Fewer wetlands, more nutrients: water quality in the face of marsh erosion

Source Article: 

Vaccare, Jessica, Meselhe, Erab, and John White. 2019. The denitrification potential of eroding wetlands in Barataria Bay, LA, USA: Implications for river reconnection.” Science of The Total Environment, 686, pp. 529–537., https://doi.org/10.1016/j.scitotenv.2019.05.475

The Problem with Nutrients

The last century has seen a doubling of nitrate in the Mississippi River, a fertilizer which humans use for farming and agriculture. These extra nutrients commonly wash into natural ecosystems and contribute to poor water quality. Eutrophication is one result of nutrient enrichment which can lead to algal blooms and “dead zones”.

“Dead zone” areas have very low oxygen concentrations and cannot support animal life, usually leading to die-offs of important animals that cannot escape the bad conditions. One report found that eutrophication has created a total area of 245,000 square kilometers of “dead-zones” world-wide, about the size of the whole United Kingdom.

Gulf of Mexico low oxygen Dead Zone from 2017. From: NOAA.

Luckily, wetlands have the ability to capture and remove excess nutrients from the system, preventing ecosystem damage from eutrophication. Microbes living in wetland soils can use nitrate as an energy source during denitrification and transform the nitrogen into a less damaging form. Wetland plants also remove nitrogen, taking it up from nitrate enriched water and using it to grow, like plants in a well fertilized garden.

Nitrogen removal is very important in coastal Louisiana where the Gulf of Mexico Dead Zone, the second largest in the world, occurs just offshore and is fed mostly by the nutrient rich Mississippi River. However, water quality in this basin will depend on the health of wetlands within the state that can perform important nutrient removal processes.

Louisiana and the case of the missing marshes

The funny thing about studying coastal Louisiana wetlands is that every time you return to study the small marsh islands scattered throughout the estuaries, they look a little bit different. Most notably, the islands are getting smaller. A lot smaller, actually, and this is happening very quickly. One study estimates that 1.5-2 meters of the marsh edge is eroded from these islands every year from all directions.

One marsh island in Barataria Bay lost the majority of land area over 20 years. Photos from Google Earth, courtesy of Yadav Sapkota, LSU.
The underside of the marsh erodes, weakening the soil structure of the marsh edge. Photo courtesy of Yadav Sapkota, LSU.

The structure holding marsh soil together is weakened by wave energy, and soil from the marsh edge slumps off and falls into the water next to the islands. This material is dispersed throughout the bay and slowly decomposes, however, the remnants of the marsh edge that are left do not erode and are buried under sandy sediment. The worry is that eroded marshes have lost their ability to remove excess nutrient inputs that enter the marsh from human impacted river systems.

Due to a combination of edge erosion and other factors, approximately 25% of Louisiana’s total wetland area has been converted to open water since the 1930s. It is important to note that open water areas do not have the soil conditions needed to host bacteria that remove excess nutrients. So, when water from the Mississippi and other human impacted rivers is added to eroded coastal basins the wetlands are not well equipped to improve water quality.

The edge of the marsh slumps off and falls into the water. Photo by Chris Neill.

Graduate student Jess Vaccare, along with her lab at Louisiana State University, wanted to study the remnants of the marsh that are left behind in the bay, and show that the buried marsh remains can provide functions similar to intact marshes. Namely, she tested if the buried marsh can still remove excess nutrients that enter the basin from human impacted rivers.

Fewer wetlands and more nitrate

Land loss in coastal wetland systems like southeast Louisiana has complex causes, and as such, stopping the cycle of loss can be very complicated. Currently managers are building large scale infrastructure that will take water and sediment from the Mississippi River and redirect it to flow over or through large coastal basins to attempt to build new land and prevent further disappearance of marsh areas.

Vaccare, however, found that the buried marsh material can still host denitrifying microbes which remove nutrients from the water. In fact, that material is twice as effective at improving water quality as the sediments found in open water areas of the bay. Additionally, the denitrification potential of the buried material is comparable to rates in other intact marshes in Louisiana and other places around the world.

If eroded marsh material is exposed to nutrient enriched water, nitrate can be removed from the system through denitrification. Although Louisiana has lost many marsh areas to erosion, there is potential to still use the submerged material left behind to improve water quality.

Although there are huge environmental changes in coastal areas around the world it is important to study how altered ecosystems function and how we may use them to our advantage.

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Alina Spera

I am a second year master's student at Louisiana State University studying wetland biogeochemistry and coastal restoration. My thesis work involves a geostatistical analysis of wetland soil and how large scale restoration projects may impact it. When I'm not in the field or lab I'm probably playing roller derby or hanging with my hamster!

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