Stress Part II: Flooding and Hypoxia

Wetland inhabitants must also deal with flooding stress. All parts of a plant must have oxygen, which causes problems when a plant is rooted in hypoxic soils and it is flooded. Gases diffuse about 10,000 times more slowly through water than through air, and wetland soils are often inundated and hypoxic. This poses an issue for supplying roots with enough oxygen since they don’t have any around them. Some root systems will have adventitious roots, which means they extend above the surface of the water or soil to allow gas exchange with the atmosphere.[1] Red mangroves have prop roots, black mangroves have pneumatophores, and both supply oxygen directly to the root system rather than relying on transport all the way from the leaves to the roots.[2]

Hypoxia can be caused by eutrophication and decomposition. Hypoxia and anoxia are dangerous to most plants and animals because most cannot live only with anaerobic (without oxygen) respiration. Bacteria can sometimes live in anoxic conditions by using different electron receptors that are more plentiful in wetland soils like sulfates. Plants can sometimes cope with hypoxia thanks to adaptations like aerenchyma development in their roots. Aerenchymous tissues are much more porous to allow gases to diffuse up to 30 times more easily through a plant! In animals, lungs can allow some fish, mammals, and aquatic gastropods (snails) to live in hypoxic waters, but many fish have gills that are not adapted to hypoxia. The Gulf of Mexico along Louisiana’s coast boasts one of the largest hypoxic zones in the world with a peak area of over 8,500 square miles in 2017, where many commercial fisheries have seen a large decline in fish catch. [3]

PHOTO- dead zone map-NOAA-700x345-Landscape
Photo from NOAA, Dead Zone 2017

Works Cited:

[1] Gilman, Sharon. “Plant Adaptations.” ci.coastal.edu/~sgilman/778Plants.htm.

[2] “Adaptations.” Adaptations :: Florida Museum of Natural History, http://www.floridamuseum.ufl.edu/southflorida/habitats/mangroves/adaptations/.

[3] “Gulf of Mexico ‘Dead Zone’ Is the Largest Ever Measured.” Gulf of Mexico ‘Dead Zone’ Is the Largest Ever Measured | National Oceanic and Atmospheric Administration, web.archive.org/web/20170802173757/http:/www.noaa.gov/media-release/gulf-of-mexico-dead-zone-is-largest-ever-measured.

Featured image is of Rhizophora mangle (red mangrove) from Flickr by barloventomagico

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Mangroves in Winter

The recent cold weather in Louisiana may have been the end of the road for some plants as temperatures dipped into the teens and stayed below freezing for full days. The hibiscus in your garden may have survived because you gave it extra insulation, but what about marsh plants? Louisiana salt marshes are home to black mangroves (Avicennia germinans), but this represents the very northernmost part of their range. Of the three mangrove species found in the continental United States [red (Rhizophora mangle), black, and white (Laguncularia racemosa)], black mangroves are the most cold-hardy, but they are still sensitive to winter weather- they generally cannot establish above 28° N and S latitude because winters are too cold (a sliver of the Birdsfoot Delta is below 29° N, so we really are at their limit).

The three mangrove species are also different in their tolerances for other environmental conditions: red mangroves establish in the intertidal zone, while black and white mangroves are found at higher elevations, and white mangroves can colonize areas with little to no soil. In Florida where all three species occur, mangrove zones can be defined from the water extending inland and up in elevation .

ZONATION1
Red mangroves are found in the intertidal zone, while black and white mangroves establish at higher elevations. Graphic from the Florida Museum of Natural History at the University of Florida (https://www.floridamuseum.ufl.edu/southflorida/habitats/mangroves/zonation/)

Black mangroves are an important component of Louisiana salt marshes, providing habitat to a variety of species. The complex root systems trap and collect sediment, limiting erosion and maintaining land. Juvenile invertebrates and fish find shelter among the roots, while seabird chicks, such as brown pelicans and roseate spoonbills, are protected from high water events and predators up in the branches.

CWPPRA projects that nourish barrier islands and create new marsh habitat help maintain black mangrove populations by providing new land for the plants to colonize; in turn, the mangroves help the new land persist in the face of wind and wave energy.