the Lake Erie were suffocating under a massive green burden of algae that exceeded by a factor of three any previously observed algal blooms in the lake. The excessive algal growth came back three years later, causing problems to 500,000 residents of Toledo whose tap water was rendered hazardous to their health, when a toxic compound from the algae made its way into the city water supply.
The toxin, that damages the liver, was produced by the high concentration of cyanobacteria called Microcystis that originated from the lake 1.
In addition to having negative impacts on human health, these massive algal blooms also cost significant amounts of money. The city of Toledo spent thousands of dollars on a special water treatment system during those critical summer weeks. And the estimated cost for making eutrophic water safe for drinking across the whole of the U.S. reaches $2.2 billion each year2.
Lake Erie is just one example of the harmful effects of algal blooms that occur because of eutrophication. Such a sudden degradation of water quality has drawn the attention of hydrologists over the past 40 years, who have tried to determine which factors causing the problem. Despite the knowledge we have gained throughout the years, more cases of eutrophication are documented every year.
What is cultural eutrophication?
Eutrophication is a naturally occurring process which depletes the oxygen levels in a body of water. Natural eutrophication is a gradual process that takes place over a long period of time – even centuries. It is related to an excess of nutrients such as phosphates gathering in a lake that encourage the growth of large amounts of plants and algae.
When these algae die and decay, high levels of oxygen are used in the process. This often leads to total oxygen depletion, which harms the aquatic ecosystem – especially fish and other aquatic organisms3. Although, eutrophication happens naturally in water bodies, changes in land use and pollution due to human activity contribute to much faster rate of eutrophication, also known as “cultural eutrophication”.
Cultural eutrophication currently affects millions of lakes and waterways throughout the world4. Usually this occurs as a result of the introduction of a range of nutrients through fertilizers, chemicals, or soaps and detergents5. A continuous increase in nutrient concentration in a lake decreases the ability of self-purification of the ecosystem and can even lead to the premature death of a body of water.
The nutrients that are mainly responsible for such changes are nitrogen and phosphorus, both commonly used in agriculture as fertilizers.
Causes and effects of cultural eutrophication
Eutrophication has become a very important problem particularly in heavily populated parts of North America and Europe and it has a range of devastating effects. The most striking effect are algal blooms. They are easily detectable with a naked eye as they turn water green and limit the sunlight that enters the water.
Limited sunlight penetration disables aquatic plants from photosynthesizing, which leads to their death. Fewer photosynthesizing plants also means less oxygen available for other organisms, and the lack of oxygen in the water results in mass fish deaths, which further pollutes the water and harms the environment6. It also changes the ecosystem balance because even if the reduced oxygen levels in the water don’t harm them, bottom dwelling species can be affected by the reduced amount of light getting to them3.
If eutrophication gets to the critical point of mass dying in the water, the microbes responsible for breaking down the dead bodies need oxygen to do their work, thus contributing further to oxygen depletion. The situation can get to a point when there is not enough oxygen available to support life anymore. These low-oxygen areas are then called hypoxic, resulting in the creation of dead zones.
- One of the best large-scale examples of a dead zone is the Gulf of Mexico. It has become a huge hypoxic zone almost devoid of marine life due to fertilizer and chemical runoff. This area reached a record size of 21,756 square kilometers in 2002. It has had huge and wide reaching effects on the local and international economy. On particularly bad years, important commercial and recreational fisheries are threatened by the hypoxic zone, and this has become a subject of debate among conservationists throughout the world 6.
- Similarly, the Baltic Sea contains one of the largest dead zones on the planet, averaging around 49,000 square kilometers for the last 40 years. Again, it is caused by algal blooms fueled by excess nutrients from human activities 7.
Eutrophication can also make environmental conditions more favorable to invasive species due to the change in the nutrient balance of the water body. A good example is the Common Carp, which is adapted to live in naturally eutrophic conditions. When the oxygen levels of a water body decrease, the carp can still function normally, even though native species suffer and decline8.
As the case of Lake Erie illustrated, eutrophication negatively affects the health of both people and animals. Some algal blooms produce dangerous toxins, such as anatoxin-a, a deadly compound quickly affecting the nervous system. Anatoxin was documented to be the cause of death of thousands of geese and ducks in the Midwest of the United States. Blooms of green-blue algae, cyanobacteria, have been linked to poisoning of livestock and wildlife all the way to the end of 19th century, and unfortunately they still pose significant risk for living organisms9.
Furthermore, cyanobacteria alter the taste of water, making water treatment for drinking more expensive. In addition to disagreeable odor and taste, abundance of undesirable organic compounds further prevents water purification processes, and contributes to the corrosion of pipes. Nitrogen released from agriculture or sewage also threatens the health of coastal coral reefs because it reduces calcification, propagates growth of algae over corals and creates the perfect conditions for coral diseases10.
What can be done to prevent eutrophication?
Regulations to reduce sources of nitrogen and phosphorus pollution have been introduced in countries suffering from eutrophication. However, climate change and predicted population growth will make control over these sources even more difficult in the future.
Some of the measures that can be applied at the moment include:
- Reduction of phosphorus in detergents.
- Adoption of sustainable agricultural practices.
- Elimination of the use of chemicals in daily life.
- Use of water purifying plants that remove nitrogen and phosphorus from the wastewater.
- Biomanipulation in affected lakes by altering a food web in favor of cyanobacteria eating crustacean – Daphnia. These tiny crustaceans help prevent excessive growth of cyanobacteria by feeding on them.
Despite increasing awareness among people and the continuous efforts of countries to improve freshwater quality, cultural eutrophication becomes serious cause of water pollution with grave impacts on the aquatic ecosystem. The prevention of eutrophication requires the cooperation of different parts of our society including experts and scientists, farmers, environmental organizations, politicians and even the public.
Ensuring our freshwater sources are clean is in the interests of all of us. Without water, life on the blue planet is not possible.