USGS Multimedia Gallery: (Video)--Effects of Sea-Level Rise on Coastal Wetlands in the Mississippi Delta

Effects of Sea-Level Rise on Coastal Wetlands in the Mississippi Delta

Featuring Dr. Karen L. McKee, U. S. Geological Survey-National Wetlands Research Center and Dr. Julia A. Cherry, University of Alabama and New College

Narration: USGS scientists are studying the effects of sea-level rise and other global change factors on coastal wetlands in the Mississippi River Delta. This work involves both field and greenhouse studies to better understand how these critical ecosystems may be affected by global change and how they manage to persist when these changes occur.

Dr. Karen McKee, USGS Research Ecologist

Dr. McKee: We have a number of different wetland types from forested wetlands to freshwater marshes, which is where I am now, to brackish and saline marshes. These are marshes that contain vegetation that are somewhat tolerant of salts in the water. In the field, we conduct studies to better understand how marshes keep up with rising sea level by building their soil surfaces vertically.

Narration: We’ve partnered with colleagues at other institutions and universities to conduct our research. Here, Dr. Julia Cherry is demonstrating how we measure marsh elevation change using an instrument called a surface elevation table.

Dr. Julia Cherry, University of Alabama

Surface elevation table.

Dr. Cherry: And we’re going to put this in the soil here where we have a marker horizon—and that’s just a layer of white clay that marks time zero or essentially the time we put it down.

Measuring soil accretion

Dr. Cherry: And then we can see how much material has accumulated above that white layer, and that is what we call accretion.

Dr. Cherry: See that?

Dr. McKee: Oh, yeah.

Dr. Cherry: So there’s a small white layer there. And we can measure the accretion above that to see how much material has accumulated through time. You can still see that Katrina layer really well.

Dr. McKee: Yeah, really.

Dr. Cherry: So this tells us that we have about three centimeters of accumulation above that marker horizon.

Dr. McKee: Plants use carbon dioxide, which is in the atmosphere, during photosynthesis to produce food for the plants, sugars, and ultimately build biomass. And as this biomass dies and decays, it contributes to the soil, and that addition of organic matter to the soil builds the soil vertically.

Dr. Cherry: This is some of the peat soil that I took just off the surface of the marsh. And you can see in it that there’s a little bit of sediment, but it’s mostly plant roots and rhizomes that are filling up the space here. So a lot of the soil volume is comprised of this plant material, roots, rhizomes, and shoot bases.

Vertical Marsh Building

Narration: During vertical marsh building, alternating deposits of mineral material and organic material accumulate over time. Starting with an unvegetated mud flat, mineral sediment deposits add new layers on the surface. As marsh plants become established, they begin to contribute organic material to the soil, building layers of peat. There may be additional deposits of mineral material during storms or hurricanes, followed by another period of plant organic accumulation. So after thousands of years, alternating layers of material create a pattern from oldest at the bottom to youngest at the top. We can see these layers by collecting soil cores through all the layers. Once extracted, the patterns in the cores can be studied and the composition of those layers can be analyzed. This type of approach allows us to better understand how these marshes have responded to sea-level rise and other changes in the past.

Collecting Soil Cores from Marshes

Reconstructing Past Marsh Changes

Dr. Cherry: So if you’ll take a look at this core. In the upper ten centimeters or so we see the darker, sandier sediment that represents the deposition that occurred during Hurricane Katrina. And just below that --this more broken up portion here with a lot of plant roots and biomass--is the area that was the active growing zone before Katrina. Then we transition into this firmer area that’s a little bit older and deeper in the core.

Dr. McKee: Well, we knew that increase in CO2 concentrations in the atmosphere stimulates productivity of plants. And so we asked the question: if CO2 concentration in the atmosphere increases, will this increase the rate of marsh building through biological accumulation of organic matter? And, so we tested that in our greenhouses in which we manipulated atmospheric levels of CO2 along with other conditions of flooding and salinity…

Wetland Elevated CO2 Experimental Facility

...to see how those soil and water factors would affect the response of the plants to carbon dioxide.

Effects of Elevated CO2 on Marshes

Dr. McKee: What we have done is we have brought in segments of marsh from the field containing intact soil and plants and established them in what we call mesocosms, which are containers in which we can manipulate flooding and salinity.

Narration: We found that by exposing marsh mesocosms to elevated concentrations of carbon dioxide that the soil surface built vertically at a faster rate compared to those mesocosms maintained at current CO2 levels.

Belowground Production of Plants

Narration: Because there were no other inputs to mesocosms, we could show that the CO2 effect on marsh elevation was due to belowground production of plant matter such as roots, rhizomes and shoot bases.

Dr. McKee: What our greenhouse and field studies are showing is that the marsh vegetation plays a key role in helping to maintain marsh elevations relative to sea level and they do this by producing organic matter that accumulates in the soil. This is very important in helping the marsh, especially marshes where there is very little mineral sediment input. What we’re seeing is that elevated concentrations of carbon dioxide in the atmosphere may be important in stimulating this belowground production of organic matter and upward, vertical movement of the marsh surface. So our work is adding to a more complete picture of how our coastal wetlands, such as here in the Mississippi River Delta, may be affected in the future by global changes such as sea-level rise and climate change.