Aquatic ScienceBrackish Seas, Brackish Lakes, and

Estuaries

The Baltic Sea is a brackish sea adjoining the North Sea.

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.

Brackish Seas

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 herring.

Water Stratification

Baltic Herring

Atlantic Herring

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.

Cod and Pike

Cod Pike

The Caspian Sea is the world’s largest lake.

It contains brackish water with a salinity about one-third that 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.

Caspian Sea Caspian Seal

Sturgeon

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.

What is an Estuary?

Most modern-day estuaries were formed during the Holocene epoch by the flooding of river-eroded or glacially-scoured valleys when sea level began to rise about 10,000-12,000 years ago.

Formation of Estuaries

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.

Estuary Zones

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.

Estuary Classification

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 degrading.

Degradation

Sedimentation from soil Erosion from deforestation Overgrazing and other poor farming practices Overfishing 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 inputs

Diking or damming for flood control or water diversion

Degradation Factors

The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates.

These typically promote excessive growth of algae.

As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish.

Eutrophication

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 their body osmolarity to their environment.

Osmoregulator: Actively control salt concentrations despite the salt concentrations in the environment. Osmolarity stays constant.

Estuarine Life

Galveston Bay Estuarine Birds

Great Blue Heron

Brown Pelican

CaracaraLaughing Gull

Great Egret

Galveston Bay Mammals

Bottlenose Dolphin

Nutria

Swamp Rabbit

You on the field trip

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 estuary.

Overfishing, pollution, sewage, and land fishing, are only some activities affecting estuaries.

Human Impact