What is an estuary?

An estuary is defined as a semi-closed coastal body of water where fresh water from rivers and streams meets and mixes with salt water from the ocean. This unique environment is characterized by its transitional nature, functioning as a critical habitat for various marine species and serving as an important ecological area for wildlife. The mixing of fresh and salt water creates a diverse range of salinity levels, which supports a wide variety of plant and animal life adapted to fluctuating conditions.

Estuaries often serve several ecological functions, providing nursery grounds for fish, protection against storms, and natural filtration systems which improve water quality. Furthermore, they play an integral role in nutrient cycling, sediment trapping, and serving as a buffer against pollution from terrestrial sources.

The other options do not accurately reflect the characteristics of an estuary, as they describe either completely enclosed bodies of water or freshwater environments that lack the important mixing of fresh and saltwater integral to an estuarine system. This distinction makes option B the correct definition of an estuary.