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Marsh madness!

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coastal mash dynamics

coastal mash dynamics

Forgive me but I just had to use this title before the end of the month!  

I call your attention to several new studies on the responses of coastal wetlands to increases in both atmospheric carbon dioxide and sea level.  “Marsh madness” doesn’t relate specifically to college basketball but, as shown by the following, there are as many uncertainties re the final outcome of the coastal science as about the outcomes of the NCAA men’s and women’s basketball conferences.

Unfortunately, the scientific uncertainties won’t be resolved in early April!

First, a new study on coastal marshes and carbon dioxide by Smithsonian scientists was summarized on March 26 in Science Daily: excess atmospheric CO2 makes some wetlands grow faster.  

“Our findings show that elevated CO2 stimulates plant productivity, particularly below ground, thereby boosting marsh surface elevation,” said Adam Langley, the paper’s lead author. Patrick Megonigal, the paper’s corresponding author, added “We found that by stimulating root growth, thus raising a marsh’s soil elevation, elevated CO2 may also increase the capacity for coastal wetlands to tolerate relative rises in sea level.” Both scientists are ecologists at the Smithsonian Environmental Research Center in Edgewater, Md.

This sounds like good news but read this disclaimer:

Though marshes appear to benefit from CO2 in the short-term, the scientists stress that increasing CO2 levels will continue to warm the Earth, melt glaciers and expand ocean water, thus accelerating sea-level rise. Ultimately, rapidly rising seas could outstrip the positive effects of COon the marshes that they have observed.

Now I call your attention to a recent paper by Sherwood B. Idso and Keith E. Idso on tidal marshes sequestering CO2 that was summarized in CO2 Science.  This time there is no disclaimer.

   
Tidal marshes typically exhibit high rates of productivity.  In the southern coastal region of North America, for example, the net primary production of these ecosystems averages approximately 8000 g m-2 yr-1 (Mitsch and Gosselink, 1993).  Tidal marshes also exhibit low rates of organic matter decomposition, because the anaerobic decomposers of these oxygen-depleted environments operate at slower rates than do their aerobic counterparts of terrestrial environments (Humphrey and Pluth, 1996; Amador and Jones, 1997).  Thus, it can readily be appreciated that as seas rise and encroach upon the land, rates of carbon sequestration in coastal marsh soils rise right along with them.

Next I call your attention to a USGS study summarized in the Weekly Carboholic on California salt marshes sequestering CO2.  Note that while salt marshes provide a net benefit re CO2, freshwater marshes emit more methane as CO2 levels increase. 

Salt water marsh restoration may efficiently sequester carbon
Deforestation. Biodiversity. Clean water. Carbon sequestration. Wetlands protection. Shoreline restoration. Most major environmental problems facing the world are connected in some way to other. For that reason, most of the connected environmental problems must be solved more or less simultaneously. Take clean water, shoreline restoration, wetlands protection, and climate change – wetlands are known to efficiently clean both freshwater and salt water, they help restore the shoreline and protect it from storm damage, and, if the 
preliminary results from a recent U.S. Geological Survey (USGS) hold, they grow fast enough to sequester significant amounts of carbon too.

Two months after starting a wetlands restoration research project in California’s Sacramento?San Joaquin River delta, the USGS researchers reported that their project was capturing 30 times the amount of carbon per square meter than Kyoto Protocol-compliant reforested agricultural land. This is because wetlands grow so fast and convert that growth into low-oxygen, slowly decomposing peat bogs and, ultimately, into new land over hundreds or thousands of years. However, freshwater wetlands have enough oxygen in the water to produce large amounts of methane. And since the USGS study hasn’t yet measured the amount of released methane, the total carbon sequestered may in fact be largely or even entirely negated by methane releases.

According to the ACS article, though, salt water marshes don’t suffer from the same problem, or at least not to the same degree. Because the salt water marshes grow as fast as their freshwater counterparts, they sequester as much carbon. But because there’s a lot more sulfate in salt water, bacteria don’t produce methane in significant amounts, and so methane emissions are unlikely to offset the CO2absorption in the marsh. And as the marshes pull CO2 out of the air, they’ll also clean the water, reduce shoreline erosion, slowly build new land, and provide food and shelter for wildlife.

Finally, I call your attention to a study in Physorg.com showing that tropical wetlands sequester much more carbon per acre than temperate marshes.  This study, by Ohio State University scientists including Bill Mitsch*, compared wetlands in Costa Rica and Ohio.  It would make sense that the subtropical wetlands in coastal Louisiana would be closer to the Costa Rican wetlands than those in Ohio. 

Len Bahr

*Dr. Mitsch has worked for years with Louisiana scientists on coastal wetlands. For example, he is cited in the above Idso and Idso reference.

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  1. Karen McKee says:

    Len,

    I realized that you were not criticizing the science, but only wanted to take the opportunity to clarify a few points. I think it's important for the public to hear from scientists who for the most part are very conservative and careful in their work and strive to present their findings in a balanced way.

    I agree that there are those who cherry-pick information to support their viewpoints. The public do not always realize this and that scientists have little control over how others use (or misuse) their data. We can only speak up to correct any major misinterpretations.

    It's great that you have this forum for people interested in the Louisiana coast to discuss points of interest.

    Regards,
    Karen

  2. Karen-
    First, I’m very glad to see you weigh into this discussion. I hope that more credible practicing researchers follow your lead.
    Second, my comments are in no way intended to denigrate either the scientists or the science. I’m interested in warning lay readers that there are well known partisans who knowingly cherry-pick snippets of factual information as propaganda to make their political case that climate change is nothing to worry about. George Will comes to mind.
    Third, I’m a huge supporter of making coastal policy on the basis of science, not politics, which is a primary theme of LaCoastPost.
    Please contiue to comment and ask your colleagues to do the same.
    Len

  3. Karen McKee says:

    As one of the authors of the study on CO2 effects on coastal marsh building, I would point out that the media often simplify scientific findings for the public. So, please don’t assume that our scientific articles report an equally simplistic (or pollyana) message.

    Our work was based on careful experiments conducted in the field as well as under more controlled conditions in greenhouses (and by separate research teams). The findings from both approaches showed the same thing: that in the short-term (2 years), elevated CO2 enhanced biological production and elevation gain in a brackish marsh community.

    Although Riverrat thinks that these findings are “not anything really new”, most scientists familiar with this literature would disagree. If you bother to read our papers, you will see that we did look at how some key factors might moderate the effect of CO2 (salinity, flooding, nutrients). Those findings were not what we predicted. So no, we don’t assume to know something without having the data to back it up.

    We interpreted our results conservatively and pointed out conditions under which CO2 might or might not have an effect on marsh elevation gain. These are not “disclaimers”, but responsible reporting of scientific findings.

    Our work needs to be confirmed by other researchers, and we must determine which marsh types may be most affected by CO2 and what other factors will modify the CO2 effect. Our previous work (published earlier this year) and the work published in PNAS suggests a much more complex situation than simply “higher C02 levels can benefit/stimulate plant growth”. For example, not all plants will respond equally to CO2 and some may even react by decreasing belowground production (and reducing marsh elevations).

    Regarding the comment about CO2 saturation: while it’s true that the CO2 effect has an upper limit (as shown in terrestrial plants), increases in stress can extend the capacity to respond to CO2 (for example, higher salinity and flooding stress accompanying sea-level rise). We definitely don’t know what this upper limit is for any coastal marsh type–because no one has conducted the research.

    The key message of our work is that coastal marshes can self-adjust to rising sea level until a threshold is reached; higher CO2 may raise this threshold, postponing marsh collapse. This type of information adds to our overall understanding of how our coastal ecosystems may be impacted in the future by global change (and lead to more accurate models predicting coastal submergence).

    Without such information, we are just making blind guesses and possibly making wrong management decisions.

  4. Riverreat-
    I agree with your comments and hasten to assure readers that this post was anything but a pollyanish view of climate change. My principal reason for calling attention to the Smithsonian paper and the others is that Friday morning on WRKF-FM 89.3 the study was described as showing that rising atmospheric CO2 makes coastal marshes grow faster. I didn’t want that (grossly simplistic) message to go unanswered. As you know, the deniers of anthropogenic climate change love to find “good news” in rising CO2 levels.
    Len

  5. riverrat says:

    While some issues may be unresolved, it seems that the implications of these studies are not anything really new. We know that higher C02 levels can benefit/stimulate plant growth, up to a point. We’re going to have higher C02 levels, so whatever benefits accrue will do so. But as the first paper and other studies point out, that won’t happen in a vacuum, but will lead to higher temperatures and more sea level rise. The projected climate change will also affect precipitation patterns, which is a key limiting factor for all plants. The IPCC report cites a number of studies showing that for terrestrial plants, higher C02 levels are not an unlimited benefit,and the plants and habitats are not limitless sinks. Past a certain point, plants can’t absorb or utilize more C02, and the effects can actually be detrimental.

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