- Significant inputs of hydrocarbons are continually released into the environment from anthropogenic and natural sources. Some of the most toxic hydrocarbon compounds are polycyclic aromatic hydrocarbons. Polycyclic aromatic hydrocarbons are known for their ability to absorb ultraviolet light and enhance toxicity. Generally, PAHs exert their toxicity via narcosis but UV-absorbing PAHs can become photosensitized and significantly exacerbate toxic effects. During crude oil spills, PAHs are released in large amounts that have potential for narcotic and phototoxic effects on aquatic organisms. As a result of the Deepwater Horizon oil spill, effort was placed on quantifying toxic effects of crude oils and oil constituents for aquatic organisms. The following studies attempted to characterize narcotic and phototoxic effects that may have occurred during the Deepwater Horizon incident; however, the results of this research are equally applicable to any PAH exposure scenario. The objectives of the following studies were to: 1) identify susceptible stages of Gulf of Mexico organisms to the photo-enhanced toxicity of PAHs, 2) determine the importance of UV intensity and exposure duration on phototoxicity, 3) determine the effect of alkylation on the phototoxic potency of PAHs, 4) validate the assumption of additivity for phototoxic PAHs mixtures, 5) evaluate the potential for narcotic toxicity and phototoxicity of fresh and weathered Macondo crude oils released from the Deepwater Horizon, and 6) assess the potential for ongoing oil phototoxicity at field sites in the Gulf of Mexico. Model organisms used in studies included the mysid shrimp (Americamysis bahia), inland silverside (Menidia beryllina), sheepshead minnow (Cyprinodon variegatus), and Gulf killifish (Fundulus grandis). Studies demonstrated that organism sensitivity to phototoxicity of PAHs decreased with organism age and increasing pigmentation. Photo-enhanced toxicity was, to some extent, dependent on the degree of organism pigmentation. Generally, high-intensity short-duration UV treatments resulted in greater toxicity than low-intensity long-duration UV treatments at similar UV doses. Fresh Macondo crude oil was more toxic than weathered crude oils, both in the presence and absence of UV light. Differences in toxicity between fresh and weathered crude oils were primarily attributed to the lighter mono and di-aromatic hydrocarbons in fresh crude oils. Phototoxic PAH concentrations were relatively similar among fresh and weathered crude oils, suggesting recalcitrance to oil weathering processes. The addition of Corexit 9500, an oil-dispersant used during the Deepwater Horizon oil spill, to crude oil in laboratory experiments increased toxicity compared to tests conducted with crude oil alone. It is anticipated that this enhanced response resulted from the increased concentrations of phototoxic and narcotic PAHs in water-accommodated fractions and the inherent toxicity of Corexit 9500. Weathered crude oil present in previously heavily-oiled Barataria Bay, LA field sites was found to pose little or no phototoxic risk in ambient environmental conditions four years after the Deepwater Horizon oil spill. Water-accommodated fractions of field-collected oil suggest slight phototoxic potential to mysid shrimp in the laboratory in highly transparent artificial seawater. When examining mixtures of phototoxic PAHs in crude oil, laboratory studies suggested that toxicity adhered to an "additive interactions" model; therefore, predictive toxicity models should consider an additivity model for assessing the toxicity of hydrocarbon mixtures. Furthermore, PAH phototoxic potency seemed to increase with increasing methylation for all phototoxic PAHs examined. In fact, phenanthrene, a non-phototoxic PAH, demonstrated a slight degree of phototoxicity when methylated. Overall, predictive models based on HOMO-LUMO gap were relatively accurate in predicting phototoxicity compared with empirical data generated in the present study. Future models should consider effects of other substituents on photo-enhanced toxicity of PAHs due to toxicity differences between unsubstituted and alkylated PAHs observed in the present studies. Data presented in this dissertation, can be used in part, as the basis for an ecological risk assessment for the photo-enhanced toxicity of oil constituents in the Gulf of Mexico during the Deepwater Horizon oil spill.