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Biological water quality testing methods

An assessment of the number and type of living organisms in surface water can also be used to monitor organic and other forms of pollution such as heat or chemical pollution. These assessments are referred to as biological indicators.

Different organisms have different tolerances to low oxygen levels or pollutants. Using the presence or absence of particular organisms, visible with naked eye, pollution levels can be assessed by specialists. An ecologist moves across a river pushing a net over the riverbed. They can then identify and count the organisms caught in the net. The more sensitive the organisms present, the better the quality of water in the river.


Environment Agency officer sampling a river

Inorganic and organic toxic chemical pollutants include substances like metals and acids, and mamufactured organics such as:

  • Pesticides
  • Polychlorinated biphenyls (pcbs)
  • Polycyclic aromatic hydrocarbons (pahs)
  • Fluorochemicals
  • Phenols

Many of these are highly toxic and can cause damage or complete destruction of aquatic ecosystems. They can also have serious impact on people too either through direct exposure to them in the environment or indirectly for example by eating contaminated food.

The specific toxic nature and impact of these chemicals varies and is influenced by a variety of properties exhibited by the chemical, for example its persistence in the environment as well as environmental conditions. The table below describes these properties in more detail.

Definition of key terms

Environmental term Description of meaning
Persistence Persistence of chemicals indicates that they are stable and long-lived in the environment, resisting degradation, for example lead, cadmium, mercury, PCBs, many fluorochemicals and other man- made organics
Xenobiotic May harm biological organisms they include many manufactured substances, especially pesticides, lead, cadmium and mercury
Biodegradation Breakdown of a complex chemical into (simpler) components by actions of biological organisms. It’s not always broken down into more benign components, for example the pesticide Dieldrin biodegrades into photodieldrin, which is considerably more toxic
Bioconcentration (Biodegrading) Extraction of chemicals from the environment, and concentration within the organism. For example, seaweed concentrates iodine from the seawater within its tissues, so it’s very useful for humans as a source of concentrated iodine. Similarly, plutonium is present at very low levels in seawater. It’s concentrated within tiny algae, (phytoplankton) that make up the producers in the open sea ecosystem (up to 3,000 times stronger in one of these algae than in the sea water). Algae also concentrate PCBs to 2,000 times the ambient sea water levels
Bioaccumulation or biomagnification Concentration of pollutant; gradient that occurs in moving from one trophic level to another, such as when an animal eats a plant or another animal

Perhaps the best-known example of a pollution event that affected humans was in the 1950s in Minemata Bay, Japan. Mercury (mercuric sulphate) was discharged untreated into Minemata Bay over many years where it accumulated in the sediment offshore. Here it was naturally converted into methyl mercury, a more soluble toxic form readily taken up by plants and animals. The level of methyl mercury found in these plants and animals increased the higher up the food chain the species was located due to bioaccumulation. When the local population ate species of carnivorous fish at the top of this food chain they were ingesting 500,000 times the normal seawater concentration of mercury. This led to a number of serious health problems and birth defects.

Other Common Pollutant Types

There are wide range of other pollutant types. These together with their likely impacts on ecosystems are described in the table.

Types of Pollution and their effects

Type of pollution Effects
Suspended inorganic solids such as silt pumped into a river
  • Substrate changed due to the riverbed being covered with silt
  • Fish gills and filter feeders become blocked
  • Fish spawning areas and eggs smothered
  • Light penetration is reduced, reducing photosynthesis and plant growth
  • Changes in the community of organisms present
  • Loss of diversity
Thermal Pollution for example. from fire runoff water
  • Water body is heated
  • Oxygen content of the water is reduced
  • Self-purification processes are accelerated if oxygen levels don’t fall too low. Some species, such as trout, salmon and pike are particularly sensitive to elevated water temperatures
  • Changes in the community of organisms present
  • A loss of diversity occurs
Inorganic chemicals
  • May be toxic
  • May change acidity or alkalinity (pH) of the water
  • Change in the community of organisms present
  • A loss of diversity occurs
Oils and fuels
  • Physical coating of animals and plants, particularly water foul
  • May be toxic
  • Reduction in oxygen levels
  • Changes in the community of organisms present
  • Loss of diversity
Organic matter such as a milk spillage
  • Reduction of oxygen levels
  • Changes in the community of organisms present
  • Loss of diversity
Nitrogen and phosphates such as a fertiliser spillage
  • Eutrophication, producing algal blooms, which can lead to deoxygenation of water bodies and changes in the community of organisms present
  • Loss of diversity
  • Many fertilisers are also acutely toxic, particularly those based on ammonia compounds


Type of pollution Effects
Toxic and persistent organic chemicals
  • Poisonous
  • Changes in the community of organisms present
  • A loss of diversity occurs
  • Bioaccumulation and persistence possible
  • Spread disease

Environmental conditions

Environmental conditions in any particular geographical area of the UK can directly influence the toxicity and fate of pollutants in a body of water. Environmental specialists within the FRS should consider these when pollutants are released into the water environment.

Hardness: in hard water, high concentrations of dissolved calcium and magnesium reduce the toxicity of metals such as cadmium, lead or copper. Such waters will also be better able to cope with an acid spill due to better buffering capacity. However, the toxicity of other substances such as ammonia is increased.

Acidity: the solubility of many metals is increased as water becomes more acidic. This can lead to negative impacts on aquatic ecosystems.

Temperature: high water temperatures naturally reduce dissolved oxygen levels. These conditions also encourage greater microbial growth, so the effect of an organic spill during the summer period may be more severe, but self-purification is accelerated.

Mixtures: pollutants can change their toxicity in the presence of other toxins. They may produce three possible outcomes:

  • Additive toxicity
  • Increased toxicity
  • Decreased toxicity

For example, the presence of chromium can increase the toxicity of nickel ten-fold whereas the presence of strontium can decrease it three-fold.