The silent disappearance of the Mississippi marshes

23/10/2024

7 minutes

oceans and climate

Coastal wetlands, crucial for biodiversity and flood protection, are under increasing pressure from climate change and human activities. The Mississippi Delta, home to 41% of the coastal marshes in the United States, is particularly threatened by rising sea levels and extreme weather events. The marshes are in danger of collapsing, triggering an ecological crisis with lasting consequences. This calls for urgent management to halt losses and restore vital ecosystems.

by Laurie Henry

Cover photo : Mississippi Delta with a marsh with a ring of healthy vegetation surrounding an area that is dying back. © Louisiana Department of Wildlife and Fisheries

A recent study conducted by researchers at Louisiana State University and published in Nature Communications reveals the causes and consequences of a massive decline in vegetation in the Mississippi region. Triggered by an acute drought in 2012, this degradation of plant cover, exacerbated by the incursion of salt water, raises concerns about the future resilience of the marshes in the face of climate change.

An ecosystem under pressure

The Mississippi Delta, located in the south-eastern United States, is a vast wetland region that is crucial to the stability of the Gulf Coast. It plays a fundamental role in preventing flooding, filtering water and providing habitats for an impressive diversity of wildlife, including migratory birds, fish and aquatic mammals. The marshes act as a natural barrier against the storms and hurricanes that regularly batter the coast. But their essential ecological functions are now under serious threat from rapidly rising sea levels and the increasing effects of climate change. According to the new study, the combination of extreme climatic factors and soil subsidence (the progressive subsidence of soil caused by compaction or erosion) has put these ecosystems under unprecedented pressure, making their future uncertain.

(a), (b) : Aerial images of summer 2016 – (c) : IImage of a Phragmites stand after dieback with a healthy stand in the background – (d),(e) : Satellite imagery of Coastwide Reference Monitoring System sampling station 0162 in the lower Mississippi Delta. © T. Elsey-Quirk et al., 2024

Rising water levels and more frequent droughts, particularly the severe one in 2012, have caused extensive damage to the vegetation that keeps the marshes stable. Phragmites australis, a species of reed that largely dominates the plant cover, is particularly vulnerable to these upheavals. The main problem is the intrusion of salt water, which not only floods the marshes but also saturates the soil, causing osmotic stress to the plants (water imbalance in the plant cells due to excessive salinity). Researchers found that in 2012, a combination of drought and salinity caused a sharp decline in this species, which is essential to the resilience of the marshes. The areas most affected, which were chronically flooded, saw their ability to recover greatly reduced.

A long-term analysis of environmental factors

To gain a better understanding of the mechanisms causing the decline of the Mississippi Delta marshes, the researchers used a methodology based on 16 years of environmental data collected between 2007 and 2022. The data comes from the Coastwide Reference Monitoring System (CRMS), a network of monitoring stations spread along the entire Louisiana coast. These stations measure various parameters such as vegetation cover, flood levels, water salinity and changes in marsh elevation. Thanks to these detailed and regular measurements, researchers have been able to establish precise links between environmental conditions and the health of vegetation, focusing in particular on the critical growth periods for plants, between March and October.

a.Location of marshes with dieback, no dieback or missing data during the dieback period, 2012-2017, based on Coastwide Reference Monitoring System data where Phragmites australis was a dominant species in the community (>20% cover), Google Earth, 2023. b. Seasonal flow of the Mississippi River, Baton Rouge, LA, 2007-2022. © T. Elsey-Quirk et al., 2024

Data analysis showed that areas dominated by Phragmites australis were particularly affected by the increased frequency and depth of flooding. In particular, the worst affected areas saw flood heights increase by an average of 15 cm, with longer periods of soil saturation, particularly between May and July. These critical periods also coincided with peaks in salinity, reaching levels that threatened vegetation. The researchers observed that the incursion of salt water in 2012, caused by a combination of extreme drought and low flow in the Mississippi River, exacerbated these conditions. In addition, tropical storms, such as Hurricane Isaac, exacerbated the situation by bringing in additional volumes of water, further saturating the already vulnerable marshes. This combination of environmental factors has contributed to a prolonged decline in vegetation, a phenomenon that the research team has been able to document thanks to access to these long-term data.

Lasting consequences for the ecosystem

The results of the study highlight the extent of the damage suffered by the marshes of the Mississippi Delta, revealing a massive decline in plant cover and very limited recovery. In 2012, during the extreme drought, Phragmites australis, which represented between 20 and 80% of the vegetation in the affected areas, suffered drastic mortality. Five years later, in 2017, these marshes had recovered only part of their plant cover, and the situation has hardly improved since. In the most affected marshes, Phragmites cover has fallen from 79% in 2011 to just 25% in 2022, a reduction of 68% over a period of 16 years. At the same time, plant biodiversity in these marshes has also declined markedly. The number of species present has fallen from 11 to 14 species before 2012, to fewer than 7 species between 2019 and 2022. These losses in biodiversity are of particular concern, as they increase the vulnerability of the marshes to further disturbance.

This decline in vegetation has had a cascading effect on the composition of the marshes. In areas where Phragmites have become scarce, more flood-tolerant species such as Colocasia esculenta have begun to dominate. However, these plants do not offer the same ecological benefits as Phragmites, particularly in terms of soil stabilisation and sediment trapping. This change in composition has direct implications for the ability of marshes to accumulate organic matter and raise their soils to cope with rising sea levels. As Tracy Elsey-Quirk explains, ‘ if these trends continue, the marshes of the Mississippi may no longer be able to fulfil their ecological role, which would lead to their permanent submersion ’. The data already shows an alarming increase in flooding rates: the proportion of time the marshes are flooded has risen from 43% in 2007 to 75% in 2022, illustrating the seriousness of the situation and the need for action to restore these ecosystems.

The black broken line represents the total annual vegetation cover (mean ± standard error) at the sampling sites. The stacked bars represent mean species cover data for all dominant and frequent species.. © T. Elsey-Quirk et al., 2024

Researchers are looking at a number of ways of dealing with this degradation of the marshes. One of the main recommendations is to reduce the chronic stress caused by rising sea levels by increasing the supply of sediment to the affected areas. This could be achieved through restoration techniques such as improving the flow of sediments via channels or using thin-layer sediment deposits to raise the surface of marshes. By increasing the elevation of the ground, these actions would reduce the frequency and duration of floods, thereby strengthening the resilience of marshes to extreme climatic events. The authors also stress the importance of proactive management to restore vegetation, in order to limit long-term impacts and prevent permanent flooding.


Source : Tracy Elsey-Quirk et al., “Vegetation dieback in the Mississippi River Delta triggered by acute drought and chronic relative sea-level rise”, Nature Communications (2024).

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