Marsh migration and beyond: A scalable framework to assess tidal wetland resilience and support strategic management

Abstract

Tidal wetlands are critical but highly threatened ecosystems that provide vital services. Efficient stewardship of tidal wetlands requires robust comparative assessments of different marshes to understand their resilience to stressors, particularly in the face of relative sea level rise. Existing assessment frameworks aim to address tidal marsh resilience, but many are either too localized or too general, and few directly translate resilience evaluations to recommendations for management strategies. In response to the deficiencies in existing frameworks, we identified a set of metrics that influence overall marsh resilience that can be assessed at any spatial scale. We then developed a new comprehensive assessment framework to rank relative marsh resilience using these metrics, which are nested within three categories. We represent resilience as the sum of results across the three metric categories: current condition, adaptive capacity, and vulnerability. Users of this framework can add scores from each category to generate a total resilience score to compare across marshes or take the score from each category and refer to recommended management actions we developed based on expert elicitation for each combination of category results. We then applied the framework across the contiguous United States using publicly available data, and summarized results at multiple spatial scales, from regions to coastal states to National Estuarine Research Reserves to finer scale marsh units, to demonstrate the framework's value across these scales. Our national analysis allowed for comparison of tidal marsh resilience across geographies, which is valuable for determining where to prioritize management actions for desired future marsh conditions. In combination, the assessment framework and recommended management actions function as a broadly applicable decision-support tool that will enable resource managers to evaluate tidal marshes and select appropriate strategies for conservation, restoration, and other stewardship goals.

Fig 1. Best management options to maximize resilience to relative sea level rise for each marsh type.

Green text indicates a positive condition, red reflects a negative one. From a restoration or adaptation practitioner’s perspective the more resilience categories that are shown in red for each marsh type, the more costly a project will likely be to implement.

Fig 2. Regional resilience of marshes as assessed by metrics of current condition, vulnerability to relative sea level rise and marsh adaptation potential.

The two categories that have a positive influence on tidal marsh resilience to relative sea level rise—current condition and adaptive capacity—range on a positive scale from 1 (low) to 10 (high). Collectively, metrics in the vulnerability category have a negative influence on resilience, so this category is scored on a negative scale from -1 (low vulnerability) to -10 (high vulnerability). MHHW stands for Mean Higher High Water. Country outline: United States Census Bureau.

Fig 3. Average marsh resilience score for each coastal state and each National Estuarine Research Reserve within the contiguous US.

Reserves with a standard deviation of zero have their boundaries contained all within one HUC-12 boundary. Country outline: United States Census Bureau.

Fig 4. Resilience scores for marsh units of the Gulf Islands National Seashore in Mississippi and Florida.

Colored polygons represent MUCs assessed in our national application of the framework. Islands labeled with letters are individual subwatershed boundaries within the Gulf Islands National Seashore. Country outline: United States Census Bureau. World Imagery source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community.

Fig 5. Resilience scores for each marsh unit, resilience category, and composite metric in the Gulf Islands National Seashore.

Letters identify marsh units within the National Seashore, as displayed in Fig 4. The two categories that have a positive influence on tidal marsh resilience to relative sea level rise—current condition and adaptive capacity—range on a positive scale from 1 (low) to 10 (high). Collectively, metrics in the vulnerability category have a negative influence on resilience, so this category is scored on a negative scale from -1 (low vulnerability) to -10 (high vulnerability). This figure serves as an example of detailed results generated by applying our tidal marsh assessment framework.