process known as Natural Resource Damage Assessment (NRDA) is used by “trustees” to determine the extent and severity of that injury. Trustees, who include representatives of the federal government, tribes, and affected state governments, must attempt to (1) quantify the extent of damage; (2) develop, implement, and monitor restoration plans; and (3) seek compensation for the costs of assessment and restoration from those deemed responsible for the injury. The goal of this effort is to “make the environment and the public whole for the injuries to natural resources and services” (NOAA, 1996).

Under common NRDA practice, losses are generally measured in ecological terms (e.g., number of acres damaged or number of fish killed) and restoration generally follows relatively straightforward equivalency approaches (e.g., acres of habitat restored or fish stocks replaced) (described in Chapter 2). The injuries to the ecosystem and the services it provides are quantified by comparing the services to a baseline when possible. When the service is well known (e.g., the income lost from the closure of a particular fishery), the assessment of injuries can be straightforward. However, for other services, their connection to ecosystem condition is less well established because baseline data have not been collected (e.g., hydrocarbon levels in marsh sediments) or baseline ecological data have not been linked to services (e.g., acreage of wetlands but not the value to fisheries).

Additional challenges to assessment arise as the spatial and temporal scale of the injured system, and the complexity of the ecosystem, increase. In these cases it becomes increasingly difficult to understand and account for the full range of ecological impacts and to translate those impacts into reductions of ecosystem services. It also becomes more difficult to determine what the baseline conditions might have been in ecosystems subject to other natural and manmade environmental changes unrelated to a specific event.

The Gulf of Mexico (GoM), often referred to as the Gulf of Mexico Large Marine Ecosystem (GoM LME), is remarkably rich and complex and provides a wealth of ecosystem services. The Gulf of Mexico provides important regulating, supporting, and cultural services, which include coastal tourism with an estimated worth of $19.7 billion per year (National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, 2011), storm surge protection by coastal wetlands, habitat for migrating waterfowl, cycling of nutrients from river discharges, and the unique cultural heritage of coastal communities. Provisioning services include food, biochemical and medicinal compounds, clean water, and energy in the form of crude oil and natural gas. In 2008, the GoM commercial fish and shellfish harvest yielded a dockside value of $659 million (1.27 billion pounds; NMFS, 2010). These commercial landings accounted for approximately 25 percent of the seafood

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