Brackish marsh
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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.[1]
Ecosystem Services
The salinity levels in brackish marshes can range from 0.5 ppt to 35 ppt.[2] Marshes are also characterised by low-growing vegetation and bare mud or sand flats.[3] Due to the variations in salinity, brackish marshes create a distinctive ecosystem where plants from either freshwater or saltwater marshes can co-inhabit.[4] The salinity levels also change with the tides, decreasing at low tide and increasing at high tide as ocean water feeds farther upriver.[5]
Biodiversity
In terms of biodiversity, a brackish marsh serves a unique ecological niche.[8] 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.[4] However, in brackish marshes both types of plants are prevalent and are in fact high in plant productivity.[4] Examples include, arrow arum (Peltandra virginica), soft rush (Juncus effusus), cattail (Typha), and sawgrass (Cladium).[2]
These plants are usually halophytic in order to survive these conditions.[9] 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).[4] 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).[10] Along with salinity, brackish marshes face high physical stress due to flooding and wave currents creating adaptive traits within the plant community.[11]
These plant communities also create an environment that provides a nursery for juvenile fish, crustaceans,[12] and birds.[13] 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.[14]
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.[14]
Algae also make up a large part of the biodiversity in brackish marshes. The most common algae, diatoms, make up a large portion of the algal community in brackish marshes.[15] Diatoms are eukaryotic microorganisms that have a cell wall that is composed of silica and can exists in freshwater or marine environments making them good candidates for brackish marshes.[15] These diatoms can be either planktic, which float freely in the water column, or benthic, which attach to a substrate.[15] Some examples of diatoms that can be found in brackish marshes are from the genera (Navicula), (Nitzschia), (Diploneis), (Cyclotella), (Cymbella), (Fragilaria), (Gyrosigma), (Tabularia), (Amphora) (Cocconeis), and many more.[16] Many different organisms in these brackish marshes depend on diatoms as a food source so they are ecologically important. Some examples of organisms that feed on diatoms are bivalves,[17] mollusks,[18] fish,[18] copepods,[18] decapod larvae,[18] and ducks,[19] as well as many others. Many organisms in these brackish marshes consume diatoms so they are very valuable to maintaining balance in these types of ecosystems.
Another group of algae that is present in brackish marshes are fucoid algae.[20] This is a type of brown macroalgae in the class Phaeophyceae.[20] Brown algae are eukaryotic stramenopiles which means that they are at one point flagellated and most people know them as seaweeds in coastal areas.[21] Examples of brown algae that have been found in brackish marshes are Fucus vesiculosus, Ascophyllum nodosum,[20] the genus Sphacelaria,[22] and many others.
Yellow-green algae can also be found in brackish marshes. Yellow-green algae are eukaryotic algae in the class Xanthophyceae.[23] An example of this is Vaucheria.[22]
Green algae can also be found in brackish marshes. Some examples of the different genera of green algae that can be found in brackish marshes are Enteromorpha, Ulothrix, Rhizoclonium, Blidingia, Percursaria, and many others.[22]
Typically, sedges and grasses dominate the vegetation in brackish marshes. Plants in brackish marshes are salinity tolerant and they tolerate frequent flooding.[24] They also have frequent tidal waves disturbing the area as well as seasonal hurricanes and tropical storms. (Julia bass) According to (Makenzie) plants in coastal marshes resist salinity by refraining from the uptake of salt via their root system. Some examples of plants that grow in brackish marshes are Panicum hemitomon, Spartina patens, Zostera japonica, Haloxylon recurvum, Juncus roemerianus, Borrichia frutescens,[24] Schoenoplectus americanus, Distichlis spicata and many others.[25]
