A bay is a water body that is partially surrounded by land. Bay usually is smaller and less enclosed than a gulf. The Bay's mouth is wider than the gulf's mouth when it meets the ocean.
Know how a bay is formed in the article below.
How is a Bay formed?
Bay can be formed in the following ways:
Plate tectonics- In the process of continents drifting together and rifting apart, formation of many large bays can happen and generally happens. One such example is the Bay of Bengal. It was formed when the Indian subcontinent crashed into Eurasian plate.
Till today the Indian plate is subducting beneath the small Burma plate and forms the Sundra trench.
Oceans overflowing coastline- This is also one of the reasons the Bays are formed. Kowloon Bay was formed in this way when the South China sea was overflowing the Kowloon Peninsula. Today the bay is entirely reclaimed by the South China Sea.
Another bay of this category is the New York Bay. It is actually two bays that are joined by a strait which is called the Narrows.
Coastline erosion into the ocean- Bays are also formed when the coastline is eroded into the ocean. Ocean waters crash into it eroding layers after layers many years resulting in formation of a Bay. Example of such a type of bay is Guanabara Bay which was formed by the Atlantic Ocean eroding an inlet in South America. It is also called the harbour of Rio de Janeiro and is one of the seven Natural Wonders of the World.
Ria- Rias are bays formed when an estuary is taken over by the ocean. These are also known as drowned rivers. One of the largest Rias is the Chesapeake Bay, on the East Coast of the United States. It is present at the dried mouth of Susquehanna river.
Fjords- These are narrow bays formed by glaciers. A glacier slices through the bedrock with a long steep canyon. There it recedes and the sea seeps into the inlet resulting in a fjord.
Bays are also formed at the shores of the lakes. The Bay of Georgia is a prominent bay in Lake Huron.
Bays can have any category of ecosystem depending on the water body that is associated with them. If it is a river it would be a freshwater ecosystem and in case it's an ocean it would be a marine ecosystem.