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Aquatic Science. Brackish Seas, Brackish Lakes, and Estuaries. Brackish Seas. The Baltic Sea is a brackish sea adjoining the North Sea.

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aquatic science

Aquatic Science

Brackish Seas, Brackish Lakes, and Estuaries

brackish seas
Brackish Seas
  • 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.
water stratification
Water Stratification
  • 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.

Baltic Herring

Atlantic Herring

cod and pike
Cod and Pike
  • 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.



caspian sea
Caspian Sea

Caspian Seal

  • 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.


what is an estuary
What is an Estuary?
  • 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 ahahhah
formation of estuaries
Formation of Estuaries
  • 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.
estuary zones
Estuary Zones
  • 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 classification
Estuary Classification
  • 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 degrading.
degradation factors
Degradation Factors
  • 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
  • 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.
estuarine life
Estuarine Life
  • 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.
galveston bay estuarine birds
Galveston Bay Estuarine Birds

Laughing Gull


Great Egret

Great Blue Heron

Brown Pelican

galveston bay mammals
Galveston Bay Mammals


You on the field trip

Bottlenose Dolphin

Swamp Rabbit

human impact
Human Impact
  • 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.