Water has many special chemical properties that make it essential for supporting life. These are mainly due to its molecular structure. Contemporary fresh waters are remarkably constant in their inorganic chemical content, varying only with inputs such as rain or melting ice caps and with evaporation. This constancy can be disrupted by pollution.
Physical Characteristics Of Fresh Water Habitat
The physical characteristics of fresh water include the appearance, temperature, turbidity and taste. These characteristics are important to human needs as they affect the consumption, quality and use of water. Water can be used for domestic, industrial, agricultural, recreational and environmental purposes.
The properties of fresh water are largely due to the fact that it is a polar liquid with hydrogen bonding between molecules. This gives it its unique structure of two hydrogen atoms covalently bonded to one oxygen atom and makes it a very good solvent for other substances.
As a result, water is able to disperse many different chemicals and minerals. The physical properties of water are further influenced by the fact that it is highly compressible at low temperatures and atmospheric pressures. However, its compressibility decreases as it rises in temperature.
Water also has the ability to absorb heat quickly. This is a function of its surface tension, which relates to the cohesive forces between water molecules. Water is also a good insulator and has good electrical conductivity, which reflects the distribution of ions in solution.
Most lakes, rivers and streams contain dissolved salts. The principal source of these is mineral matter that leaches into the water from underlying rock strata, or from chemical reactions with soluble substances. In addition, living organisms metabolise organic matter to produce food and energy, and this is deposited in the water as a form of colloidal matter (known as labile or refractory organic matter).
The overall chemical composition of water is relatively constant, varying only with input from hydrothermal waters, drainage from the melting ice caps and rainfall, as well as output through evaporation. This constancy is largely maintained by circulation and mixing.
Generally, fresh water is slightly alkaline, with a pH of between 7.5 and 8.5. The alkalinity is mainly the result of carbonates and bicarbonates along with free and ionised carbonic acid. This buffering system is essential to the stability of aqueous ecosystems. It is also a key factor in the productivity of many types of aquatic environments. In particular, it is critical to the survival of higher plants.
Chemical Characteristics Of Fresh Water Habitat
Water is a unique compound with the ability to dissolve many polar and ionic substances. This characteristic makes it one of the most important of all the substances that have been formed by nature and which make up the planet. All living organisms are dependent upon aqueous solutions for the processes of life.
The chemical characteristics of fresh water vary widely. This is partly because of the great volume of water on the planet and also because natural processes alter the physico-chemistry of water over time and space. Some of these changes are comparatively rapid, such as the dissolution of salts when lakes evaporate – this produces the characteristic taste of seawater in some freshwater systems. Other changes are slower, such as the accumulation of organic matter in natural waters.
Most natural waters contain a large number of dissolved chemicals, some rapidly soluble and others slowly soluble, and these are determined by the minerals and rocks with which they come into contact. Water is also a very good solvent, meaning that it can carry and diffuse many gases (such as dissolved oxygen) from the atmosphere.
Water has a very high surface tension which, in combination with its low specific gravity, limits the amount of material it can hold on its surface and this explains why it is so difficult to find solid mineral particles in flowing water. Consequently, the clarity of freshwater systems is determined principally by the suspension in water of mineral and organic materials derived from erosion of the underlying rock substrate; the production of such material by living aquatic organisms; or by the re-suspension of dead organic material.
The physico-chemistry of water is very much affected by temperature and this has both direct effects on the behaviour of chemicals and indirect effects through its impact on the environment. For example, temperature affects the rate at which a chemical is taken up by aquatic organisms through directly affecting the rate at which it diffuses into the organism.
Water pH has a lesser-known influence on the trans-epithelial electrical potential (TEP) across the skin and gills of aquatic animals which could also impact on their sensitivity to chemicals. Similarly, water alkalinity, in combination with pH, has a dominating influence on the speciation of toxic metals such as Cu, Cd and Pb in water, which will affect their bioavailability through chemical speciation (Stehly & Hayton, 1990).
Biological and Microbiological Characteristics Of Fresh Water Habitat
The microbiological characteristics of fresh water are important indicators for determining its quality. They are used as a benchmark to judge the efficacy of treatment processes in delivering drinking water of acceptable microbiological quality. They are also essential in determining the health risk associated with pathogenic microorganisms and defining a safe level of exposure to them. Indicator bacteria such as fecal coliforms and enterococci are routinely monitored in source and treated drinking waters to assess their suitability for human consumption.
Bacteria are unicellular organisms that lack a membrane-bound nucleus and other internal structures, and as such can adapt to almost all ecological habitats on Earth. They are the dominant microorganisms in terrestrial and aquatic ecosystems and occupy a wide range of biogeochemical roles. Some bacteria cause disease in humans, animals and plants, but most are beneficial ecological agents whose metabolic activities sustain higher-level life forms. They are symbionts of many plants and invertebrates, where they carry out important functions like nitrogen fixation and cellulose degradation.
In the euphotic zone of standing and running waters, bacterial communities consist of both oxygenic (autotrophic) and anaerobic (heterotrophic) autotrophs, and various lineages of bacteriorhodopsin-bearing photoheterotrophs. These prokaryotic assemblages can be classified according to their growth strategies and cell sizes. The smallest free-living microorganisms are the ultramicrobacteria, followed by opportunistically growing bacteria and filamentous bacteria that resist protistan grazing.
Oxygen is highly soluble in water and gets dissolved as dissolved oxygen (DO). Its solubility decreases with increasing temperature of the water, which is why DO is one of the most important parameters in freshwater quality assessment. It is a critical factor in the survival of aquatic aerobic organisms.
The DO is influenced by many factors such as the temperature, organic matter content, water pH, nutrient status and turbidity. In addition, it is influenced by the presence of biofilm on the surface of the water, as well as the redox state. The concentration of DO is determined by balancing the demand of the aquatic organisms for DO with the supply available in the water. This balance is achieved by a combination of biotic and abiotic mechanisms, including photosynthesis, oxidation, and diffusion.
Environmental Influence On Fresh Water
Our planet is covered with water – but not all of it is fresh. About 71 percent of our world’s water is salty, locked in glaciers and ice at the North and South poles or deep underground in an aquifer. The rest is fresh water – a relatively small amount in lakes, rivers and streams. Fresh water sustains ecological systems, supports agriculture and industry, and provides recreation and drinking water for humans. It also influences the condition of ground water and coastal systems downstream.
Fresh water starts out as water vapor that evaporates from the surface of oceans, lakes and other bodies of water. This water vapor then collects in drifting clouds that eventually release it back to Earth as rain, snow or fog.
Most of the fresh water that falls to Earth is recycled – it may pass through natural wetlands, woodlands and manmade wetlands before reaching streams, rivers or lakes. The biological communities that reside in these ecosystems act as filters, removing many chemicals and sediments. These ecosystems also supply most of the energy and organic matter to these fresh waters, through the nutrient cycles of decomposition and plant growth.
Despite their importance, freshwater ecosystems are under stress. In particular, the extent of fresh surface water varies widely, reflecting a range of stresses, including hydromodifications that restrict fish migration and the withdrawal of freshwater for drinking and other human uses. The ROE’s freshwater condition indicators focus on the state of these habitats – their physical, chemical and biological characteristics. The indicators include those that assess the health of streambed sediments, the acidity of lakes and streams, cyanobacteria in large rivers and streams, and the concentration of nitrogen and phosphorus in the nation’s largest river basins.
Many of these water quality issues can be traced to pollution, caused by agricultural chemicals, improperly managed industrial operations and inadequately treated sewage and urban waste. The pollutants wash into lakes, rivers and streams, where they can then enter the aquatic food chain and cause a number of health problems for wildlife and humans. Sometimes the effects are immediate, but other times they may be seen only after the build-up of toxic substances in the system for years.