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Life in Freshwater

Pollution in Lentic Waters

Types of pollution

Iron
Eutrophication
Red Tides
Acid Rain

See also River Pollution e.g for organic pollution

Iron as a pollutant

This is also called ochre pollution due to the colour it turns the water. Iron compounds exist in rocks and soil in many forms. If these are prevented from coming in contact with the air by being below the surface of the water table then no pollution occurs. However, if the water table drops, e.g. over consumption of water, or the materials are brought to the surface, e.g. by the movement of water or mining operations, the iron compounds become oxidised and begin to taint the water a rusty colour. Sometimes this appears as a floculant material especially once the oxidised material enters bigger areas of water and it combines with bacteria (one of the few organisms able to cope). It can also enter rivers. The oxides collect on the bottom but also all over the plants preventing photosyntheis. Coating animals means that gills will get clogged and they will soon die. Those animals feeding by scraping material of surfaces will ingest the iron and die.

The end result is a major depletion in the variety of species and few groups of animals will survive. An exception can be the Alder Fly and a few non-biting midge larvae.

Ochre Pollution can be both naturally occuring and created by human activity. Either way the results can be devastating.

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Eutrophication

Basic nutrients are necessary for the growth of plants. If certain nutrients become too prolific some plant species develop especially well and can soon dominate a water body. Phosphate and nitrate are two such nutrients and these have arisen mainly from run-off from agricultural land where chemical fertilisers have been added. Other sources are possible but these have been the main culprits leading to eutrophication , the increase in nutrient levels of water.

Eutrophic lake with blanket weed covering the surface
Eutrophic lake with blanket weed covering the surface

Phosphate has been the biggest problem. Eutrophication will be accompanied by the sudden rise in plants such as blanket weed (filamentous algae) that may completely choke a pond or small lake. The surface of the water may be blanked off so that animals cannot get access to the surface to gain oxygen. Large fish cannot move and worst still is when the lower algal mats die and start to decay. The increase in dead organic matter will demand oxygen for the bacteria to decompose the material and deoxygenation of the water will ensue. Fish and larger invertebrates will die.

Phosphate is the main cause of eutrophication as it accumulates with time whilst nitrate, a serious contender for major eutrophication, tends to denitrify with time and so does not accumulate to the same extent. In some small African and Asian communities living by the side of lakes mild eutrophication has become a benefit. Local sewage enters the lake increasing phytoplankton and algal growth. Certain invertebrates increase in numbers that help to feed fish that are tolerant of mildly deoxygenated conditions. This all depends on local conditions (people and the environment) but the benefit is the potential increase in fish, an important source of protein for the villagers. On the other hand, Lake Victoria in Africa , is a case of a very large body of water (69,000 km 2, one of the largest in the world). It was not thought that the tropics would be such a problem for eutrophication as agricultural run-off was not applicable. However, during the 1960's studies showed a steady increase in the amount of phytoplankton. By 1990's this was ten times what it had been in the late 1950's. The reasons are complex but essentially is due to an increase in sediment entering the lake after forests were cleared around the edge. This increased phosphate levels in the benthic region. This is not the only change. Alien fish species, like the Nile Perch, were added to the lake and along with climate change the general food web of the lake seems to have changed.

Eutrophication is also a natural process. For example as lakes fill in with sediment the phosphorous content rises. Also the lake will become shallower and as the hypolimnion gets smaller so deoxygenation is a greater chance in summer. Shallow lakes tend to be more fertile, as can be seen by the amount of vegetation growing around the fringe and even across the middle. However, the real test of high productivity is in the catchment. If water flows across fertile and eroding soils then we would expect there to be a greater eutrophication. Poor quality soil and rock such as granite would not be so productive.

Eutrophication is can be dealt with by a) treating the symptoms and b) removing the causes. In the first of these the algae and plants can be removed to restore drinking water but the problems will reoccur. Preventing the nutrients from entering the water is not as easy as it may seem. Phosphorous can be precipitated and controlled more easily than nitrate but in all cases it will depend on the sources of these.

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Red Tides

We have seen above that the high loading of nutrients into freshwater bodies causes eutrophication. Invariably this is an increase (or bloom) of algae. In some cases this may be blue-green algae, some of which are toxic. Over half of the blue-green algal blooms the potency can be so high that a cow drinking the water at the edge could die quickly from nerve and liver failure. For reservoirs this is not a good thing and can be expensive to remove. Phytoplankton may be removed but can leave a bad taste in the water. Copper sulphate has been used to try and minimise algal growth, but is both expensive and undesirable.

The "red tide" is a phrase that has been applied to a phenomenon when a planktonic organism (often a dinoflagellate) blooms in huge densities such that the water goes red. In most cases the organism is highly poisonous and soon can obliterate the life in the lake. The poison is a liver toxin but it is unclear why teh dinoflagellate produces this.

Red Tide in a brackish lake
Red tide in a brackish lake

Red tides will occur in lakes but are not as common as the blue-green algal bloom. They are associated more with brackish water and so will not influence reservoirs in quite the same way. However, both types are becoming more frequent and is a worrying trend. Even if the water is not drunk the toxins can get into the food web; consumed by shellfish or other species eaten by humans.

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Acid Rain

Acid Lake in Norway
Acid Lake in Norway

Whilst eutrophication increases productivity acid rain decreases it.

Before the latter part of the twentieth century the lakes in Scandinavia were rich in aquatic life including vegetation and a diversity of animals. The invertebrates provided food for trout and salmon but by the 1980's and 1990's this started to become a rarity. In the recent decades sulphur dioxide gases produced by industries burning fossil fuels has been released into the atmosphere. Primarily this has come from countries like the UK. This acidic gas dissolves in the clouds to fall, many miles from where it was produced, as acid rain. This greatly increases the acidity of the lake killing many of the species living there. This is coupled with the release of aluminium from the soil as the acid rain percolates through and drains to the lakes.

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