Answer: C. Maintains a constant salinity level.
Estuary is a natural water body that forms when a fresh water source like river meets with the saline water source like ocean. It is a transition zone between freshwater river and salt water ocean. The water in the estuary is brackish which means it is moderately saline. But the extend of salinity often varies, depending upon the supply of water from freshwater from river source and salt water from oceanic source. The estuaries does not maintain a constant salinity level. The organisms living in the estuary water ecosystem are adapted for moderate salinity. But with the increase in the salinity level the organisms are not able to survive and reproduce.The increase in amount of salt in water also depletes the oxygen level in water. Therefore, organisms may either migrate to other locations or die.