Final answer:
An estuary is a biotic community where fresh water from a river meets the salt water of the ocean. It is formed when geographical barriers separate the two bodies of water. Estuaries act as protected areas and have varying salinity levels.
Step-by-step explanation:
An estuary is a biotic community where fresh water from a river meets the salt water of the ocean. It is formed when geographical barriers, such as sandbars or shoals, separate the two bodies of water. The mixing of fresh water and salt water in estuaries creates a diluted saltwater known as brackish water.
These estuaries act as protected areas where many marine organisms, including crustaceans, mollusks, and fish, give birth to their offspring. The salinity in estuaries varies based on the rate of freshwater flow. High tides bring salt water into the estuary, while low tides reverse the current of salt water. The short-term and rapid variation in salinity in estuaries poses a challenge for the plants and animals that inhabit them, leading to unique adaptations.