Colonization and Sea Level Rise Effects on Carbon Storage in Freshwater Wetlands of Southeastern United States

The Impact of Late Holocene Land Use Change, Climate Variability, and Sea
Level Rise on Carbon Storage in Tidal Freshwater Wetlands on the Southeastern United States Coastal Plain

Miriam C. Jones, Christopher E. Bernhardt, Ken W. Krauss, Gregory B. Noe

Summarized by James Myers who is a graduate student at Binghamton University earning his masters in teaching for earth science. As an undergraduate he majored in environmental
chemistry. Not long after he decided he wanted to become an educator and work towards
creating the next generation of environmental scientists. In his downtime he enjoys playing
guitar, camping, and watching hockey.

What data were used: Sediment cores were collected along the Waccamaw River in South Carolina and the Savannah River in Georgia. The sites were chosen because they have similar landscapes, ranging from freshwater, to moderate salinity, and oligohaline marsh. Four piston core samples were taken from the Waccamaw River, one that was found in freshwater, one in moderately salt-impacted water, and two from the Sampit River, one from a heavily salt-impacted area and one from an oligohaline marsh. Four other cores were collected along the Savannah River using a peat corer. These core sites were also from freshwater, moderately salinated, highly salinated, and an oligohaline marsh.

Three maps of the Savannah river, Waccamaw River, and an inset map showing the location of both rivers along the southeastern United States.
Maps designating the locations of the sites sampled. The sites are roughly 150 km away from each other, along the southeastern coastline of the United States. The Savannah River sites are found further upstream compared to the Waccamaw River sites. The cores at both locations were assigned numbers from one to four. The lower numbers are further upstream and are lower in salinity.

Methods: The cores were dated using radiocarbon analysis on macrofossils and bulk sediment which helped determine which samples were from the colonial era. Time scales were reported with calibrated years before present from 1950. Core compression was apparent within the samples, and bulk density (weight of sediment in a given volume) and accretion rates (how fast sediment accumulates) were adjusted to account for this. Carbon content was calculated using the loss on ignition method. Carbon accumulation rates were calculated by multiplying the percent carbon by the bulk density and accretion rate determined from an age-depth model. Pollen analyses were run to understand which plant species lived at these sites over time, as this method revealed what the environment must have been like if certain plants and trees were able to survive.

Results: The core samples from the Waccamaw river dated between the last 1,100-4,200 years. The oldest sample was the heavily salt-impacted site, which began as a back swamp environment, where fine silts and clays settle after flooding which create a marsh-like landscape. This was determined from the presence of Nyssa, Taxodium, and Poaceae pollen. The accumulation rates are low, but still higher than the freshwater sites. Upper freshwater and oligohaline sites were also found to have been back swamps due to the presence of Alnus in the freshwater core, and Liriodendron tulipifera seeds found at the oligohaline marsh site, as well as Nyssa, Taxodium and Alnus pollen found at both sites. The accretion and accumulation rates are similar to the heavily salt-impacted site. Freshwater environments are characterized by low accretion and carbon accumulation. Higher accretion and carbon accumulation rates are found around 1700-1400 calibrated years before present, and can be seen in the cores with a decrease in hardwoods and increasing Nyssa, Taxodium, and Liriodendron evidence. The largest observed changes happened around 400 years ago, the same time of colonization and the increase in agriculture within the regions. The changes are marked in the cores by large increases in accretion, organic matter, and carbon accumulation. Another indicator of this is the increase of Poaceae, while evidence of Nyssa, hardwoods, and Taxodium diminish. Poaceae pollen and the presence of Scirpus and Carex seeds suggests a change to oligohaline marsh in relation to the increase of land use in the area. Reforestation efforts over the last 100 years show a decrease in accretion and carbon accumulation in all sites. The Savannah River cores were found to be roughly five to six thousand years old. The results from the cores along the Savannah River were found to be very similar to those from the Waccamaw River.
The study revealed that the same zones were also back swamps and that the freshwater core showed low accretion and carbon accumulation. The presence of Alnus designated this back swamp environment. Around 2,000 calibrated years before present, the sites show various changes in biota, but very little change in accretion and carbon accumulation rates. The largest change in the Savannah samples are found around 400 years ago, as was seen in the Waccamaw cores. All sites showed a decline in Nyssa, and an
increase in Poaceae, and what the researchers call weedier taxa, such as Scirpus, Sagittaria, and Polyganum. Both the Savannah River and the Waccamaw River both show stark increases in carbon accumulation and accretion rates right at the start of when colonization and agriculture increased in these regions dramatically, as well as when sea-level rise began to increase during the Holocene. The lowest accretion rates were found further inland, which is tied to an expansion of the marsh. Reforestation efforts coincided with lowered accretion rates, which increased the vulnerability with a rise in sea level. The tidal freshwater forested wetlands are vulnerable to the smallest of salinity changes.
Why this study is important? Wetlands like the ones studied in this research, are important for coastal communities because they help mitigate flooding and support many organisms, as well as fisheries, which provide millions of dollars in commercial and environmental goods and services. Wetlands are also important carbon sinks and help control the amount of CO2 in the atmosphere. Sea level rise today will affect these ecosystems and the people living near them. The results of this research are important for understanding the future long-term resilience of these ecosystems and what measures will be best suited to support these environments.
The big picture: The paper looked at evidence within sediment cores to understand the changes in carbon accumulation and accretion within two southeastern United States rivers. Core evidence indicated that there were increases in accretion and carbon accumulation rates with the emergence of colonization and agriculture in the area. Reforestation efforts in the last 100 years showed a decrease in accretion. The findings were then compared to sea level rise data to show that these environments become more vulnerable with increased sea level rises over the last 200-100 years. This research will be helpful in understanding the effects sea level rise in the future will have on this environment and the surrounding communities.
Citation: Jones, M. C., Bernhardt, C. E., Krauss, K. W., & Noe, G. B. (2017). The impact of late Holocene land use change, climate variability, and sea level rise on carbon storage in tidal freshwater wetlands on the southeastern United States coastal plain. Journal of Geophysical Research: Biogeosciences, 122(12), 3126–3141.

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