How can the freezing of surface waters make the water that is left unfrozen denser

Explanation:

When an aqueous solution freezes, the unfrozen solution becomes denser. That's because some of the water has been removed as ice. However, the solutes remain dissolved in a solution with a smaller liquid volume. The concentration of the solution increases, so the density increases.

this is because water expands as it gets cold hence floating leaving the unfrozen water dense

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estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.