Most aquatic organisms are adapted to a narrow range of salinity. Why must aquatic organisms living in coastal areas be adapted to survive in both low and high salinity?

During high tide, freshwater mixes easily with ocean water, lowering salinity.
During low tide, the ocean water becomes concentrated, increasing salinity.
Ocean currents bring in deep-ocean water with high salinity concentrations.
Temperature fluctuations can cause changes in the salinity levels of the water.