distances in fairly still waters. Because they cannot swim against currents, zooplankton are more important in lakes than in running water, as running water usually carries them downstream faster than they can reproduce. However, they can be abundant in large slow flowing rivers. Zooplankton are heterotrophic and are significant sources of energy and nutrients to carnivorous invertebrates and some vertebrates.
Heterotrophic organisms utilize the organic substances produced by autotrophic organisms as energy sources and as the raw materials for the synthesis of their own biomass. Decomposers (or reducers) are a subclass of the heterotrophic organisms and consist of chiefly bacteria and fungi, which ultimately break down material of biological origin to the simple compounds originally fixed by the autotrophic organisms.
Insects are the most diverse group of animals on earth – most insects are terrestrial, while some have life stages that are aquatic (such as dragonflies and mosquitoes). Some insects are entirely aquatic (such as the aquatic beetles). While most aquatic insects live on or near the bottom of waterbodies, though some can swim into the water column. Most aquatic insects have gills and need water with dissolved oxygen, while others, such as mosquito larvae, breathe through the surface film of still waters. Stream and river insects are crucial in the processing of organic matter. Some scrape biofilm; others shred larger leaves into smaller particles, while still others filter or collect these smaller particles. This chain of processing reduces large organic matter to successively smaller and smaller particles.
Fish display every major feeding type: (i) herbivorous fish feed on periphyton or macrophytes, or may even filter phytoplankton from the water, (ii) carnivorous fish feed on mollusks, worms, insects, zooplankton, and other fish, (iii) omnivorous fish may feed on specific types of prey, or feed indiscriminately on nearly anything they can consume. Due to this diversity in modes of feeding, different fish can occupy different places in a food chain.
As with feeding behavior, some fish occupy specific habitats, while others can be found in a wide variety of lakes and rivers. The distribution of fish can be influenced by a large number of factors, including (i) oxygen concentration, (ii) temperature, (iii) the presence of macrophytes, (iv) the availability of suitable substrate for spawning, and (v) current speed in streams and rivers. Changes in fish habitat (such as reduction of flooding due to damming) can favor some types of fish, and disadvantage others.
See also: Aquasphere.
Aquatic Plants
Aquatic plants (also known as hydrophytic plants, hydrophytes) are plants that have adapted to living in aquatic environments and are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that grows in or near water and is emergent, submergent, or floating. The three types of aquatic plants are (i) submerged aquatic weeds. submerged plants are rooted in the pond bottom and grow up through the water column, (ii) emergent aquatic weeds, and (iii) free floating aquatic weeds.
Aquatic plants include kelp from the ocean, and freshwater plants such as algae, water hyacinth, and duckweed. Aquatic plants are considered to be a sub-category of biomass and the plants, like wood products, dried vegetation, and crop residues, have the potential to produce biomass fuels.
Aquatic plants have adapted to living in or on aquatic environments and, because living on or under water surface requires numerous special adaptations, aquatic plants can only grow in water or permanently saturated soil. As opposed to typical plant types, aquatic plants do not have a problem in retaining water due to the abundance of water in its environment. This means that the plant has less need to regulate transpiration which requires less energy and increases the possible benefits.
Enhancing the growth rate of these plants by increased nutrient supply, for example, from carbon dioxide in flue gases, or growing suitable plants in conjunction with municipal wastewater treatment facilities, has received some attention. While quantitative data and costs for this resource are limited, full development of the aquatic biomass energy potential is not expected to approach 1% of total US energy requirements.
Algae are tiny aquatic plants have the potential to grow extremely fast in the hot, shallow, saline water found in some lakes in the desert Southwestern United States. Forms of algae thrive on carbon dioxide, and emissions from power plants have been used to feed the plants, which are then used in biofuels.
See also: Algae, Aquasphere, Biomass.
Aquiclude, Aquitard, Aquifuge
An aquiclude is a water-bearing layer in which both the horizontal and vertical flow components are so small that they can be neglected. The groundwater flow in an aquiclude is assumed to be zero.
Impervious rock in the unsaturated zone may retain water infiltrating from the surface to produce a perched water table that is above the main water table and from which water may be extracted. However, the amounts of water that can be extracted from such a formation are limited and the water is vulnerable to contamination.
An aquitard is a water-bearing layer in which the horizontal flow component is so small with respect to the vertical flow component that it can be neglected. The groundwater flow in an aquitard is assumed to be predominantly vertical. An aquifuge is a geological formation with low permeability and porosity. This does not transmit any groundwater and does not contain groundwater in appreciable quantities.
See also: Aquifer, Geohydrology, Groundwater Aquifer.
Aquifer
An aquifer is a water-bearing subsurface zone in which the vertical flow component is so small with respect to the horizontal flow component that it can be neglected.
The groundwater flow in an aquifer is assumed to be predominantly horizontal. The term aquifer is synonymous with water-bearing formation. An aquifer may be porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. Economically important amounts of water may vary from less than a gallon per minute for cattle water in the desert to thousands of gallons per minute for industrial, irrigation, or municipal use.
Aquifers are important reservoirs storing large amounts of water relatively free from evaporation loss or pollution. If the annual withdrawal from an aquifer regularly exceeds the replenishment from rainfall or seepage from streams, the water stored in the aquifer will be depleted. Lowering the pressure in an aquifer by overpumping may cause the aquifer and confining layers of silt or clay to be compressed under the weight of the overburden. The resulting subsidence of the ground surface may cause structural damage to the aquifer and to surface buildings, damage to wells, and other problems.
Depending on the permeability of the layers bordering the aquifer, a distinction is made between: (i) a confined aquifer, (ii) a semi-confined aquifer, (iii) an unconfined aquifer, and (iv) a semi-unconfined aquifer.
A confined aquifer is an aquifer in which groundwater is held under pressures greater than atmospheric pressure by upper and lower confining layers, forcing water to rise in wells to heights above the top of the aquifer (artesian wells; also known as an artesian aquifer). The confined aquifer is typically sandwiched between two impermeable layers of rock or sediments and are recharged only in those areas where the aquifer intersects the land surface. The pressure of the water in confined aquifers is higher than atmospheric pressure, which is why when a well is bored into the aquifer, the water rises up the well tube, to a level higher than the aquifer.
If the water level in an artesian well stands above the land surface, the well is a flowing artesian
