The goal of the research conducted in this dissertation was to define and test methods to incorporate oil spill effects into an ecosystem-based assessment model. It was instigated by the Deepwater Horizon oil spill, an unprecedented oil spill in the United States for both depth and volume, with unknown implications for the health of the region. Using an ecosystem-based assessment model like Atlantis, with integrated oil spill dynamics, was the ideal candidate to predict long-term impacts such as decreased abundance or population recovery time. However no previous methodology existed for doing so in any ecosystem-based assessment model. Therefore, first I conducted a literature review to gather data across fish species on lesion frequency and fish body growth impacts from oil exposure. The two data sets were then fitted to four different dose-response models, and an effect threshold log-linear “hockey-stick” model was selected as the best fit and most parsimonious for both lesions and growth. Next, I conducted a similar analysis comparing macrofaunal and meiofaunal abundances to oil exposure concentrations in the Gulf of Mexico collected after Deepwater Horizon. I confirmed that these data had the domed relationship between invertebrate abundances and oil concentration observed in previous invertebrate oil studies. This domed relationship indicates that abundance increases at low to moderate oil levels, and declines at high oil levels. To drive this relationship in an Atlantis ecosystem model, three scenarios were tested in combination with oil toxicity: 1) Mississippi nutrient loading, 2) increased detritus from marine oil snow sedimentation and flocculent accumulation, and 3) predators altering their behavior to avoid oil exposure. At the Atlantis polygon resolution, only scenario 2, increased detritus from marine oil snow sedimentation and flocculent accumulation, generated the domed relationship for invertebrate abundances. Lastly, the “hockey-stick” model for fish mortality and growth was applied to both fishes and invertebrates in combination with scenario 2 for an integrated long-term assessment of the Gulf of Mexico. Newly available fish exposure data were used to generate an uptake-depuration model for this assessment. The combined effect forcings on vertebrates and invertebrates proved to have more severe long-term implications on population size and recovery than simulations with only fish forcings. Large demersal fishes, including elasmobranchs, were the most severely impacted by large biomass declines in the model spill region. Sensitivity analyses indicated that there was the potential for no recovery during 50 years of simulation in the spill region for many functional groups. Analysis of the synergy between fishing mortality F and toxicity from an oil spill identified that some guilds are more sensitive in an oil spill simulation to varied F than others. Snappers are the most sensitive to increased fishing mortality, while groupers respond the most to a reduction in fishing mortality. The invertebrate guild and small pelagic fishes responded the least to different values of F. Changing F also had implications for guild recovery – some guilds only fully recovered to control scenario biomass when F was reduced. A few functional groups were unable to survive with the combined effects of oil toxicity and increased F, and went extinct before the end of the 50-year simulation. Overall, this work provided the first framework for initial integrated modeling of oil spill impacts in an ecosystem-based assessment model, a potentially important component to future ecosystem-based fisheries management. The “hockey-stick” dose response model is applicable beyond Atlantis modeling, and can be tuned to fit specific events based on available data. I have also identified the importance of including marine oil snow sedimentation and flocculent accumulation to accurately drive the response of benthic invertebrates. Findings from the combined vertebrate and invertebrate simulations should help inform research efforts in the Gulf of Mexico and future oil spill response efforts.

The goal of this research is to understand the impacts of the 2010 Deepwater Horizon oil spill and the 1970-1980 IXTOC-I oil spill and other anthropogenic activity (e.g. dam construction) on surface water primary productivity by measuring sedimentary biogenic silica. It is known that sedimentary biogenic silica is distinct from mineral – bound silica, therefore it has been used as a proxy record for surface water primary productivity (e.g. diatom blooms). The Deepwater Horizon oil spill resulted in a widespread Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) event. The IXTOC-I oil spill was one of the largest oil spills in history and it is likely that the MOSSFA event occurred as a direct result. MOSSFA is characterized by increased deposition of surface derived components and dramatic changes in post-depositional chemical (redox) and biological (benthic meio- and macro-fauna) conditions. Sedimentary biogenic silica provides an independent record of the surface derived portion of MOSSFA inputs. Occurrences of MOSSFA after IXTOC-I and Deepwater Horizon were compared by collecting sediment cores from the northern Gulf of Mexico (Deepwater Horizon) and the southern Gulf of Mexico (IXTOC-I). An age model for each core was developed using short-lived radioisotopes (i.e. 210Pbxs). Sedimentary biogenic silica was significantly elevated in sedimentary intervals affected by the Deepwater Horizon spill. This suggests that a significant portion of the surface biological materials entrained during the MOSSFA event were sourced by diatom production. However, only one core (of three from the oil spill influenced area utilized in this study) from shallower depth had elevated levels of sedimentary biogenic silica in the sedimentary interval associated with IXTOC-I. Also, the down-core profiles of sedimentary biogenic silica from the other cores collected in the southern Gulf of Mexico are consistent with the history of dam construction (1949 to 1989) on the Grijalva and Papaloapan river systems. These two river systems are the dominant freshwater and nutrient sources for primary production in the Bay of Campeche region in the southern Gulf of Mexico and therefore the dominant control on diatom productivity and sedimentary biogenic silica distribution. Consequently, distribution of annual fresh water outflow and nutrient supply has transitioned from seasonal (before 1940’s) to stable (after 1980’s). Overall, sedimentary biogenic silica provides an independent record of surface derived MOSSFA inputs and serves as a proxy for other anthropogenic influences related to surface primary productivity variability.