Project Summary

The Elkhorn Slough National Estuarine Research Reserve (ESNERR) is located in California. From 2009–2012, scientists conducted a study to determine which marsh sites at the reserve could be able to withstand the effects of sea level rise. With this information, conservation and restoration strategies can be developed to support the most resilient marsh sites.

Background

ESNERR is located 23 miles north of Monterey, California, on approximately 1,700 acres. It contains the second largest salt marsh in the state (San Francisco Bay is the first). Human activities have caused various amounts of degradation to the salt marsh, including altered sediment fluxes and decreased sediment inputs because of land use changes and sea level rise. A 2009–2012 project examined the ability of salt marshes in Elkhorn Slough to be resilient to sea level rise with these major alterations to sediment supply and salt marshes.

Implementation

ESNERR scientists collaborated with the National Oceanic and Atmospheric Administration, National Geodetic Survey, and U.S. Geological Survey on this study. These scientists examined tidal dynamics, sediment accretion rates, and salt marsh elevation to determine which marsh sites at the ESNERR might be able to withstand the effects of sea level rise. Marshes will need to elevate in order to withstand sea level rise, which is directly correlated to sediment accumulation. This project, funded by the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET), developed and implemented tools for use in managing salt marshes in the face of existing and projected stressors. Project activities throughout the funding period included:

• Developing a method to map the ancient, natural extent of Elkhorn Slough salt marshes. Team members analyzed 85 three-meter sediment cores in a 200m x 200m grid; one core was representative of about 10 acres. These sediments were classified as marsh, tidal flat, or other habitat types based on visual examination, organic content, and carbon/nitrogen ratios.
• Evaluating accretion rates. Thirteen shallow sediment cores were collected from healthy and degraded marsh locations within the Slough and analyzed for sediment composition and accumulation rates using radiometric techniques. Scientists monitored existing sediment accretion by using sediment-erosion tables, feldspar marker beds, and net-sediment tiles.
• Developing visualization tools to help managers set conservation targets. Team members used the Sea Level Affecting Marshes Model (SLAMM) and remote sensing data from previous projects to model future marsh sustainability. Coastal managers and stakeholders were part of the project’s refinement process, providing insight for the decision support system. Products are based on sea level rise scenarios developed by the Intergovernmental Panel on Climate Change. Team members determined impacted shoreline areas and affected property owners using these resources.
• Modeling migration scenarios for salt marshes and management strategies. Team members developed numerical models based on marsh response to sea level changes and sediment dynamics to assist decision-makers in developing conservation strategies.

Outcomes and Conclusions

Between 2009 and 2012, multiple workshops were held for regional and local officials, landowners, agricultural representatives, and other coastal decision-makers. These workshops provided stakeholders with iterative input into the design and implementation of final products. This project developed and implemented tools to support salt marsh management strategies in the face of global climate change and sea level rise. Project leads believe that this research demonstrates the ability of NERRs to act as sentinel sites for climate change.

Modeling of historic natural marsh extent in ESNEER revealed that the current extent of the marsh is low relative to baselines from earlier maps, but high relative to the past 200–3,000 years. Based on historic data, current sediment accumulation rates at most of the ESNEER marsh sites appear to be adequate to keep pace with sea level rise. However, the marshes continue to undergo “marsh drowning.” Modeling and visualization tools developed as part of the project allowed team members to view possible scenarios for marsh migration. SLAMM analyses showed the best option for marsh migration would be south of the estuary, an area currently occupied by farmland.

Citation

Gregg, R. M. (2021). Designing Salt Marsh Conservation Strategies in Elkhorn Slough National Estuarine Research Reserve [Case study on a project of the Elkhorn Slough NERR]. Version 2.0. Product of EcoAdapt's State of Adaptation Program. Retrieved from CAKE: https://www.cakex.org/case-studies/designing-salt-marsh-conservation-strategies-elkhorn-slough-national-estuarine-research-reserve (Last updated October 2021)