The Deepwater Horizon oil spill constituted an ecosystem-level injury in the northern Gulf of Mexico. Much oil spread at 1100-1300m depth, contaminating and affecting deepwater habitats. Factors such as oil-biodegradation, ocean currents and response measures (dispersants, burning) reduced coastal oiling. Still, >2100km of shoreline and many coastal habitats were affected. Research demonstrates that oiling caused a wide range of biological effects, although worst-case impact scenarios did not materialize. Biomarkers in individual organisms were more informative about oiling stress than population and community indices. Salt marshes and seabird populations were hard hit, but were also quite resilient to oiling effects. Monitoring demonstrated little contamination of seafood. Certain impacts are still understudied, such as effects on seagrass communities. Concerns of long-term impacts remain for large fish species, deep-sea corals, sea turtles and cetaceans. These species and their habitats should continue to receive attention (monitoring and research) for years to come.

Exploratory discharge of produced water and drilling cuttings from oil and gas production has become a major source of pollutant to the biota. The aim of this study is to review previous research on the environmental effect of such exploratory wastes with emphasis on the Niger Delta region. Drilling waste is composed of Aliphatic Hydrocarbon, Polynuclear Aromatic Hydrocarbon (PAH) and heavy metals such as cadmium, chromium, lead, mercury, arsenic , copper, Iron, lead, manganese, zinc, barium and strontium among others which are toxic to the environment. The composition and characteristics of naturally occurring chemical substances in produced water (PW) are closely associated to the geological characteristics of each reservoir. The toxicity of produced water effluent can be reduced when treated before discharging into the sea. Sulphide reducing microorganism which are associated with produced water pose threat to the environment. Barite and bentonite present in most drilling fluid were found to reduce plant growth. Studies in some part of the Niger delta have shown high level of some heavy metals associated with exploratory waste with concentrations higher than world health organization (WHO) standard; these have negative impact on the environment such as massive destruction to aquatic lives and agriculture.

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Environmental regulatory edicts within the EU, such as the regulatory framework for chemicals REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), the Water Framework Directive (WFD), and the Marine Strategy Framework Directive (MSFD) focus mainly on toxicity assessment of individual chemicals although the effect of contaminant mixtures is a matter of increasing concern. This discussion paper provides an overview of the field of combined effects in aquatic ecotoxicology and addresses some of the major challenges related to assessment of combined effects in connection with environmental risk assessment (ERA) and regulation. Potentials and obstacles related to different experimental, modelling and predictive ERA approaches are described. On-going ERA guideline and manual developments in Europe aiming to incorporate combined effects of contaminants, the use of different experimental approaches for providing combined effect data, the involvement of biomarkers to characterize Mode of Action and toxicity pathways and efforts to identify relevant risk scenarios related to combined effects are discussed.

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