Aquatic ecology and Biodiversity

Unit 10- Coastal ecosystems

10.3.4. Classification of organisms
The estuarine organisms are classified based on the salinity tolerance levels as follows:
Oligohaline organisms
These organisms include most of the freshwater forms inhabiting rivers which cannot tolerate variations in salinity of more than 0.1 ppt and which are not found at the head of the estuary. Some oligohaline species at the head may tolerate salinities up to 5 ppt and a few even as high as 19 ppt.
True estuarine organisms
These organisms are euryhaline forms and are restricted to estuaries only. These are adapted to the wide variations of salinity and are known to live both in the upper and middle reaches of the estuaries with low saline conditions.
Euryhaline marine organisms
These organisms extend their distribution from the sea to the upper reaches of the estuary. They can tolerate salinity as low as 15 ppt and a few even 5ppt.These organisms form the majority of the total estuarine biota.
Stenohaline marine organisms
These organisms live on the open sea shore and at mouths of the estuaries. They do not enter the estuaries below salinities of 25 ppt.