What is Brackish Water?

Find out all about brackish water. What is it? What lives there? We’ve got all the answers in today’s post.

brackish water

With less salt than sea water and more salt than freshwater, brackish water is the result of the mixing of these two types of water. Brackish water generally has around 0.5 to 30 grams of salt per liter—this isn’t a very specific definition—so there is wide range of salt concentrations that can be found in brackish water.

The scientific term for the “saltiness” of the water is salinity. Therefore, brackish water can be said to have a high variance of salinity. The salinity of a body of brackish water can vary widely over time. For instance, an aquifer which was at one point at 5g of salt per liter, can become saltier e.g. 25g salt/liter.

The occurrence of rain can cause the freshwater and saltwater boundaries to move, often changing the volume of brackish water in the process. This same result can occur from the addition of freshwater.

Brackish water is most commonly found in estuaries, places where a freshwater river meets the sea. The resulting combination of freshwater and salty seawater is brackish water.

Well-known estuaries which influence water levels, leading to high-level brackish water fish and brackish water plants in surrounding bodies of water, include the Amazon River, the Thames Estuary, Chesapeake Bay, the Fleet Lagoon, the lower Hudson River, Saint Lawrence River, and Hampton Roads.


Estuaries generally have shallow bodies of water which are exposed to sunlight throughout. Because of their optimum conditions for habitation, estuaries often function as the first homes for various types of aquatic lifeforms before they leave for the ocean. The hospitable conditions of estuaries allow for a diverse range of organisms to live within the estuary, such as algae, marsh grasses, brackish fish, crabs, shrimp, and oysters.

However, some seas and lakes are themselves already brackish. For instance, the Caspian Sea (somewhat misleading called “Sea” when it is an actual fact the world’s largest lake) is comprised of brackish water. Its salinity (saltiness) is around a third of that found in typical seawater.

A lot of brackish water bodies can be a very hostile living environment for organisms. Consequently, many species which exist in brackish water are specifically adapted for saltwater survival. However, this is far from the whole story.

Salty brackish water is often incredibly fertile, meaning that the fish in this aquatic environment are normally healthy and fat. The organisms that survive in brackish water generally have specific adaptation mechanisms in order to cope with the high concentration of salt. They thrive in these conditions, but have to remain adaptable to the changing salinity of brackish water.


A few ordinarily saltwater fish species such as striped bass and redfish, can live in freshwater as well, and consequently they are able to explore areas of brackish water.

The arrowhead and fahaka pufferfish are freshwater organisms which are also often found in brackish water. These fish can grow up to 43cm in length, and as with all pufferfish, can inflate themselves when intimidated by other organisms. They carry the poisonous tetrodotoxin and mainly feed on mussels and snails.

Gobies are small (less than 10cm long) fish which are commonly found in brackish water habitats in both tropical and temperate climates. They thrive in lower rivers, salt marshes, and mangrove swamps. The larvae of the freshwater varieties are sent downstream to more brackish waters, only returning many weeks or even months later. Gobies use their teeth in order to scrape algae from rocks.

Scats, although able to survive in freshwater environments when young, prefer brackish water settings when they have fully matured. These fish scavenge for algae and feces. The largest examples of scats have grown up to 38cm in length. They also have a remarkable longevity, being capable of living for more than 20 years in the right brackish water conditions.

Gars, or garpikes, are a relatively large fish species with highly scaled and elongated bodies. They primarily inhabit freshwater but several variants also live in brackish waters, often being able to travel through these bodies of water with wide ranging salinity.


Certain other saltwater fish are able to survive in non-especially salty brackish areas, even when they wouldn’t otherwise go into freshwaters areas.

Archerfish are a brackish water-inhabiting fish which are also predators of insects. They shoot water at small prey in order to knock them out. They are accurate at between 1- 2 meters from their prey, but are known to try to hit their target from as far away from 5 meters, taking multiple shots if they fail initially.

Many plants that thrive in freshwater conditions can also do well in brackish environments, such as anacharis, anubias, cabomba, cryptocoryne, hornwort, wisteria, and vallisneria.

However, since there is a high degree of variance between brackish waters, some being significantly saltier than others, the survivability of these plants often depends on the specific salinity of the brackish water in question.

To this end, anacharis, also often known as waterweeds, are known primarily to be found in freshwater bodies, but can also be present in brackish water. Similarly, anubias plants, which can be either aquatic or semi-aquatic, are known mainly to grow in streams and rivers, but are also present in marsh environments.

Camboma, also referred to by its vernacular name fanwort, are commonly used as aquarium plants due to their aesthetic attractiveness and their rapid-growth qualities. They can survive in freshwater and brackish water with low salt-level salinities.

The C. ciliata species of crypotocoryne is one of a limited number of plants used in aquaria that are able to tolerate a high salt concentration that, to the majority of aquarium plants, would be fatal. Consequently, this species of cryptocoryne can be located in brackish water.

The extent to which water can be considered “brackish” is most accurately described by specific gravity, a measure of how dense the water is relative to the density of pure water. Tools such as a hydrometer or a refractometer are used to measure the specific gravity in a body of water.

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