Dr. Jim Morris on the mechanisms of how tidal marshes keep pace with sea level rise, and Dr. John Callaway with research on carbon sequestration in the San Francisco Bay’s tidal wetlands
Back in the late 1800s as the Sacramento-San Joaquin Delta was being reclaimed for agriculture, a somewhat parallel process was happening to the shoreline of the San Francisco Bay as the tidal marshes that fringed San Francisco Bay have been destroyed as a result of progressive diking and filling for agricultural, salt pond, and commercial development. As a result, about 90% of the wetlands in the San Francisco Bay as well as the upper estuary have been lost.
The California Eco Restore program plans to restore 30,000 acres of habitat in the Delta with 9000 acres being tidal or subtidal habitat. The Baylands Ecosystem Goals project envisions restoration on a larger scale: In 1998, the first iteration of the project set a goal for 100,000 acres of tidal marsh restoration; to date, 7,000 acres have been restored and another 30,000 acres are planned.
California’s cap and trade program for carbon has focused a lot of attention on the effectiveness of wetlands for carbon sequestration. Dr. John Callaway with the University of San Francisco next describes the results of studies of the ability of the tidal wetlands in the San Francisco Bay to sequester carbon.
DR. JIM MORRIS: Limitations of vertical marsh accretion rate as determined by suspended sediment concentration and sea level rise
Sea level rise threatens tidal marshes, which by their very definition, are at sea level. Can our marshes keep up? Dr. Jim Morris from University of South Carolina began by noting that sea level rise is accelerating and is only expected to continue to accelerate, so there’s a lot of interest in what will happen to coastal wetlands. “Everybody accepts that these are important ecosystems; they have high ecosystem service value and it’s going to be a real loss if we lose the wetlands that we have,” he said.
Marshes do track sea level, he said, noting that the marsh that he works at in Plum Island, Massachusetts is at least 4000 years old as determined from carbon-14 dates, and that marsh today is right at the top of the tidal frame at an elevation of mean high-high water. “It wouldn’t be at that relative elevation if it wasn’t keeping up with sea level,” he said. “But there’s certainly a limit, and we’d like to be able to predict what is the limiting rate of sea level rise above which marshes won’t keep up. There’s not one answer because it really depends on sediment supply, tidal range, and some other factors.”
He began by explaining how marshes track sea level rise. “Marsh biomass responds to changes in sea level, and the change can be positive or negative – it really depends on the relative elevation of the marsh itself within the tidal frame,” he said. “This is important, because the plants trap sediment, and the more plants you have, the more sediment they will trap. They also generate biovolume. Sedimentation is a function of inundation time, so there’s a feedback there between the relative elevation of the marsh, the productivity of the marsh and the sedimentation; sedimentation is a function of the suspended sediment concentration.”
Plants trap sediment by trapping sediment that’s suspended in the flood water and also by encouraging the settlement of particles in floodwaters as they move through the vegetation canopies; the denser the canopy, the more sediment is trapped, he said. The amount of sediment trapped is proportional to the biomass density of the plants and the depth of the surface bellow mean high-high water.
“The settlement of particles onto the surface is a function of the amount of time that the surface is under water, and that’s a function of the relative elevation,” he said. The sedimentation rate is proportion to the length of time the surface is inundated and the concentration of suspended sediment, times the depth and settling velocity.
Dr. Morris noted that a the part of a marsh that is at mean sea level would be inundated 50% of the time, and that the deeper you go, the more sediment is accumulated. As an illustration, he showed pictures of ceramic plates which were set out on the marsh at different elevations and left there for a number of days. “You can see very clearly, the deeper you go, the lower you go in elevation, the more sediment accumulates on the plates,” he noted.
“These plants make a considerable amount of biovolume,” he said. “Where we’ve been measuring elevation change for probably 20 years or more in control and fertilized plots, we see very clearly that where we have increased the biomass of plants by fertilizing, we also increase the biovolume of roots and rhizomes. The fertilized plots gained elevation at a much, much higher rate because they trapped more mineral sediment and they made more biovolume.”
Dr. Morris said this led to the development of the Marsh Equilibrium Model, explaining that it is a simple model that calculates the maximum possible sediment load delivered to a given site, the mineral and organic parts of sediments, and from that, it calculates bulk density and from that, it calculates volume which it uses to measure accretion rates.
He then spent some time explaining the math behind his model (view video presentation for the details.) “I can use this model and I can ask what is the accretion rate for any feasible equilibrium elevation, where the accretion rate equals the rate of sea level rise, so I can basically solve this model in reverse, I can say, for an elevation that’s at mean sea level, what does the accretion rate have to be to keep up with the sea level rise that would put the equilibrium elevation at mean sea level,” he said. “The equilibrium is kind of a neat idea, if you understand how it works.”
“Sea level rise is accelerating,” Dr. Morris reminded. “It’s important to remember that when you say sea level is going to rise 1 meter in 100 years, it doesn’t mean it’s going to rise one centimeter a year, it’s going to mean that by the 100 year mark, it’s going to be rising much faster than 1 centimeter, and really that’s what’s going to place a lot of marshes in trouble.”
DR. JOHN CALLAWAY: Carbon Sequestration in natural and restored tidal wetlands in San Francisco Bay
Dr. John Callaway with University of San Francisco gave a presentation on carbon dynamics and sequestration, focusing on measurements that have been made in the marshes around San Francisco Bay over the last 10 years. He noted that the critical aspect of marshes is their elevation relative to current sea level, and mineral sediments as well as organic matter play an important role as this is what will maintain the marshes into future.
“In thinking about carbon, there are really two issues to think about,” he said. “What is the role of organic matter in maintaining marshes and building marshes under increases in sea level rise, and then what effect does all that have on climate change as marshes are sequestering carbon in their soils.”
Because of the state’s carbon trading program that has recently been set up, there is a lot of interest in understanding how restored wetlands can be used to sequester carbon and when he started doing this work about eight years ago, there was very little data. “People needed to know if we were going to assign credits, how many credits should wetlands receive, and how variable are the carbon sequestration rates around the estuary,” he said.
Wetlands do accumulate carbon, and estuarine wetlands, whether they are marshes, mangroves, or seagrasses, accumulate more carbon in the soil than most any other ecosystem, he said. “Sea level rise is the mechanism that’s driving all of this, and accommodating and allowing for all of this carbon to accumulate,” he said.
He presented a chart of carbon sequestration as measured in tidal wetlands around the world. “This shows you a scatter of that data from a whole range of different coastal areas, but on the Pacific Coast, just a handful of data points. So we needed to know how San Francisco Bay would fit into this range of data.” He also noted that the data were collected over a wide range of time scales. “Carbon sequestration rates that we measure are really dependent on the time scale. If we measure over a very short term, we’re likely to measure a very high rates, but over longer terms, a lot of the surface carbon is going to decompose, so we have to think about the time scale that’s really relevant.”
A range of salt marshes were selected for the study: Whale’s Tail marsh in the south bay, a 70 or 80 year old marsh; China Camp and Petaluma River, two really pristine remnant natural salt marshes; Coon Island, a mix of salt and brackish marsh; and Rush Ranch and Brown’s Island, two brackish marshes in Suisun Bay. “We excluded the Delta because of issues associated with methane and we were just measuring sequestration within the soil itself, not emissions,” he said.
“At each site, we collected cores from the lower marsh to the upper marsh, because the elevation and the period of inundation is really going to affect plant biomass and it’s going to affect the rate of sequestration as well as the mineral inputs,” Mr. Callaway said. They collected six cores at each site, and ended up with more than 50 cores from across the bay. They used radioisotope cesium and lead 210 to date them.
“Cesium gives you dates back to the early 60s; it gives you average rates of accumulation on a 40 to 50 year time scale,” he said. “Lead 210 goes back to about 100 years, and for the policy issues of carbon sequestration, 100 years was sort of the time scale of interest that we were looking at. But we wanted to look across those two different time scales.”
“Dating the cores, we can get the accretion rates, and then we combine those data with the soil bulk density as well as the soil carbon content, and those together, then we can look at the mineral sediment inputs on mass base, as well as the organic matter and the carbon sequestration rates over different time scales,” he explained.
He presented a slide showing the mean accretion rates over the last 50 years, noting that these are just the vertical accretion rates. “The vertical accretion rates are all around 3 or 4 millimeters per year, with the exception of Whale’s Tale, which is about 6 millimeters per year,” he said. “Whale’s Tale is a relatively young salt marsh, and probably was at slightly lower elevations and is the reason why we’re seeing those higher rates of accumulation there. We’ve measured accumulation rates over the shorter term in many different areas and found that lower elevations are going to get much higher rates of accumulation within the marshes.”
“The good news is our marshes are keeping pace with sea level rise, which has been just a couple of millimeters per year over the last 30 or 40 years,” he said. “This shows you the data for those same areas, from Whale’s Tail to China Camp, Petaluma, Coon Island, Rush Ranch and Brown’s, going from salty to fresh to brackish, and then from the low marsh, the mid marsh, and the high marsh, and the different bars. The black bars are cesium, which are the accumulation rates over 50 year period, the gray bars are the lead accumulation rates over 100 year period.”
He noted that the cesium rates are always higher because it accumulates on the surface and then it gets consolidated and compacted. “Both the compaction and the decomposition lower the measured rates over longer periods,” he said.
Mr. Callaway noted that the mid to high marsh all averaged about 3 millimeters per year, or .3 centimeters per year – just slightly more than the rates of sea level rise. The lower marsh was more variable and higher, especially over the shorter time period, due to the mineral sediment coming in and building up the marsh more quickly, he said. “The good news beyond just keeping pace currently is that the lower marshes are showing rates of about 6 millimeters per year; that indicates that if we have the concentrations of sediment that are coming into the bay currently in to the future, the marshes are likely to be pretty resilient to sea level rise, and will be able to keep pace as sea level rise begins to increase.”
“We then took those accretion rates, combined them with the soil data and these are the carbon sequestration rates we measured within the areas around the bay,” he said. “Since the accumulation rates weren’t so variable, there was some variation in carbon concentrations, very little variation in the density of carbon in the soil in our salt marshes or in other salt marshes, and so not a whole lot of variation in carbon sequestration rates.”
The cesium measured around 100 grams of carbon per square meter per year, or about 1 ton to 1.5 or slightly more tons of carbon or CO2 equivalence per hectare. He noted that Brown’s Island was the one site with slightly higher rates, but with all of the other salt marshes and Rush Ranch, there were no significant differences. “That was surprising to me,” he said. “In many other systems, they found dramatically higher rates in brackish marshes, 2 to 3 or 4 times the carbon sequestration in brackish marshes, and I was expecting that in our marshes, but that wasn’t the case. Very little variation.”
There wasn’t much variation across the marsh either, he noted. There were slightly higher rates at the low marsh where the marsh is building actively, he said. “Not so much variation across the marsh as well. Slightly higher rates at the low marsh, where the marsh is really building actively.”
Comparing how this data compares to the data from the other areas in the country, the vast majority of the data were cesium or even shorter term measurements. “We’re right within the range,” he said “Many of these really high values that have been reported are probably very short term measurements, and I think most salt marshes are much more likely in the range of just below 100 to around 100 to maybe 200 grams of carbon per square meter per year, so we’re probably pretty typical. It’s because they’re tracking sea level rise, and so if you just look at the elevation that’s accumulating times the typical carbon densities, that’s really what these salt marshes are accumulating in terms of carbon sequestration.”
Moving beyond these local sites, the larger question is how to get an estimate across a larger area, such as the bay or a coast, he said. USGS is currently undertaking a project that is studying six areas across the country that have intensive data – the San Francisco Bay, the Louisiana Coast, the Everglades, Chesapeake Bay and other sites, and trying to make improved estimates of landscape level carbon sequestration. “It’s using a model to try to evaluate how things vary across the marshes, and then using a large amount of remote sensing data, other landscape scale data, and elevation data, so that we can get feedback between elevation and sediment inputs. This is just starting, but San Francisco Bay is one of the sites, and so we’ll be trying to improve the estimates regionally within the bay as well as across the country.”
Methane also has to be considered, because if all this carbon is accumulating, but you are emitting methane gases that counteract, that really is a challenge, he said. “There definitely is a gradient across there, so methane is not the elephant in the room, it’s the cow in the room, and that’s the big challenge is methane being emitted from these sites.”
The good news is that data accumulated from salt marshes and tidal marshes around the country show that with increasing salinity – that above half the strength of sea water, there’s very little methane produced, so what’s accumulating in the ground in probably a good measure of CO2 accumulation, he said.
In the brackish marshes, initial results from studies using flux towers at Rush Ranch show that methane emissions from those brackish marshes are very low, and probably are not enough to counteract the accumulation rates in the soil, he said. “In the freshwater marshes, below 10 ppt, it’s much more variable and it really will depend on the site, but methane could have a significant counteracting effect in some of those freshwater marshes, in terms of looking at the overall CO2 balance.”
“A slight increase in sea level rise in terms of carbon sequestration is actually a good thing because it is that keeping pace with sea level rise and putting carbon into the soil as the marsh builds that is what’s allowing for all of this carbon sequestration to happen in tidal marshes, so as long as the marshes keep pace, they will actually build more and more carbon under increasing levels of sea level rise,” Mr. Callaway said. “The challenge is that when we get to the threshold of where they can’t keep pace, then we’re going to lose that function of carbon sequestration. I don’t think we’re going to lose the carbon that’s in the ground – as long as it stays flooded, that carbon is going probably stayed reduced and in place and won’t be converted back to CO2, but we won’t have that storage continuing into the future.”
Within the San Francisco Bay, Suisun Bay, and the Delta, there are enormous opportunities for restoration and potential credits for carbon at those restored sites, he said. “The challenge is that most of these sites have subsided substantially, they are significantly below the elevation where vegetation can get established, and so they are going to have to build elevation very quickly to get up to where vegetation can get established and can start to sequester carbon.”
In the study, they looked at two restored salt marshes, Muzzy Marsh in Corte Madera and Cogswell Marsh in Hayward, and for the salt marsh sites, there was more organic matter accumulation. He noted that it is organic matter, not carbon content. It is somewhere between 10 and 20 percent organic matter, and the older restored sites are rather similar. “Once they get to be pretty well established, they are probably going to accumulate carbon at a relatively similar rate. Maybe slightly elevated like we saw with Whales Tale in the south bay but not dramatically higher,” he said.
For highly subsided sites such as the salt ponds in the south bay or island in the Delta, studies show those sites have accumulated vertically very rapidly. “We’ve followed sites in the island ponds and over three years they accumulated anywhere from 20 to 40 centimeters of materials, so many, many centimeters per year compared to a few millimeters per year in a natural marsh,” he said. “So they do accumulate materials very rapidly; the thing is that it’s all mineral material that’s coming in or organic material from outside the marsh that’s accumulating with that sediment, and it’s not until vegetation gets established that carbon sequestration is really going to start occurring.”
“There is definitely the potential that there’s going to be some increase in some elevated levels of carbon sequestration as the marsh is just getting established,” he said. “But maybe it’s 200 grams of carbon rather than a 100 grams of carbon per square meter; it’s not going to be 10 times the rate that we’re seeing. If we look at the current price of carbon in terms of the carbon trading, the current price of carbon is somewhere about $13 a ton of CO2 equivalents, and these marshes are accumulating less, even at the optimistic level, 2 to 3 to 4 tons of CO2 equivalents per acre, so we’re not talking thousands or even many hundreds of dollars per square acre, we’re talking a small amount of funds at the current price of carbon in terms of credits.”
“That’s just carbon sequestration in the ground,” he said. “There are other ways to think about the carbon credits in terms of avoided losses or protection of carbon stores in the soils, and then hopefully the price of carbon will go up, and it will create more of an incentive. That is just one economic aspect of the marsh to think about, in terms of the credits they might receive.”
“To conclude then, carbon definitely is a key component,” he said. “Organic matter accumulation really is a key part of soil vertical accretion and it’s what allows marshes to keep pace with sea level rise. Tidal wetlands do accumulate very high rates of carbon compared to other ecosystems in the marshes and the bay and are pretty typical of salt marshes from around the county. If sea level rises slightly, they are going to increase even more carbon into the future. Carbon credits, tidal wetlands definitely, it could provide some incentives but it’s not an enormous incentive under the current price of carbon.”