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Mangrove marshes, or mangrove swamps, are coastal wetlands found in tropical and subtropical regions across the globe. They are characterized by halophytic (salt loving) trees, shrubs and other plants growing in brackish to saline tidal waters.Mangrove marshes are rich in biodiversity, but are under multiple environmental threats, including sea-level rise, atmospheric carbon dioxide, and human development.

Overview
As a major blue carbon ecosystem, mangrove marshes help mitigate the effects of climate change by sequestering carbon at highly efficient rates.They also provide raw materials and food for humans, coastal protection from storms and erosion, water purification, and tourism and recreation. These wetlands are often found in estuaries, where fresh water meets salt water and are infamous for their impenetrable maze of woody vegetation.

Characterization
Functional root types of mangrove marshes include prop roots (Rhizophora spp.), large and small pneumatophores (Sonneratia/Avicennia spp.), knee roots (Bruguiera spp.) and plank roots (Xylocarpus/Heritiera spp.), which develop along various hydrological and geomorphological gradients in many forests. Important differences in root morphology among these root types can also extend below ground.

Locations
Mangrove marshes are located where mangrove forests transition into salt marsh habitats. Mangrove-marsh ecotones are located all around the world, with the Indo-Malaysian region considered to being the center for the evolution of Mangrove vegetation.

North America
In the United States, mangrove marshes also exist in the northern Gulf of Mexico, the Atlantic Coast of Florida, and along the Gulf Coast to Texas. Florida's southwest coast supports one of the largest mangrove swamps in the world.

Asia
Hong Kong is home to hundreds of mangrove marshes, many of which are contaminated with heavy metals in the fine-grained (<63 μm) and sand-sized (2 mm–63 μm) fractions of surface sediments. These mangrove marshes are located in the Sai Kung district, the Tolo region, and the Deep Bay region. Higher concentrations of heavy metals were found in the fine-grained than the sand-sized fractions of the sediment; however, the differences between these two fractions became less significant when the swamp was more contaminated. Typically, mangroves are limited in their northward expansion because of winter freeze events. However, as temperatures increase, there will be fewer freeze events, allowing mangroves to expand into salt marsh habitats.

Flora
To distinguish the general mangrove community among other flora and fauna, the term “mangal” was proposed. There is existing discussion of the exact number of global mangrove species, but there is a consensus of 50 to 70 species. As mangrove marshes are comprised of mangrove trees, only three species of mangrove grow in the continental United States: red, black, and white. Rhizophera mangle (Red Mangrove) has distinctive, arching roots. Avicennia sp. (Black Mangrove) consist of pneumatophores, or root projections, that help supply the mangrove with air within the soil. The Black Mangrove often grows more inland. Laguncularia racemosa (White Mangrove) do not have outstanding root structures, and grow farther inland than the Black Mangrove

Fauna
Nutrients transported by fresh water runoff from the land consistently replenish mangrove marshes, resulting in a wide diversity of animals in these regions. Mangrove marshes are flushed by the ebb and flow of tides, and consist of blooming populations of filter feeders, decomposers, and bacteria, such as barnacles, oysters, worms, and protozoa, among many other invertebrates. Wading pelicans, birds, and crocodiles feed on the aforementioned species, further continuing the mangrove marsh food chain.

Exploitation
Ecotones are typically considered valuable areas to monitor how climate change impacts habitats. Thus, the decline of the mangrove marsh ecotone is a great concern to the scientific community. Because of human activities, the mangrove-marsh ecotone is transforming, and salt marsh habitats are increasingly declining. Recent assessments on the existence of global mangrove marshes suggests that there has been a reduction of mangrove-cover by 3% between the years of 1990 and 2010 due to land conversion for coastal development, rice production, and aqua cultural projects.

Sea Level Rise
Mangrove marshes are threatened by sea level rise. In order to survive rising sea level and flooding, mangrove marshes begin to build vertically. To avoid submergence, vertical soil development must equal relative sea-level rise, which is the combination of ocean height increase and local subsidence (both shallow and deep). In turn, mangroves often influence soil accretion processes through the production and accumulation of organic matter. If mangrove marshes cannot accumulate enough sediment and organic matter to combat the rate of sea level rise, they will eventually drown. Additionally, rising waves and increased incidences of storms can erode salt marsh boundaries. Salt marshes are also threatened by nutrient pollution from excess fertilizer runoff. Nutrient-enriched salt marshes destabilize creek banks that are prone to collapsing into unvegetated mudflats.

Atmospheric Carbon
Mangrove marshes store carbon at efficient rates, but because of fossil fuel use, atmospheric CO2 concentrations are increasing at a faster rate than mangrove marshes can sequester. This leads to a cascade effect where high atmospheric CO2 concentrations induce climate change and global warming, thereby promoting mangrove range expansion and sea level rise, which harm mangrove marsh health. While healthy mangrove marshes can take up excess nutrients effectively, too much nutrient pollution can damage salt marshes.