Eutrophication, the hyper-enrichment of nutrients such as phosphorus and nitrogen in a waterway, is the main cause of algal blooms. Nutrient enrichment causes hyper-productivity and promotes algae growth, resulting in massive blooms that can ultimately consume a water body; devastating the ecology of fresh water ponds and streams. The algae blocks precious sunlight needed for photosynthesis and growth of other plants, it chokes them of nutrients. While the algae grow very quickly, each algae is short lived, causing rapid increase and high concentration of decaying organic matter. When the algae decays it consumes a high percentage of the dissolved-oxygen in the water; the outcome of which is hypoxic conditions, lowering the oxygen levels to below 30% and well under the healthy concentration for aquatic plant and animal life (above 80%). With insufficient dissolved oxygen in the water, animals and plants may die off in large numbers. In bad cases some varieties of blue-green algae such as Anabaena, Aphanizomenon, and Microcystis, which are actually bacteria (cyanobacteria), can produce deadly toxins called cyanotoxins which cause digestion problems, allergic reactions, dysfunctions of the liver, they have toxic impact on nerve cells and can initiate carcinogenic reactions. There have been several cases involving both animals and humans that have resulted in death from exposure to this toxin.
But not all alga's are bad; some varieties of algae can be very beneficial to ponds
Detergents:
In our environmentally conscious society, many people are turning to grey-water for watering their gardens and other outdoor activities. Most grey-waters, being from washing machines or bathtubs, contain detergents. Though these detergents can be beneficial to some ground plants, adding a variety of naturally-occurring ingredients for plant growth, they can also be devastating to the ecology of ponds and streams. The detergents contain high phosphate content (up to 40%) which cause major problems for pond ecosystems. Increasing the nutrient supply in a process called eutrophication. This nutrient enrichment causes hyper-productivity and promotes algal growth and blooms that can consume a pond. The algae can block out light needed for the growth of other plants and release toxins into the water. The algae also rob oxygen of other plants, consuming it as it decomposes. Some blue-green alga's produce toxins that are harmful not only to the aquatic animals living in the ponds, but also the land animals that drink the polluted water. To reduce the chances of phosphate pollution, countries around the world have banned the use of phosphates in detergents and have built thousands of treatment plants to remove phosphorous from waste water.
Organic Matter:
Organic matter such as decaying flora and fauna as well as facial material and sewerage that has washed into water bodies can cause rapid increase in both aerobic and anaerobic decomposer bacteria which increases nutrient availability in the water and quicken the decomposition of organic matter. This increase in nutrients (eutrophication) causes accelerated growth of planktonic green and blue-green algae, again, resulting in algal blooms. From the effects of the algal bloom, light cannot penetrate into deeper layers of the water column and reach the submerged flora which then begins to die, adding to the decaying organic matter in the water…>
Acids & Industrial Waste:
Industrial and domestic uses of fossil fuels (by combustion) release acidic substances and aerosols into the atmosphere, which eventually settle and fall to the ground along with various other acid gases. These gases can end up in water ways, being deposited directly or washed into them by run-off rainwater from polluted soils. The presence of these acids can cause the water to become acidic; lowering its pH below 6.0(critical pH for most aquatic species). Acidified water can cause a decline in population of decomposer bacteria and fungi; this retards the rate of decomposition of organic matter causing a nutrient deficiency in the ecosystem. This low pH and lack of nutrition results in a reduction of abundant aquatic plant and animal species. While the number of acid sensitive species decreases, there is an increase in acid tolerant species, one of which is algae. A large variety of algae can tolerate harsh acidic conditions and become very abundant in highly acidic freshwater; such as the Cladophora genus of green algae. This can result in large algae blooms. With nothing else competing with it, the algae can overtake total bodies of water, devastating ecosystems.
Waste Heat Deposition:
It is not uncommon for industries, such as power plants, to use water from rivers and lakes to cool their boilers and equipment. The heated water then is returned to the water body from which it is sourced. This 'waste heat' can cause major problems for plant life within this body. At higher temperatures the solubility of oxygen is greatly decreased, thus the availability of oxygenised water needed by the aquatic flora for respiration is significantly reduced. Plants show an increase in respiration as well as a reduced rate of photosynthesis and a inhibition of enzyme activity. The lack of oxygenised water also causes a reduction in the activities of aerobic decomposers which results in the decrease of organic matter decomposition and as a result there is reduced nutrient availability in the water. The increase in temperature causes a decline in species diversity. The ecosystems primary productivity declines as the productive aquatic plants are replaced by less productive more heat tolerant species which then become gradually dominant; green algae is replaced by less-productive blue algae.
