"The effects of the Deepwater Horizon oil spill on natural resources are dependent on multiple factors including oil composition, oil quantity, dispersal techniques, and contact with organisms. Broadly speaking, when offshore, impacts may occur in the upper meter or so of the water column, mid-level mixing layer (through dispersal of oil and toxic components) and at the sea floor. When onshore, impacts may occur to shorelines, nearshore waters, and coastal habitat. To help quantify the magnitude of impact and injures, modeling efforts will be supported through data collected during the spill."
National Oceanic and Atmospheric Administration: http://response.restoration.noaa.gov/