4.which of the following statements isn't true about the open ocean
a. the open ocean has low level nutrients
b. organisms in the deep ocean are exposedbto frigid temperatures and total darkness.
c. the open ocean begins at the low-tide mark and extends to the end of the continental shelf
d. most of the photosynthesis activity on earth occurs in the open ocean within the photic zone.

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.