Tidal flats are coastal wetlands that exist in the intertidal zone, a dynamic environment alternately submerged and exposed by the rhythm of the tides. These areas represent a highly productive interface between marine and terrestrial ecosystems, often forming in sheltered areas like estuaries, bays, and lagoons. These soft-sediment environments are complex ecosystems that support a vast array of life and provide numerous benefits to the planet and human communities. They face increasing pressure from human activity and climate change.
Physical Formation and Sediment
The development of a tidal flat requires a specific set of geological and hydrological conditions, most notably a low-relief coastline and the presence of a significant tidal range. A tidal range, the difference between high and low tide water levels, must be large enough to regularly flood and expose the shallow seafloor. The periodic movement of water, or tidal currents, is the primary mechanism for transporting and depositing fine-grained sediments in these areas.
The resulting flat environment is composed of different sediment types, which determines whether it is classified as a sand flat or a mud flat. Sand flats form in areas with stronger currents or wave action, depositing larger, coarse-grained particles. Mud flats are found in protected, low-energy zones where fine silt and clay particles settle out during the slack tide. The fine sediments of mud flats retain more water, preventing rapid desiccation at low tide, but they limit oxygen percolation, often leading to oxygen-poor (anoxic) conditions just below the surface.
Unique Biological Community
Life in the tidal flat is dictated by the extreme fluctuations in the environment, requiring organisms to possess specific morphological, physiological, and behavioral adaptations to survive. Organisms must cope with dramatic daily changes in temperature, salinity, and the risk of desiccation when the tide recedes. The animals living within the sediment, known as infauna, such as clams and marine worms, retreat into burrows to avoid drying out and escape predation.
Mobile organisms, like fiddler crabs, utilize burrows as a refuge from the harsh conditions, sometimes sealing themselves inside to survive the low-tide period. Organisms living on the surface, or epifauna, often have hard shells or exoskeletons that help reduce water loss and protect them from physical damage. The high productivity of the flats supports a large biomass of invertebrates that form the base of an extensive food web. This dense concentration of food makes tidal flats significant feeding grounds for numerous fish species and migratory shorebirds.
Essential Ecosystem Services
Tidal flats provide functions that extend far beyond their boundaries, contributing to the health of coastal ecosystems and human well-being. One major function is their role in nutrient cycling and water purification, acting as natural filters for pollutants and excess nutrients carried in from inland runoff. The organisms within the sediment, particularly filter-feeding bivalves, process large amounts of water, removing suspended matter and organic waste.
These systems also provide significant coastal protection, functioning as natural physical buffers that absorb the energy of waves and storm surges. The soft sediments and low gradient dissipate wave action, reducing erosion on the adjacent shoreline and protecting infrastructure further inland. Tidal flats also contribute to climate change mitigation by acting as a “blue carbon” sink, sequestering and storing carbon in their sediments. Their high productivity and nursery function also support provisioning services, maintaining commercially harvested fish and shellfish populations.
Threats and Conservation
Tidal flats worldwide are facing rapid degradation and loss due to human-caused disturbances and the accelerating effects of climate change. Coastal development and land reclamation projects represent a major threat, directly converting or destroying habitat to build infrastructure. This urbanization often leads to “coastal squeeze,” where the flats cannot migrate inland as sea levels rise because human-made structures block their movement.
The habitats are also impacted by pollution, which can destabilize the ecosystem. Sea level rise poses a direct threat, as increased water depth and stronger currents lead to the erosion of sediment, potentially submerging the flats and making them less accessible to foraging birds. Conservation efforts focus on restoring degraded habitats and implementing sustainable management practices, such as ensuring sediment supply is maintained and creating protected areas that allow the flats sufficient space to migrate landward.