations at 100 m depth, elevated concentrations at 150 m depth, and then decreased or constant concentrations at further depths. Vertical profiles showed maximum particulate PAH concentrations at 100 m that remained similar throughout the water column to 350 m depth. Lower and middle molecular weight PAHs – which attach to marine particles more easily – dominated sinking PAH concentrations, confirming marine particles’ role in vertical PAH fluxes. The team determined that if vertical sinking was regarded as the only removal pathway, PAHs’ would exhibit residence times of about 15 days in the upper euphotic zone.
“Our results showed that the sinking particles including marine snow can efficiently transport PAHs from the surface oceans to the seafloor, and it is a key factor controlling PAHs’ removal from the upper oceans,” said study author Puspa Adhikari. This study is the first to measure particulate PAH concentrations and sinking PAH fluxes simultaneously using large-volume in situpumps and drifting sediment traps, which allowed researchers to directly estimate particulate PAH loss due to vertical fluxes and better understand vertical sinking’s role in open-ocean PAH cycling.
Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi: 10.7266/N7N29TXF.