Aquatic ecology and Biodiversity

Unit 10- Coastal ecosystems

10.3.1. Classifications of estuaries
1. Estuaries based on geomorphology:
•Coastal Plain Estuaries are formed by sea level rising and filling an existing river valley. Chesapeake Bay in Maryland and the harbour of Charleston, South Carolina.
•Tectonic Estuaries are caused by the folding or faulting of land surfaces. Eg. the San Francisco Bay area in California.
•Bar-built Estuaries form when a shallow lagoon or bay is protected from the ocean by a sand bar or barrier island. Examples the Eastern Seaboard and the Gulf Coast of North America.
•Fjords are U-shaped valleys formed by glacial action. Fjords are found in Northern Europe, Alaska and Canada.
2. Estuaries based on circulation
•Salt-wedge estuary or Positive estuaries-A salt-wedge estuary is highly stratified. Salt water moves into it in the shape of a wedge, with fresh water flowing over it. The Mississippi River estuary is an example of this type
•Well mixed estuaries or Neutral estuaries- The vertically homogenous or well-mixed estuary is characterized by low inflow of fresh water and large tidal ranges.
•Evaporate estuaries or Negative estuaries- In desert climates where the amount of freshwater input to the estuary is small and the rate of evaporation high, a negative or evaporate estuary results.