Brackish water represents a unique aquatic environment, characterized by a salinity level that falls between that of freshwater and seawater. While freshwater typically contains less than 0.5 parts per thousand (ppt) of salt, and seawater ranges from 35 to 38 ppt, brackish water generally exhibits a salinity between 0.5 and 30 ppt. This intermediate salt concentration makes it a distinct habitat, often found where these two major water types converge.
Natural Coastal Mixing Zones
Coastal areas are key locations where brackish water naturally forms, where rivers meet the ocean. Estuaries, such as bays, sounds, inlets, and fjords, are dynamic mixing zones. In these areas, the constant ebb and flow of tides introduce seawater, while rivers continuously supply freshwater, creating a fluctuating salinity gradient. This constant mixing means that salinity levels can vary significantly across an estuary and even change with the tidal cycle and river discharge.
River deltas are also brackish environments. They develop at the mouth of a river where its flow slows and deposits sediment. Here, the extensive network of channels and wetlands allows for the widespread intermingling of river water and ocean water, leading to brackish conditions across a broad area. Coastal lagoons, which are shallow bodies of water separated from the open sea by a barrier, also frequently become brackish. Limited exchange with the ocean and significant freshwater input from land contribute to their reduced, yet still present, salinity.
Mangrove forests thrive in the intertidal zones of tropical and subtropical coastlines, often within estuaries and deltas. They feature salt-tolerant trees adapted to brackish conditions. The intricate root systems of mangroves trap sediments and slow water flow, further influencing the mixing of fresh and saltwater and creating localized brackish habitats. The interplay of riverine discharge and tidal incursions ensures a continuous spectrum of salinity, providing a habitat for species adapted to these specific conditions.
Inland and Underground Occurrences
Brackish water also occurs inland and underground through different mechanisms. Certain inland saline lakes, particularly those in endorheic basins, can become brackish. In these closed basins, water flows into the lake but does not drain out to the sea, leading to the concentration of dissolved salts through evaporation. Over long periods, this process can result in lakes with salinity levels ranging from brackish to hypersaline, depending on the rate of evaporation and the mineral content of the incoming water.
Underground aquifers can also host brackish water. Saltwater intrusion is a common cause, where over-pumping of freshwater from coastal aquifers reduces the water pressure, allowing denser seawater to move inland. This infiltration creates a brackish interface within the aquifer. Additionally, some deep “fossil aquifers” contain ancient water that became trapped underground, and over geological timescales, mineral dissolution from surrounding rock can lead to brackish conditions.
Global Examples of Brackish Water Bodies
Many brackish water bodies exist worldwide, each with unique salinity characteristics. The Baltic Sea in Northern Europe is one of the largest brackish water bodies globally. Its brackish nature stems from a large freshwater input from surrounding rivers and limited, narrow connections to the saltier North Sea, which restricts the inflow of oceanic water and leads to reduced salinity. Similarly, Chesapeake Bay on the East Coast of the United States is a prominent estuary where the Susquehanna River and other tributaries mix with Atlantic Ocean water, creating a gradient of brackish conditions along its length.
The Amazon River Delta, where the Amazon River meets the Atlantic, features extensive brackish zones. The immense volume of freshwater discharged by the Amazon significantly influences the salinity of the adjacent ocean, creating a large plume of brackish water that extends far offshore. In North America, the Everglades in Florida is a wetland ecosystem where freshwater from the Kissimmee River basin slowly flows southward, eventually mixing with saltwater from the Gulf of Mexico and Florida Bay, resulting in brackish marshes and estuaries. The Venice Lagoon in Italy provides another example, as it is a semi-enclosed body of water connected to the Adriatic Sea by several inlets, with freshwater input from rivers contributing to its variable brackish state.
The Unique Ecosystems of Brackish Waters
Brackish water environments support unique ecosystems with specialized plant and animal species adapted to fluctuating salinity. Organisms in these habitats, such as certain fish, shellfish, and aquatic plants, possess physiological adaptations that allow them to tolerate wide ranges of salt concentration. For instance, some fish can regulate the salt content in their bodies, while specific plants have mechanisms to excrete excess salt.
These transitional zones often serve as important nursery grounds for marine species. Many commercially important fish and shellfish begin their life cycles in the protected, nutrient-rich brackish waters of estuaries before migrating to the open ocean as adults. Beyond supporting biodiversity, brackish ecosystems provide important ecosystem services. They can filter pollutants and sediments from freshwater runoff before it reaches the ocean, contributing to water quality. Furthermore, coastal brackish habitats, like mangrove forests and salt marshes, help protect shorelines from erosion and storm surges, acting as natural buffers against coastal hazards.