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13.7: Eutrophication

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  • Eutrophication is a process in which there’s an excess nutrient input to water. This process increases the rate of supply of organic matter in an ecosystem and stimulate aquatic plant growth1. At normal levels, these nutrients feed growth of organisms called cyanobateria or algae. But with too much nutrients, cyanobacteria grow out of control. Excess algae blocks the sunlight needed by bottom-dwelling plants and leads to decrease in oxygen in the water and consequently leads to negative outcomes.

    Eutrophication is often linked to anthropogenic activities. Runoff from agricultural operations includes fertilizers such as nitrogen and phosphorous. Once these nutrients enter the ocean, the balance of ecosystems changes and production of plants increases. One such plant is blue-green algae, or cyanobacteria. Cyanobacteria consume these nutrients and increase in productivity.

    Some of the negative effects of this excessive algae production, termed algal blooms, are:

    • production of extremely dangerous toxins that can kill animals and people
    • creation of "dead zones" (low oxygen or hypoxic zones) in the ocean
    • raise treatment costs for cleaning water
    • harm industries and communities that rely on the affected watershed

    Categories of Pollution

    Point source: Factories and sewage treatment plants are the most common types of point sources. Some factories discharge their wastes directly into a water body and others treat it themselves before it is released. However, unregulated discharges can cause severe damage to human health and the environment. An example of consequences of unregulated discharge are water pollution, unsafe drinking water and restrict activities like fishing and swimming.1 Intoxication or health problems are due to exposure or drinking water with algal toxins. Symptoms can include nausea, vomiting and throat irritation when exposed to algal toxins. When water is consumed in sufficient quantities, the toxins can affect the liver and nervous system.2 This can also indirectly affect the economy because of loss of working days due to such health problem.

    Non-point source: agricultural runoff

    Agriculture runoff is the largest cause of eutrophication in the Delta. More than 200 million pounds of pesticide are applied to California farms every year which are washed into the delta.


    Eutrophication can lead to hypoxia in the water column. Hypoxia event occurs when there is low oxygen level in the water. This incident is a consequence of eutrophication due to an excess of nutrient input (nitrogen and phosphorus) in the water that stimulates the growth of phytoplankton and consequently affect fishes and other organisms. Human activities have increased the rate of eutrophication through point source and non-point discharge of nutrients such as nitrogen and phosphorus.

    As plant and animal biomass increase, species diversity decreases and the affected area will become overpopulated by phytoplankton feeding off the increased algae. This will also change the dominant biota in the region.

    Turbidity is the clouding of water due to sediment. It can be caused by excessive phytoplankton, algae growth, urban runoff, or sediments from erosion. These suspended particles, in addition to making water look dirty, also help promote the toxins in water as heavy metals and toxic organic compounds can attach easily to the suspended sediment. These suspended particles also absorb heat from the sun, making turbid waters warmer. This also reduces oxygen content in the water, as more oxygen is dissolved in colder waters. The suspended particles also scatter light, decreasing photosynthetic activity of plants and algae, which results in a positive feedback loop for decreasing oxygen even more. Some biological impacts include: fish eggs and larvae will be covered and suffocated, and gills will become clogged and damaged. Thus, turbidity is a plausible and extremely harmful effect of eutrophication.

    DEAD ZONE locations

    The Black Sea is one of the dead zones that have been identified. The dead zone resulted from the contaminants from the Danube River which courses from Germany. During 1960’s to 1989, huge input to watersheds from several sources occurred. The nutrient sources are rising fossil fuel use and NOx input from atmospheric sources, intensive fertilizer use in farming practices, sewage input to water systems. This resulted to loss of fisheries and marine habits disrupted and reduced tourism.




    The Gulf of Mexico is essentially a large drain for the network of rivers known as the Mississippi-Atchafalaya River Basin (MARB), which includes major rivers such as the Mississippi and the Missouri. MARB passes through 31 states, and agriculture is the dominant industry in several of those states, which is where the overflow of nutrients originates. The eutrophication process in the Gulf of Mexico is cyclical and grows in the summer and shrinks during the winter due to decreased agriculture only to return the following summer. This dead zone along the northern edge of the Gulf stretching along Texas and Louisiana measured 13,080 square kilometers in the summer of 2014, and it is the largest dead zone in the United States