Salt marsh

A salt marsh is a specialized tidal wetland located in the upper coastal intertidal zone.

This zone exists between the land and the sea. It is regularly flooded by seawater or brackish water, which is a mix of fresh and salt water. These ecosystems are dominated by dense stands of salt-tolerant plants. These plants include various herbs, grasses, or low shrubs. These species are terrestrial in origin, meaning they evolved on land. They are essential for maintaining the stability of the marsh by trapping and binding sediments.

The formation of a salt marsh is a gradual process of sediment accretion. Accretion is the gradual buildup of material like mud or sand. It begins on low-energy shorelines known as tidal flats. These flats are often nourished by sediment from inflowing rivers and streams. When river water reaches these low-gradient flats, the discharge rate reduces. The rising tide creates a backwater effect that helps suspended sediment settle onto the surface.

Mats of filamentous blue-green algae can help by fixing silt and clay particles to their sticky sheaths. This increases the erosion resistance of the sediment. Pioneer species, such as glasswort (Salicornia spp.), then begin to colonize the surface.

These plants retain sediment around their stems and leaves to form low muddy mounds. These mounds eventually coalesce into depositional terraces. A sub-surface root network then binds the sediment and aids upward growth.

Salt marshes exhibit distinct patterns of zonation based on elevation and salinity.

Zonation is the way different plant communities arrange themselves in specific areas. In the lower marsh zone, tidal flooding is frequent. This results in fairly constant soil salinity. Plants here must tolerate high salt concentrations, periodic submersion, and low oxygen levels in the mud. In the upper marsh zone, tidal inflow is much less frequent. This leads to lower and more variable salinity levels. Rainfall can reduce salinity, while evapotranspiration can increase it during dry periods. Because of these differences, different species occupy specific microhabitats. In the New England salt marsh, smooth cordgrass (Spartina alterniflora) dominates the low marsh. Moving landward, one might find salt hay (Spartina patens), black rush (Juncus gerardii), and the shrub Iva frutescens.

These wetlands are incredibly productive and support complex food webs.

The food web begins with primary producers like vascular plants, macroalgae, diatoms, and phytoplankton. Primary consumers, such as zooplankton, molluscs, and insects, eat these producers. Secondary consumers then feed on the primary consumers. Many marine fish use the high grasses as nursery grounds to grow safely before moving to open waters.

Birds also rely on the marsh to raise their young. The high grasses provide sanctuary from predators and an abundant food supply. This food includes fish trapped in pools, insects, shellfish, and worms. Many halophytic plants, which are salt-tolerant plants, are not eaten by higher animals. Instead, they die and decompose. This decomposition feeds a broad chain of organisms ranging from bacteria to mammals.

Salt marshes occur in many different geomorphological settings.

Deltaic marshes are associated with large rivers. Examples include the Mississippi River Delta in the United States, the Camargue in France, and the Ebro delta in Spain. Back-barrier marshes form on the landward side of barriers and are common along the eastern United States and the Frisian Islands. Large, shallow coastal embayments can also host marshes, such as Morecambe Bay in Britain or the Bay of Fundy in North America. In New Zealand, most salt marshes occur at the heads of estuaries where there is high sedimentation and little wave action. In tropical regions, these marshes are often replaced by mangroves, which are dominated by salt-tolerant trees rather than herbaceous plants.

Global studies show that salt marshes are widespread but face many threats.

Scientists have mapped salt marshes in 99 countries. One study estimated the global extent at approximately 90,800 square kilometers, or 9,080,000 hectares. Historically, these areas have been endangered by poorly implemented coastal management. Land has often been reclaimed for human uses. Pollution from upstream agriculture or industrial coastal uses has also caused damage. Additionally, sea level rise caused by climate change is a major threat. It can lead to the erosion and submersion of otherwise tidal marshes.

Despite these challenges, there is growing recognition of their value. Since the 1980s, there has been an increase in salt marsh restoration and management. Environmentalists and society now acknowledge their importance for biodiversity and ecological productivity. They also provide vital ecosystem services, such as carbon sequestration. This is the process of capturing and storing atmospheric carbon dioxide. By protecting these wetlands, we help maintain the natural balance of our coastal environments.