Final answer:
Liverpool lies on the Mersey Estuary, which is a biologically productive area where the river meets the sea, creating a habitat with mixed salinity levels.
Step-by-step explanation:
The estuary on which Liverpool lies is the Mersey Estuary. Estuaries are where fresh water from rivers meets the sea, creating areas with brackish water - which is a mix of fresh and salt water. The Mersey Estuary is a prominent geographical feature of Liverpool, providing the city with its historical significance as a major port.
The biodiversity and salinity levels within an estuary can vary, yet they typically serve as productive ecosystems and crucial habitats for many species, including crustaceans, mollusks, and fish. These regions are not only biologically productive but also economically important for cities built around them, such as Liverpool.