Gulf of Mexico
Example of synergism in coastal restoration. Fish move among complex habitats such as seagrass meadows and salt marshes in coastal seascapes (dark gray arrows). By restoring these habitats in close proximity to one another we might improve the habitat values, productivity, and carrying capacity of coastal seascapes for fish and fisheries (light gray arrows and ellipses). Symbols courtesy of the Integration and Application Network,

LA - Ten years of Gulf Coast ecosystem restoration projects since the Deepwater Horizon oil spill

In 2020, the National Academies of Sciences, Engineering, and Medicine (NASEM) Gulf Research Program created the Committee on Long-Term Environmental Trends in the Gulf of Mexico. Our committee was tasked to consider the synthesis of additive, synergistic, and antagonistic cumulative effects resulting from ecosystem restoration following the 2010 Deepwater Horizon(DWH) oil spill.

This anticipated multidecadal restoration was made possible by dedicated settlement monies, distributed over the past decade as governed by the RESTORE Act of 2012 and other legal vehicles, which are today approaching one-half spent or committed. Thus, in our view, it is important to take stock of progress and, looking forward, to make recommendations regarding strategies for evaluation and management.

It is timely to examine the collective effects of coastal restoration across the five US states because the spatial and temporal coordination of restoration throughout a geographic region can substantially increase the return on investment (1). By the end of 2021, more than 570 environmental restoration projects were underway or completed. These included at least 152 focused on habitat restoration and enhancement, 82 on species restoration, and 47 on water-quality restoration and management, conducted by numerous state, federal, nonprofit, and other entities.

Synergistic and antagonistic interactions are a basic and cross-cutting concept addressed throughout our recent report (2). Ecological synergies occur when, for instance, living vegetated shoreline and oyster reef restorations interact to create seagrass meadows between them, or seagrass meadows and salt marshes interact to increase fish productivity (3). Conversely, coastal restoration utilizing freshwater and sediment diversions of river water could prove to have antagonistic effects. While mineral sediments may increase wetland volume and area, the changes in salinity, water quality, and substrate could be deleterious for marsh persistence, oysters, and other estuarine biota in the diversion area (46).

A robust consideration of restoration-related changes must also include the impacts of acute events (e.g., hurricanes) and long-term environmental trends (e.g., sea-level rise) on the valuable resources along the dynamic, spatially variable coastline of the US Gulf of Mexico (GoM). Yet, while the US West and East Coasts have hosted environmental synthesis centers since 1995 and 2011, respectively, the Gulf Coast does not have a scientific body performing similar functions. Here we argue that the time for synthesis of data and products from GoM restoration projects is now and the need is urgent.

One way to synthesize and evaluate large-scale restoration involves applying the concept of cumulative effects. The cumulative effects of restoration are the collective additive, synergistic, antagonistic, or null effects of all restoration activities that occur within a setting defined by common or connected characteristics of hydrology, geomorphology, ecology, ecological function, and biodiversity. There remain three critical challenges to the measurement of effects that must be addressed to encompass the full cumulative effects of large-scale restoration:

1). understanding and accounting for synergistic and antagonistic effects of restoration;

2) incorporating long-term trends and acute events in background environmental conditions; and

3) evaluating the spatial and temporal scale of restoration effects relative to other environmental changes.

In ecosystem restoration practice, inherent constraints make carefully designed experiments the exception, not the norm, and this tradition appears to have continued across the GoM since settlement-funded restoration began. The GoM itself or individual estuaries, bays, and watersheds are the experimental units for restoration effects and cannot be replicated to support a formal experimental design. Restoration since the spill has proceeded partially in a coordinated fashion but also has occurred in an ad hoc manner due to the manifold participants including five US states, multiple federal agencies, and two new organizations (NASEM’s Gulf Research Program and a federal agency, the Gulf Coast Ecosystem Restoration Council). Eighteen funding streams empowered by the settlements (appendix A of ref. 2) produce intersecting timelines of regulatory and environmental requirements.

In our report we reviewed the available data describing the nonstationary environmental trends facing the region. Against that backdrop, we then assessed approaches for evaluating the cumulative effects of restoration at multiple scales in the absence of an overarching, a priori study design by the restoration programs. Here we summarize the rationale for and essential features of an approach to synthesize the cumulative effects of restoration as recommended in our report, an approach rooted in lines of evidence and comparative analysis methods already proven on the Gulf Coast and elsewhere, which can and should be started now.

Synergism and Antagonism in Ecosystem Restoration

Cumulative interactions—whether they are between drugs in the human body, or ecological interactions within an ecosystem, or among restoration projects—are generally considered as additive (the sum of their parts), synergistic (interactions are greater than the sum of the parts), antagonistic (interactions are less than the sum of the parts), or null in effect. Any of these interactions may be viewed as positive or negative, depending on the objectives, values, and perspectives of assessors. Understanding the potential end results of these interactions and how they can enhance restoration, both through positive synergies and by avoiding negative antagonistic interactions, should be a desirable goal for all restoration funders.

Although understanding remains incomplete and worthy of pursuit, some evidence exists for each of these types of interactions among and between human-constructed ecosystem restoration projects (Fig. 1). Ecosystem restoration may intentionally facilitate synergistic interactions among habitats (7) or species (8, 9).

Incomplete understanding poses a vulnerability for effective planning and engineering. Activities may blunt or, conversely, augment the expected gains from restoration. This has the potential to either reduce the cost effectiveness of restoration projects if unexpected antagonistic effects occur or to generate larger returns than expected if unexpected synergistic effects occur. Syntheses of cumulative effects focused on certain types of restoration method (e.g., living shorelines), or of two or more methods used in proximity to one another, therefore have the potential to directly inform advances in future ecosystem-restoration applications. A spatial example is that effects driven by localized environmental conditions that may not be replicated elsewhere may lead to a false indication of the value of restoration. A temporal example is when effects occur in the short run but disappear over longer durations (or vice versa, time lags that delay the appearance of restoration effects). The true expected outcomes of restoration may be confused with the activities implemented or with preliminary initial outputs (e.g., the number of acres treated with restoration measures).



Heida L. Diefenderfer

Larry D. McKinney

Walter R. Boynton

Kenneth L. Heck Jr.

Barbara A. Kleiss

Deepak R. Mishra

Holly Greening

Albert A. George II

Bethany A. Carl Kraft

Catherine L. Kling1


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