James G. Speight - Encyclopedia of Renewable Energy

Aquaculture itself has both good and bad aspects for the environment. It takes pressure off wild fisheries, many of which have been severely overfished. In some areas, however, it contributes to pollution of the water.

Economically, using geothermal energy to heat water for aquaculture can have many benefits. Places that use water that has already been used for heating or electricity generation can heat their fish ponds essentially for no cost. They can also enjoy the economic benefit of selling the fish or prawns that they produce. Fish grown in geothermally heated water grow faster than fish in unheated water, so some fish farmers can grow extra fish crops for sale. Heated water makes it possible to grow fish in winter when it ordinarily would not be possible. Selling tropical fish for the pet store market can be quite profitable. Developing nations can export their fish produce for good prices, bringing foreign capital into the country.

See also: Aquasphere, Geothermal Energy.

The Earth is a unique planet insofar as there is an abundance of water that is necessary to sustaining life on the Earth, and helps tie together the atmosphere, the land (the geosphere), and the oceans and rivers (the aquasphere) into an integrated system. Precipitation, evaporation, freezing and melting, and condensation are all part of the hydrological cycle, which is a never-ending global process of water circulation from clouds to land, to the ocean, and back to the clouds. This cycling of water is intimately linked with energy exchanges among the atmosphere, ocean, and land that determine the climate of the Earth and cause much of natural climate variability. The impacts of climate change and variability on the quality of human life occur primarily through changes in the water cycle.

The water systems of the Earth, often referred to as the aquasphere or the hydrosphere, refer to water in various forms: oceans, lakes, streams, snowpack, glaciers, the polar ice caps, and water under the ground (groundwater). An important aspect of the water systems is an aquifer which is a water-bearing (water-rich) subsurface formation (a subsurface zone) that yields water to wells. An aquifer may be porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. Aquifers are important reservoirs storing large amounts of water which, in theory, should be relatively free from evaporation loss or pollution. However, in practice, this is not always the case.

The aquasphere consists of a variety of non-oceanic water systems ( Table A-23) that are essential to life, and all are interrelated, and the many interactions between the systems of the Earth are complex, and they are occurring constantly and simultaneously although the effects are not always obvious.

Table A-23Sources of non-oceanic water and water systems.

The oceans play a key role in the water cycle insofar as the oceans hold 97% v/v of the total water on the Earth and 78% v/v of the global precipitation occurs over the oceans, and it is the source of 86% v/v of global evaporation. Besides affecting the amount of atmospheric water vapor and hence rainfall, evaporation from the sea surface is important in the movement of heat in the climate system. Water evaporates from the surface of the ocean, mostly in warm, cloud-free subtropical seas. This continuing event cools the surface of the ocean, and the large amount of heat absorbed the ocean partially buffers the greenhouse effect from increasing carbon dioxide and other gases. Water vapor carried by the atmosphere condenses as clouds and falls as rain, mostly in the intertropical convergence zone (ITCZ), far from where it evaporated. Condensing water vapor releases latent heat which drives much of the atmospheric circulation in the tropics. This latent heat release is an important part of the heat balance of the Earth, and it couples the energy and water cycles of the Earth.

The intertropical convergence zone, known by sailors as the doldrums or the calms because of the monotonous, windless weather, is the area where the northeast and southeast trade winds converge. The zone encircles Earth near the thermal equator, although the specific position of the zone can vary on a seasonal basis. When the zone lies near to the geographic equator. it is referred to as the near-equatorial trough. When the intertropical convergence zone is drawn into and merges with a monsoonal circulation, the zone is sometimes referred to as a monsoon, a usage that is more common in Australia and parts of Asia.

The major physical components of the global water cycle include the evaporation from the ocean and land surfaces, the transport of water vapor by the atmosphere, precipitation onto the ocean and land surfaces, the net atmospheric transport of water from land areas to ocean, and the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes.

On land, the situation is more complex, and includes the deposition of rain and snow on land; water flow in runoff; infiltration of water into the soil and groundwater; storage of water in soil, lakes and streams, and groundwater; polar and glacial ice; and use of water in vegetation and human activities. Processes labeled include precipitation, condensation, evaporation, evapotranspiration (from tree into atmosphere), radiative exchange, surface runoff, ground water and stream flow, infiltration, percolation, and soil moisture. Furthermore, in the water systems (particularly in the lakes) the term eutrophication becomes important. Eutrophication is the deterioration of the esthetic and lifesupporting qualities of lakes and estuaries, caused by excessive fertilization from effluents high in phosphorus, nitrogen, and organic growth substances. Algae and aquatic plants become excessive, and when they decompose, a sequence of objectional features arises.

Water for human consumption (and, in many cases as on farms for animal consumption) from such lakes must be filtered and treated. Diversions of sewage, better utilization of manure, erosion control, improved sewage treatment, and harvesting of the surplus aquatic crops alleviate the symptoms.

There are some extremely dramatic examples of Earth systems interacting, such as volcanic eruptions and ocean tsunamis, but there are also slow, nearly undetectable changes that alter ocean chemistry, the content of the atmosphere, and the microbial biodiversity in soil. Each part of the Earth, from inner core to the top of the atmosphere, has a role in making Earth suitable for the existence of billions of lifeforms (1 billion = 1 x 10 9). Within the aquasphere, the phenomenon known as (i) precipitation, (ii) evaporation, (iii) freezing, (iv) melting, and (v) condensation are part of the hydrological cycle – also known as the water cycle – which is a continuous global process of water circulation from clouds to land, to the ocean, and back to the clouds.