Many organisms living in estuaries experience freshwater and saltwater conditions each day with the rising and falling of tides. Explain how these changing conditions challenge the survival of these organisms (see Concept 44.1).

Eestuaries are areas where freshwater from rivers interacts with seawater.This transitional zone between river and sea occurs when saline seawater interacts with freshwater, resulting in brackish water. Estuaries are places where river and ocean environments meet.

Osmolarity is a measure of the overall solute concentration of a solution.Low-osmolarity solutions contain more water molecules than high-osmolarity solutions, while high-osmolarity solutions contain fewer water molecules than solute particles.

Osmoregulation is the active control of an organism's fluid osmotic pressure to prevent excessive dilution or overconcentration of its fluids. It stops fluid from becoming excessively diluted or too concentrated. Osmotic pressure is a measure of water's tendency to osmose into one solution from another.

To prevent excessive changes in cell volume (under saltwater conditions), organisms living in estuaries need to be able to compensate both for water gain (in freshwater conditions) and water loss (in saltwater conditions).