Why is the mixing of fresh water and salt water in the bay important?

When the salt water is mixed with fresh water, it is called brakish water which have the salinity 0.5 to 35 ppt.

Explanation:

The mixing of fresh and salt water is important for aquatic organisms.

The catadromous fish born in marine habitat but they spend their rest of life in the fresh water where growing and maturation takes place.

Ex - Eel is the common example of catadromous fish.

Like wise,

Anadromous fish born in fresh water and spends their life in the sea and return to fresh water for spawning.

For example Salmon, smelt, shad, are common examples.

Thus mixing of water is important for breeding and growing of these fishes.

More over, the brakish water contain world's most diversified life forms.