many other situations, the complex interconnections of the ecosystem are not well understood, and thus quantification of the full impact of the spill on all components of the ecosystem may be difficult, if not impossible. Challenges to the assessment of damages under NRDA increase with the spatial and temporal scale of the injury and the complexity of the ecosystems involved, which makes it more difficult to understand and account for the full range of ecological and societal impacts. Quantification of the level of injury is further complicated when an event occurs in an ecosystem that is subject to natural and anthropogenic environmental changes that are unrelated to the specific event. In these cases, the injury caused by a specific spill must be assessed relative to dynamic or shifting baseline data.

The DWH oil spill was a large-scale event with impacts in the entire water column, from deep to shallow, and in open waters to coastal marshes and beaches. The ecology of the GoM system is complex and incompletely understood. It is a highly dynamic system, with a number of natural and human processes causing changes that result in a constantly shifting baseline. All of these conditions make attribution of injury and recovery planning challenging.

The interconnected nature of the coastal and marine ecosystems in the GoM makes it difficult to isolate injury to a single resource or single ecosystem process. Impacts tend to spread through the system because of physical processes, such as currents that spread oil from place to place, and biological processes that carry pollutants through the food chain and, as with migrating waterfowl, potentially cause effects far from the site of the oil spill. In addition, an understanding of social and economic processes is needed to determine how biophysical changes in ecosystems translate to injury to various segments of the public. Consequently, understanding of highly complex interconnected systems such as the GoM requires a more holistic view of ecosystems and the role of the people within them.

Two concepts articulate this more holistic view: ecosystem-based management and ecosystem services. Ecosystem-based management accounts for the complexity of interactions within an ecosystem (including those involving humans) rather than focusing on single resources or species in isolation (Christensen et al., 1996). It is a central element in the recently released National Ocean Policy Implementation Plan3 as well as an evolving theme in current fisheries management efforts in the United States (Sissenwine and Murawski, 2004). Ecosystem services are the benefits that the public receives from natural resources and the ecological processes provided by ecosystems (Daily, 1997; MEA, 2005).

Ecosystem services are produced as a result of the normal functioning of the ecosystem— the interactions of plants, animals, and microbes with the environment. Ecosystem services include provisioning services (the material goods provided by ecosystems, including food, feed, fuel, and fiber), regulating services (climate regulation, flood control, and water purification), cultural services (recreational, spiritual, and aesthetic), and supporting services (nutrient cycling, primary production, and soil formation). These services have immense value to society and are essential to the well-being of all people.

The GoM provides a broad array of provisioning, regulating, supporting, and cultural ecosystem services. Coastal tourism (a cultural service), for example, has an estimated worth



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