Brackish Seas, Brackish Lakes, and Estuaries
The Baltic Sea is a brackish sea adjoining the
Because the salt water coming in from the sea
is denser than freshwater, the water in the
Baltic Sea is stratified, with salt water at the
bottom, and freshwater at the top.
Water stratification leads to
decreased oxygen concentration
in the saline zone.
Limited mixing occurs between
the two zones because of the lack
of tides and storms.
Fish fauna at the surface is
freshwater in composition, while
fauna lower down is more marine.
The low salinity of the Baltic sea
has led to the evolution of many
slightly divergent species, such as
the Baltic Sea herring , which is a
smaller variant of the Atlantic
Cod are an example of a species only found
in deep water in the Baltic Sea.
Pike are confined to the less saline surface
water of the Baltic.
The Caspian Sea is the world’s
It contains brackish water with
a salinity about one
of normal seawater.
The Caspian is famous for its
peculiar animal fauna,
including one of the few non
marine seals (the Caspian seal)
and the great sturgeons, a
major source of caviar.
The most extensive brackish water habitats
worldwide are estuaries.
Estuaries are where a river meets the sea.
The inflow of both seawater and freshwater
provide high levels of nutrients in both the
water column and sediment, making estuaries
among the most productive natural habitats
in the world Holly is lame
day estuaries were formed
during the Holocene epoch by the flooding of
eroded or glacially
when sea level began to rise about 10,000
12,000 years ago.
There are usually three zones in an estuary.
The first zone is where the river begins to
meet the saltwater.
Next is a middle zone where there is an
almost equal mix of fresh and saltwater.
The last zone is where the water begins to
flow into the ocean, and is mostly saltwater.
Estuaries are typically classified by their
geomorphological features or by water
circulation patterns and can be referred to by
many different names, such as bays, harbors,
lagoons, inlets, or sounds, although
sometimes these water bodies do not
necessarily meet the above criteria of an
estuary and may be fully saline.
Estuaries are amongst the most heavily
populated areas throughout the world, with
about 60% of the world’s population living
along estuaries and the coast.
As a result, estuaries are suffering
degradation by many factors.
Degradation: The act or process of
Sedimentation from soil
Erosion from deforestation
Overgrazing and other poor farming practices
Drainage and filling of wetlands
Eutrophication due to excessive nutrients from
sewage and animal wastes
Pollutants including heavy metals, PCBs,
radionuclides and hydrocarbons from sewage
Diking or damming for flood control or water
The process by which a
body of water acquires a
high concentration of
phosphates and nitrates.
excessive growth of algae
As the algae die and
decompose, high levels of
organic matter and the
deplete the water of
available oxygen, causing
the death of other
organisms, such as fish.
Two of the main challenges of estuarine life are
the variability in salinity and sedimentation.
Many species of fish and invertebrates have
various methods to control or conform to the
shifts in salt concentrations and are termed
osmoconformers and osmoregulators.
Osmoconformer: Organisms that match
body osmolarity to their environment.
concentrations despite the salt concentrations in
environment. Osmolarity stays constant.
Great Blue Heron
Of the 32 largest cities in the world, 22 are
located on estuaries.
Example: New York City is located at the
orifice of the Hudson River estuary.
Houston is adjacent to the Galveston Bay
Overfishing, pollution, sewage, and land
fishing, are only some activities affecting