Brackish marshes develop from salt marshes where a significant freshwater influx dilutes the seawater to brackish levels of salinity. This commonly happens upstream from salt marshes by estuaries of coastal rivers or near the mouths of coastal rivers with heavy freshwater discharges in the conditions of low tidal ranges.
The salinity levels in brackish marshes can range from 0.5 ppt to 35 ppt. Marshes are also characterised by low-growing vegetation and bare mud or sand flats. Due to the variations in salinity, brackish marshes create a distinctive ecosystem where plants from either freshwater or saltwater marshes can co-inhabit. The salinity levels also change with the tides, decreasing at low tide and increasing at high tide as ocean water feeds farther upriver.
In terms of biodiversity, a brackish marsh serves a unique ecological niche. Its vegetation is a byproduct of its salinity levels. High salinity serves as an evolutionary barrier for most plants, creating a less diverse number of plant species as an ecosystem moves from fresh to saltwater. Thus, there are only a few colonies of saltwater native plants in freshwater and almost no freshwater plants in saltwater ecosystems. However, in brackish marshes both types of plants are prevalent and are in fact high in plant productivity. Examples include, arrow arum (Peltandra virginica), soft rush (Juncus effusus), cattail (Typha), and sawgrass (Cladium).
These plants are usually halophytic in order to survive these conditions. For example brackish sites in Georgia, U.S., are dominated by species such as smooth cord grass (Sporobolus alterniflora), big cordgrass (Spartina cynosuroides), and black rush (Juncus roemerianus). Other communities are cabbage palm (Sabal palmetto), sand cordgrass (Spartina bakeri), black rush (Juncus roemerianus), saltgrass (Distichlis spicata, Paspalum distichum), and mixed halophytes (Batis maritima, Salicomia virginica). Along with salinity, brackish marshes face high physical stress due to flooding and wave currents creating adaptive traits within the plant community.
These plant communities also create an environment that provides a nursery for juvenile fish, crustaceans, and birds. Fauna use the shallow habitat and the turbidity of the water to protect themselves from predators. Similarly the surface of the marsh is covered with vegetation which is used by the nekton species for shelter, leaving enough space to move underneath between the stems.
The trophic levels within a brackish marsh has been shown to depend on the amount of macro organic matter in the upper level of soil. This macro organic matter is believed to be the food source of detritivore benthic animals that support higher trophic levels. These materials build up as the marsh matures, making age another factor in the biodiversity of a brackish marsh.
Brackish marshes are very important for flood control. However, they are often subject to heavy pollution and habitat degradation from land reclamation. For example, the Indian River Lagoon has suffered significant man-made changes since the 1940s. Marshes were often dredged or impounded to prevent mosquitoes, however this led to the disruption of connectivity by replacing marshes with open water. These changes prevented fires that allowed invasive species to move into the remaining marshes.
Brackish marshes can be restored by human intervention. Studies have found that given that restoration is properly carried out, fish do not discriminate between restored or natural marshes.
As in most habitats, the greatest threat towards brackish marshes are humans. Traditionally, direct human activities such as dredging and development are the main cause of destruction. Pollution has also been a threat to brackish marshes through chemical run-off. Once degraded, it could take at least 30 to 90 years for restored marsh soil to become equivalent to a natural marsh in terms of nitrogen and organic carbon profile. In some cases, these process could take over 200 years to achieve the wetland soil characteristics of certain communities.
For conservation, the key is to restrict human activities. Installing a passive management system could help restore certain species using brackish marshes' role as an ecological nursery. For some areas, periodic livestock grazing could help create a better habitat for certain species of birds. Brackish marshes are a unique type of wetland and the local circumstances are paramount to consider for either conservation, biodiversity, or restoration.