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How do changes in salinity affect marine organisms living in estuaries?
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Marine ecosystem
Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.
In estuaries, where freshwater and seawater mix, organisms must contend with fluctuating salinity levels that create significant osmotic pressure. Because the internal salt concentration of an organism's cells must remain within a narrow, functional range, these environmental changes pose a constant physiological challenge.12
To survive these shifts, organisms have evolved various strategies:
- Osmoregulation vs. Osmoconformation: Some animals, known as osmoconformers, allow their internal osmotic pressure to match the surrounding environment. Others, called osmoregulators, actively control their internal salt concentrations to keep them constant despite external changes.1
- Tolerance Levels: Organisms that can tolerate a wide range of salinity are termed euryhaline, while those with limited tolerance are called stenohaline.1
- Physiological Adaptations: Many species, such as anadromous fish, require time to adjust their internal systems when moving between fresh and salt water. They often congregate in brackish estuarine waters to avoid physiological shock while their osmotic regulating systems adapt.3
- Active Transport: To maintain balance, many marine organisms actively transport inorganic ions, such as sodium and chloride, across membranes—a process that consumes significant energy.1