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Introduction to freshwater pollution

Freshwater - an introduction in its ecosystem

Types of freshwater pollution

Toxicity response

Acids & alkalis







Organic pollution



Plants and animals can’t live without freshwater, because all organisms are made up mostly by water. A tree for example is about 60% water by weight and most animals are about 50-65% water. Also each of us needs huge amounts of water.

Where can freshwater be found?
Only a tiny fraction of the planet’s abundant water is available to us as freshwater. About 97.4% by volume is found in the oceans and is too salty for drinking, irrigation, or industry (except as a coolant).
Most of the remaining 2.6% water is freshwater and locked up in ice layers or glaciers or it’s too deep underground to be reached or too salty to be used.
Thus, only about 0.014% of the earth’s total volume of water is easily available to us as soil moisture, usable groundwater, water vapor, and lakes and streams.

Freshwater life zones
Freshwater life zones occur where water with a dissolved salt concentration of less than 1% by volume accumulates on or flows through the surfaces of terrestrial biomes. Examples are lentic bodies of freshwater like lakes, ponds, and inland wetlands, and lotic systems like streams and rivers. The major components of a freshwater ecosystem are producers (plants with roots and phytoplankton), consumers (zooplankton, fish, and turtles), and decomposers (bacteria and fungi). Their interaction with abiotic components (penetration of light, water currents, dissolved nutrient concentrations, and suspended solids) forms an aquatic ecosystem. The producers supply O2 to the aquatic systems through photosynthesis. This O2 is then used by the producers, consumers and decomposers through aerobic respiration. The CO2 enters an aquatic system from the atmosphere and through aerobic respiration by producers, consumers, and decomposers and it’s removed by photosynthesizing producers. The concentrations of dissolved O2 and CO2 in water vary greatly with depth because of differences in the photosynthesis and aerobic respiration rates.

Freshwater ecosystems and pollution
The concentration of dissolved O2 highly depends also on the amount of pollutants, because most water pollutants cause low oxygen levels in freshwater. These pollutants make it difficult for species to live, and many aquatic organisms, especially fish, die when dissolved oxygen levels fall below 5 ppm.
There are a few natural sources of pollutants present in aquatic ecosystems. But mostly, freshwater ecosystems may become unbalanced by factors due to human activities. Human activities affect the bioavailability of chemicals to organisms, cause temperature fluctuations, and modify rainfall, pH and salinity.
Water plays a key role in diluting pollutants and because of that superiority as a solvent, it also means that water-soluble wastes pollute water easily. For instance, runoff from nearby land provides freshwater life zones with an almost constant input of organic material, inorganic nutrients, and other pollutants. Some 1500 substances have been listed as pollutants in freshwater ecosystems.

you can find a generalized list of pollutants that occurs in freshwater, and here you can find more information about toxicity responses and how toxicity is measured.

Freshwater species
The types of species that could become affected by water pollution in freshwater ecosystems are:

Insects are usually the most sensitive group, followed by crustaceans, fish and amphibians. Any changes could have harmful or disastrous effects for them. Adverse effects, such as the presence of toxic substances in industrial effluents, may affect many components of the aquatic ecosystem. The extent of which will depend on both biotic and abiotic site-specific characteristics.

For more questions about water look at our water questions overview: over one hundred questions on water!

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