A key feature of estuaries is:

low light conditions and high productivity

salt water and fresh water

frequent algal blooms

little or no vegetation

salt water and fresh water

Explanation:

The estuaries are located on the contact between a fresh water body and saline water body, or rather a place where a river enters the sea. Because of this, the estuaries have a mixture of saline water and fresh water. They are also shallow and warm. The sunlight penetrates to their bottom, and they are rich in nutrients. This results in the estuaries to offer several different living conditions, so there are lot of different species living in them.