Estuaries are the delicate water bodies where the fresh water from the river and salty water from the sea water meet and mix-up. They are responsible for transporting and mixing the nutrients. The difference in sedimentation of the water of rivers and sea-water, the wind action and the tidal activity allow the trapping and transport of the nutrients. Reefs, deltas and barrier islands protect the estuaries from the ocean forces.

Over 68 percent of the fresh water on the Earth is found in icecaps and glaciers, and just over 30 percent is found in ground water. Only about 0.3 percent of our fresh water is found in the surface water of lakes, rivers and swamps.

Water is basic necessity for all terrestrial forms of life.

The percentage of water in human body is $70\%$. 

A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around the world.

Chilka Lake is a brackish water lagoon, which is the largest brackish water lake in India. It is spread over Puri, Khurda and Ganjam districts of Odisha state on the east coast of India, at the mouth of the Daya River, flowing into the Bay of Bengal, covering an area of over 1,100 square kilometre. Brackish water or briny water that has more salinity than fresh water, but not as much as sea water. It may result from mixing of sea water with fresh water, as in estuaries, or it may occur in brackish fossil aquifers.

Water covers 71% of the Earth's surface. It is vital for all known forms of life. On Earth, 96.5% of the planet's water is found in seas and oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products. 

The process of losing water by plants is called transpiration. It is the process where plants absorb water through the roots and then give off water vapour through pores in the leaves.

About 97.5% of the water available on earth is found in seas and oceans as saline water and is not available to us for use directly. Only 2.5% of the total water resources of the world consist of fresh water. Majority of it is frozen in the ice caps at the two poles and on snow-covered mountains. Only limited amount of fresh water is available to us as a renewable resource through water cycle but its distribution is uneven. So, the correct answer is '2.5'.

Water is a renewable resource which can be replenished due to the activity of rainfall. The resource can be degraded due to the addition of the pollutants from the sources like industries, domestic, agricultural, etc. The wastes which are added to the water bodies can be cleaned using the help of microbes in the treatment plants. So, water resource is degradable but the purity of the water can be maintained. 

Groundwater is formed by rainwater that accumulates below the soil levels. This forms the water table below the soil. Around 76% of the freshwater source is groundwater. Apart from this, it is also present in ponds, lakes glaciers etc.

Earth is made up of 70% water and the remaining is land. This water is present in the oceans, frozen icecaps and glaciers, groundwater water and lakes, rivers and ponds. Oceans and seas hold 97.5% of all the available water on earth. Thus the correct answer is option is A.

The oceans hold 97 percent of the Earth's water, the remaining three percent is the fresh water found in glaciers and ice below the ground or in rivers and lakes. Of the three percent that is not in oceans, 69 percent is locked up in glaciers and ice caps. The 69 percent of 3 percent is roughly 2 percent of the total fresh water available on the Earth's surface. This much fresh water is locked up in glaciers as ice and snow. About 90 percent of frozen water is in Antarctica and Green land.

Salt water or Marine wetlands are the coastal wetlands, many of which are being destroyed for obtaining space for agricultural lands and coastal developments. Important saltwater or marine wetlands are estuaries and mangrove swamps. Estuaries are one of the most highly productive ecosystems, which provide food and habitat to several marine and other organisms.

The Oceans hold about 97% of water, and the Glacier’s and polar ice caps hold another 2%. The polar ice caps contain 77% of the world's fresh water supply and another 22% is trapped in the ground which leaves only 3% of the world’s fresh water supply in lakes and rivers.

• In the atmosphere, water exists as a gas (water vapor from evaporation), as a liquid (droplets of rain and liquid water that coats solid particles), and as a solid (snow and ice).
• The water in the atmosphere makes up only a very small percentage of the total water on Earth i.e 0.001%.
• Hence Out of the total at any time, water present in the atmosphere is 0.001%.
• So, the correct answer is '0.001%'.

Percentage of fresh water in the hydrosphere is 2.5%. Out of this, about 68% of the fresh water is found in icecaps and glaciers and about 30% is found in ground water. About 98.5% of water in the hydrosphere is salt water.

Over 68 percent of freshwater is found in polar ice & glaciers, while 30 percent is found in ground water, 0.3 percent is found in surface water of lakes rivers & ponds

Sea water has a salinity of 3.5% i.e. the sea water contain about 35 grams of dissolved salt per kg of water ( about 1 litre of water ). This salinity is because of the rivers that carry chemical/salts dissolved out of rocks and soil. NaCl makes the most of this dissolved salt. 

Percentage of fresh water in the hydrosphere is 2.5%. Out of this, about 68% of the fresh water is found in icecaps and glaciers and about 30% is found in ground water. About 98.5% of water in the hydrosphere is salt water.

The oceans hold about 97% of water and the glaciers and polar ice caps hold another 2% of the total water on Earth. The polar ice caps contain 77% of the world's fresh water supply and another 22% is trapped in the ground, which leaves only 3% of the world’s fresh water supply in lakes and rivers. 

Estuaries are unique plant and animal communities that have adapted to brackish water mixture of fresh water draining from the land and salty seawater. However, there are also several types of entirely freshwater ecosystems that have many similar characteristics to the traditional brackish estuaries. Estuaries may be considered as ecotones between the fresh and marine habitats.

Around 71% of earths surface is covered with water, oceans and seas contain maximum amount which is around 96.5%

About 71% of earths surface is covered by water & about 96% of water is present in oceans

• 70% of the Earth's surface is water. Of this 70%, 98% is salt water, leaving 2% as fresh water. Of the 2% that is fresh, about 90% is frozen(i.e 1.97%of the total water). This frozen water is locked up in the Antarctic ice sheets and glaciers on the Alps, etc.
• Hence Water occurs in the frozen state over the land is 1.97%.
• So, the correct answer is '1.97%'.

The amount of potable water is decreasing because of over usage. Water scarcity is the lack of sufficient available water resources to meet the demands of water usage within a region.

The amount of water on earth today compared with the amount of water on earth at the time it was formed about $5$ billion years ago is about $50$ per cent of what it was originally.

All of the following factors are leading to shortage of water.