The Deepwater Horizon oil spill was primarily an offshore, oceanic event. Much of the oil entered the surface waters and was mixed throughout the epipelagic zone (1-200 m). Large amounts of oil also formed deep, subsurface plumes (500-1500 m). In addition, dispersant was applied at both the surface and at the wellhead, and its distribution generally followed the oil (Kujawinski et al. 2011). The oil and, to a lesser extent, the dispersant represent a large injection of carbon to an otherwise oligotrophic environment. The addition of this organic carbon has the potential to alter the carbon cycling and the particle flux of the pelagic ecosystem. In addition, an unprecedented amount and variety of polycyclic aromatic hydrocarbons (PAH) entered the Gulf of Mexico waters during the oil spill. The carcinogenic, mutagenic and teratogenic properties of PAHs make them a critical group of organic pollutants (Samanta et al., 2002) that are listed as priority pollutants by national and international environmental agencies (e.g., EPA-US, EEA-EU). While some components of the oil were transported to the surface or dissolved in the water column (Reddy et al. in press), sinking of particles is the major removal pathway of PAHs from the pelagic system as these compounds with low aqueous solubility readily adsorb onto the particles and are then exported to the seafloor as the particles sink. Thus removal of PAHs via sinking particles may play an important role in the distribution, dynamics and sinks of PAHs in the open ocean. The vast majority of animals in these zones in the offshore waters are plankton (phytoplankton, microzooplankton, mesozooplankton, and ichthyoplankton) and marine microbes. These organisms are the base of the food web and play a key role in the export of carbon and organic matter to the seafloor benthic communities. Marine microbes can utilize the carbon added to the ecosystem though whether the carbon is then available to the planktonic community or converted to carbon dioxide remains unclear. Understanding how the microbial and planktonic community responded to this event is critical as it is necessary to interpret any observed changes at higher trophic levels (i.e. fish). This information on the biologically-mediated fate and transport of the oil is a critical piece missing from the models accounting for the oil that was released. The proposed project would build upon our own research and the research of others to quantify the biological response to the Deepwater Horizon event. There is previous evidence that microbes responded by increased respiration both in the deep plumes (Valentine, et al., 2011) and in the surface waters (Edwards et al. 2011). A great deal of survey work has been conducted by Drs. Sutor, Benfield, and others to assess the plankton community in the affected areas offshore and those data are being analyzed presently. These data, in conjunction with the data we propose to collect in this project, will determine if there is a lasting impact of the oil spill on the plankton community and a potential alteration of carbon export to the benthos.
The paucity of PAH flux data in the open Gulf of Mexico water column and the absence of flux measurements in the water column critically limits our understanding of the transport and fate of PAH and further hinders the development of a PAH mass budget in the region. PAH enters the ocean mainly through direct discharge from oil spill accidents, natural seepage, riverine discharges, continental run-off and atmospheric deposition (Wakeham et al., 1980, Dach et al., 2002) and it is surprising that even then so little data exist about PAH in the Gulf of Mexico waters. . The persistent (low biodegrading) nature of the PAH coupled with the huge fishing industry in the Gulf make them a critical organic contaminant that needs to be studied thoroughly. The existing void of PAH inventory and flux data is largely a result of ease of contamination and the lack of a method that allows direct estimation of pollutant export from the mixed layer. Here we plan to use water column inventories of the suspended pool using large volume (>500L) in situ pumps, radioactive disequilibria between two natural radioisotopes (238U-234Th), sediment traps and bottom core samples to understand the removal flux of PAH from surface ocean to seafloor and their accumulation rates in bottom sediments. Measurement of pollutant fluxes in the region will provide us with a necessary first step to better constrain related models of pollutant cycling and also provide ideas about their removal rates and residence time in water column which will be invaluable for understanding the impact of any future oil spill (Dachs et al., 2002; Palm et al 2004). It is critical that we continue to monitor and further explore the potential impacts and recovery of the offshore planktonic community both to understand the immediate impact of the DWH oil spill, but also to better understand the rates and processes that drive these communities which have been studied very little up until now. In the event of another oil spill in the Gulf of Mexico, we will have a better understanding of how the system functions and so will be able to make better choices in the response and restoration phases to minimize impacts on this important natural system. We propose to conduct two research cruises, one in each year, to continue to monitor the microbial and planktonic communities and to assess rates of particle flux and the rates of removal of PAHs from the system via this process and accumulation of PAHs in bottom sediments. We will conduct surveys and collect samples in the field each spring to assess the physical and chemical water column properties as well as the abundance and distribution of microbes and plankton. We will conduct experiments in the field to measure the rates of bacterial production, primary production, respiration, and grazing. We will deploy the floating sediment traps at three depths in the water column to get a direct estimate of particulate organic carbon (POC) and PAH fluxes on time scale of days and also utilized 238U-234Th disequilibria ion the water column to understand the removal rates and remineralization of POC and PAH on time scales of weeks to months. An in situ pump system will be utilized to obtain high-volume water samples to get an estimate of the PAH inventory in the suspended particle pool while core samples will be collected to get an estimate of PAH accumulation in bottom sediments using 210Pb based sedimentation rates. We will work closely with the Earth Scan Laboratory (ESL) at LSU to utilize remote sensing data to place our observations in a larger physical and biological context. ESL is a satellite receiving station for polar-orbiting (MODIS, AVHRR) and geostationary (GOES-E) satellites specializing in ocean remote sensing. The ESL will provide us with sea surface temperature, sea surface height, and ocean color products that will enable us to assess the location and motion of oceanic features (eddies and the Loop Current) impacting our measurements.