Final answer:
An estuary is a zone where land and sea come together, and fresh and salt water meet. It is a biologically productive region where many organisms begin their lives. Organisms in estuaries have adaptations to deal with the varying salinity and oxygen content of the water.
Step-by-step explanation:
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the ocean. It is a zone where land and sea come together, and where fresh and salt water meet. The mixing of fresh water and salt water in estuaries results in a diluted (brackish) saltwater.
Estuaries are biomes that occur where a river or source of fresh water meets the ocean. They form protected areas where many young offspring of crustaceans, mollusks, and fish begin their lives. The salinity of estuaries varies based on the rate of flow of freshwater sources, and high tides bring saltwater into the estuary once or twice a day.
Organisms living in estuaries, such as plants and animals, have developed adaptations to deal with the varying salinity and oxygen content of the water. For example, some plants called halophytes can tolerate salty conditions and have filters in their roots to remove salt. Animals like mussels and clams have behavioral adaptations to function in the rapidly changing environment.