Sediment-preserved diatom assemblages can distinguish a petroleum activity signal separately from the nutrient signal of the Mississippi River in coastal Louisiana M.L. Parsons a,⇑ , R.E. Turner b , E.B. Overton c a Coastal Watershed Institute, Florida Gulf Coast University, 10501 FGCU Blvd. South, Fort Myers, FL 33965, USA b Department of Oceanography and Coastal Sciences, Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA 70803, USA c Department of Environmental Studies, Louisiana State University, Baton Rouge, LA 70803, USA article info Article history: Available online 28 June 2014 Keywords: Gulf of Mexico Eutrophication Phytoplankton Hypoxia Oil spill abstract We analyzed the preserved diatom assemblages in dated sediment cores collected from five locations in the Louisiana Bight to test if there was a signature of petroleum extraction activities (hopanes and bar- ium) distinct from the well-documented effects of nutrient loading. The results of a multi-dimensional scaling analysis indicate that the diatom assemblage changes documented throughout the 40 year record could be explained by three variables: barium and hopanes concentrations, and Mississippi River nitro- gen loading. The results of a canonical correspondence analysis demonstrated that these signals could be distinguished through correlations with specific diatom species. The abundance of Actinoptychus senarius, for example, was negatively correlated with barium and the Pseudo-nitzschia delicatissima complex was positively correlated with nitrogen loading. These results provide a ‘‘proof-of-concept’’ demonstration that diatom assemblages preserved in the sediments can be used to study the effects of petroleum extrac- tion activities, and that these ‘petroleum signals’ may be distinguished from other significant influences such as nutrient loading. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Oil extraction activities have played significant economic and ecologic roles in coastal Louisiana since the 1940s. Canals, for example, were dredged in coastal wetlands for navigation, pipeline installation, and to transport drilling equipment to inshore and offshore oil fields, resulting in wetland loss (Scaife et al., 1983). Oil production in Louisiana nearshore waters spiked in the early 1970s, and began migrating farther offshore in the 1980s and 1990s (Louisiana Department of Natural Resources; U.S. Depart- ment of the Interior, Bureau of Safety and Environmental Enforce- ment). Produced water discharges increased steadily over this time frame (Turner et al., 2004), whereas the percentage of extracted oil that was spilled has decreased since the 1970s – until the 2010 Macondo oil spill. The State of Louisiana mandated baseline mon- itoring of background petroleum levels in 1999, resulting in several monitoring efforts over the next decade (e.g., Gisclair and McMillan, 2001; Gisclair and Iqbal, 2005). Although Louisiana has had an extensive oil industry for decades, studies on the potential impacts of oil spills on the coastal ecosystems were limited prior to 2010. The few ecological studies were primarily concerned with marsh plants (Pezeschki et al., 2000), zooplankton (e.g., Carman and Todaro, 1996), and benthic food webs (e.g., Carman et al., 2000). This dearth of studies meant there were many unknowns in assessing the impacts caused by large oil spills in the northern Gulf of Mexico. The 2010 Macondo oil spill brought these short comings to light. The Macondo oil spill began on April 20, 2010 when the Deep- Water Horizon drilling rig failed catastrophically, resulting in the release of 4.9 million barrels of oil (Camilli et al., 2010). This oil spill was likely the largest in U.S. history and, coupled with the depth at which the spill occurred (>1500 m), created an unprece- dented large and unique set of ecological impacts for the Gulf of Mexico, including coastal Louisiana. Approximately 75 linear km of Louisiana coastline was oiled, primarily on the seaward margins, with additional significant oil landings in northern Barataria Bay (Lin and Mendelssohn, 2012; Silliman et al., 2012). The initial impacts of the oil landfall included marsh grass mortalities and increased margin erosion (Silliman et al., 2012). In addition to the immediate impacts of the oil spill, there is concern as well for long-term (i.e., decadal) effects. For example, ecological impacts http://dx.doi.org/10.1016/j.marpolbul.2014.05.057 0025-326X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 12395907526. E-mail address: mparsons@fgcu.edu (M.L. Parsons). Marine Pollution Bulletin 85 (2014) 164–171 Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul