DELTA LEAD SCIENTIST: Sea level rise and the Delta’s wetlands

This article has been updated to reflect feedback from the study’s authors.

Wetlands an ‘important battle partner’ in managing sea level rise, plus how atmospheric rivers might factor in

At the March meeting of the Delta Stewardship Council, Delta Lead Scientist Dr. Laurel Larsen spotlighted recent research on the effect of sea level rise on wetlands.

Dr. Laurel Larsen began the article spotlight with a question:  How do we estimate which parts of coastal landscapes will be inundated with future sea level rise?  Coastal planners often use the ‘filling the sink’ approach:  They use GIS elevations and draw higher and higher lines across the bathymetry, with areas higher than the new sea level escaping inundation or vice versa.

However, that approach gives a worst-case scenario for inundation because it discounts the fact that some types of ecosystems, such as wetlands, build elevation over time while sequestering organic carbon.  If conditions are right, they can effectively keep up with sea level rise.

“So in this way, I like to think of wetlands as a tremendous battle partner to have in the fight against sea level rise,” she said.

This is one reason wetlands factor importantly into coastal resilience and adaptation plans like Delta Adapts or Bay Adapt.  But wetlands have limits and can only be pushed so far before they succumb to rising seas.  So effective coastal adaptation planning needs predictive tools to identify wetland management strategies to help ensure the long-term persistence of wetlands.

The most common predictive models of wetland resilience simulate processes through which wetland soils are generated and lost and keep track of wetland elevations relative to the mean sea level.  The models are based on the idea that there’s an optimum water depth at which wetland plants are most productive, meaning they rapidly produce organic matter in the form of stems, roots, and leaves.

The plot in the lower right of the graphic shows elevation relative to sea level on the y-axis and the plant growth range on the x-axis.  There is a range of depths at which wetland plants can tolerate conditions, so the productivity goes to zero at an upper and lower elevation, as depicted by the bell-curve line.

Plants go through natural and seasonal cycles when they shed material.  In the forest, leaves and organic material on the forest floor are exposed to a lot of oxygen, which makes the decomposition rapid and soil accumulation slow.  However, in wetlands that regularly have standing water, oxygen at the soil-water interface is much more scarce, so decomposition is slower.

“What this means is that the plant material tends to stick around longer in the form of highly organic wetland soil, which we call peat in the Delta,” said Dr. Larsen.  “And this can accumulate at a rate much faster than in environments like forests with more oxygen.”

On the plot, a wetland at equilibrium with sea level is the point where the horizontal red line intersects with the wetland productivity curve; this is the point at which soil accumulates at the same rate as the sea level rises.  So if sea level rise were to decelerate suddenly, the wetland would be more exposed, the soil would be oxygenated more frequently, and plants would be less productive, resulting in slower soil accumulation.  That slower growth would make the wetland’s elevation relative to sea level lower, and eventually, it would come to equilibrium again to match that lower level of sea level rise.

In the case of accelerating sea level rise, the plants become more productive, there’s less oxygen at that soil-water interface, and soil accumulates faster, allowing the wetland to catch up with sea level rise.

“Now, that’s true up to a point when the wetland passes through the red point on the top, which is its maximum growth and soil accumulation rate,” said Dr. Larsen.  “The horizontal dashed line, if it continues to move up, loses contact with the curve.  And this means that with future increases in sea level rise, the wetland can only slow down its soil accumulation processes as it’s on the other side of that maximum.  And as its elevation continues to decrease, it just produces organic matter soil at a slower and slower rate.  So essentially, it will eventually drown at that point.”

So the maximum on this plot, the vertical red-dashed line, separates wetlands into two kinds: resilient for wetlands that could keep up with sea level rise in green and unsustainable for wetlands that will inevitably drown in red.   These processes are relatively easy to model, and the models can be calibrated to individual wetlands with different types of species and different soil characteristics.

The Delta Adapts project used a marsh sustainability model called WARMER II to estimate the vulnerability of Delta wetlands to sea level rise and found that all marshes will succumb to drowning under six feet of sea level rise in 100 years.  A recent paper, An assessment of future tidal marsh resilience in the San Francisco Estuary through modeling and quantifiable metrics of sustainability by Morris et al. (2022), used a new model called the Coastal Wetland Equilibrium Model (CWEM) that was adapted specifically to the Delta. CWEM improves upon previous marsh sustainability models by more realistically estimating how belowground organic matter, which is a major component of marsh soils, changes with time. The authors also came up with a new way to measure marsh resilience to sea-level rise called the tipping point, the time at which a marshʻs ability to build its elevation starts to decline.

The authors also incorporated estimates of sediment deposition from atmospheric rivers into the modeling. They did this by using the thickness of sediment deposited on Browns Island after passage of a category 5 atmospheric river and combining it with the recent frequency of major atmospheric rivers in the Delta region.

“So what did Morris et al. (2022) find?  They found that a Delta marsh of average elevation will survive 100 years or more even at a centennary sea-level rise of 200 cm (6.6 feet); however most Delta marshes have a tipping point of between 41 and 64 years,” said Dr. Larsen.  “This is a rosier picture than what was found by the Delta Adapts study, but still illustrates the ultimate vulnerability of the Deltaʻs microtidal marshes to sea-level rise. The more optimistic CWEM projections are due to an improved parameterization of organic matter and the incorporation of the full elevation range of tules into the modeling. The addition of triennial additions of 6 mm of sediment from atmospheric rivers into the modeling boosted marsh resilience by increasing the proportion of marshes that survived from 51% to 72% and decreasing the proportion that drowned from 49% to 28%.”

“The authors’ takeaways from these findings are that it is imperative not to wait to restore wetlands, as we lose that important partnership with wetlands that I discussed earlier once they pass through their tipping point,” she continued.  “They also pointed out that models like this can help identify which wetlands might be near the tipping point, as those wetlands might be saved by adding slurries of sediment to them to raise their elevations – an expensive intervention, but one that is done in the Bay and can buy us time.”

“Finally, the authors stress that these sobering statistics require planners to think in more than just one dimension (the vertical dimension) and identify places for wetland restoration where wetlands may be able to retreat laterally by moving inland, to higher elevations. Science articles like this illustrate just how important it is to think about wetland restoration in a coordinated and holistic manner that takes into account spatial considerations and a future elevation profile that might be very different from today. It is fantastic that we have the DPIIC Restoration subcommittee to take on these challenges.”

Councilmember Diane Burgis asked how saltwater intrusion associated with sea level rise might impact vegetation and species in the marshes.

“Salinity intrusion does have a very noticeable shift in the species compositions within wetlands, and we do see an inland shift of salt-tolerant wetland species,” said Dr. Larsen.  “Generally, the more freshwater wetlands are the ones that tend to produce more organic matter and have more organic soils.  They produce a lot more below-ground matter in the form of roots that then get preserved in the soil.  So this organic process of soil generation I talked about is strongest in the freshwater wetlands.”

“The flip side of this is that saltwater wetlands are really good at trapping sediment suspended in the water, so you tend to get less organic soils.  And there’s a little bit of this feedback process that I described at play there.  So there are more stems that are more effective at trapping that sediment that’s in the water column.”

Chair Virginia  Madueño asked how the atmospheric rivers we’ve experienced this year have impacted wetlands.

Dr. Larsen said that climate models project that atmospheric rivers will become more frequent and intense, containing more moisture.  “In this paper, those atmospheric rivers provide a net benefit to wetlands because a big driver of whether wetlands can keep up with sea level rise is how much sediment is available to be deposited on them that originated from elsewhere.   In the Delta, we’ve seen a major decline in suspended sediment in the rivers since the plug of material liberated during the Gold Rush era moved through the estuary.  That’s a concern for fish because many of our native fish species require turbid waters to evade predation.  But it’s also a concern for wetlands because having less sediment carried from other regions means there’s less available to support the vertical growth of wetlands.  So atmospheric rivers could be helpful.”

“One thing that hasn’t been looked at is the extent to which they mobilize nutrients in the watershed.  That could be a double-edged sword because it can support productivity, but on the other side, too many nutrients can contribute to harmful algal blooms.  So we are interested in seeing more research on the effects of these atmospheric rivers.”

“The other thing is that these atmospheric rivers have connected new wetlands to the Delta that are usually disconnected, particularly those in our bypasses.  We know from over a decade of research that is a really good thing for supporting food webs and providing shelter and rearing areas for our native fish species.”

Integrated Modeling Framework Workshop

The two-day Integrated Modeling Framework Workshop, “One Delta, One Science, One Modeling Framework Workshop was held at the beginning of March.  The plenary session featured visionary presentations that inspired ideas about incorporating human behavior into models, quantifying uncertainty, and the concept of the best available tools to complement the best available science.  Panels discussed how local agencies and investigators are dealing with decision-making under deep uncertainty, assimilating data into models, designing effective workflows that deal with big data, computational complexity and the need to generate information quickly, and combining information from many different types of models of climate, water, ecosystems, and humans to address large scale and difficult challenges.

In the afternoon, participants identified the grand challenge topics that would be significantly advanced by more coordination around modeling and improved modeling tools.  They then ranked these topics for their feasibility and their impact.

“For example, they highlight some of the lowest hanging fruit that could have a big impact on our ability to manage the Delta’s resources with relatively limited investment of resources,” Dr. Larsen said.  “One topic that came up on top is envisioning alternative scenarios for managing salinity in the Delta and evaluating trade-offs associated with those.”

On the second day of the workshop, participants addressed detailed elements of what goes into a common modeling framework in terms of the modeling tools, best practices, and governance of a modeling community.  On the third day, a subset of participants mapped out strategies for developing research proposals to advance the integrated modeling framework to address some of the high-impact high-feasibility challenges identified on the first day of the workshop.  Another subset of participants convened to distill workshop findings and work toward drafting a white paper to be circulated widely and presented to the Council and DPIIC, among other agencies.  The white paper will summarize the key findings from the workshop and include recommendations for the next steps for the Delta science community.

Upcoming workshops

On May 19, a joint symposium with the UC Davis Coastal and Marine Sciences Institute will focus on the implications of rising temperatures for coastal marine and estuarine ecosystems.  The symposium will feature four main sessions that focus on physical environments and how they influence biotic responses to heat waves, how organisms are responding, how communities and ecosystems are responding, and management and governance, including the effects of heatwave events on economics and how heatwave events can be integrated into future plans.  The symposium will be a hybrid event.

The Adaptive Management Forum has been scheduled for May 4-5.  The biennial event will focus on governance for adaptive management, with the aim of fostering learning and discussion around governance needs to support effective, equitable, and inclusive processes in the Delta.  Dr. Larsen noted that adaptive management provides a structured approach to adaptation in the context of rapid, often unprecedented and unpredictable environmental change.  Its success depends on support from the larger social, regulatory, and institutional context, referred to as the governance system.

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