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Eutrophication Of The Oceans

written by: Dr Mike C•edited by: Jason C. Chavis•updated: 6/14/2010

Understand facts about eutrophication, the ingress of nutrients into a water body, such as a rivers, lakes or the oceans. The presence of additional nutrients in the water can trigger explosive growth in the population of algae which may have catastrophic consequences for other marine life

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    Feeding A Hungry World

    The global population of mankind is now in the region of seven billion souls. Although hunger and famine are a daily reality to many in certain regions of the world, most people in the West have ready access to plentiful supplies to a wide range of food items. These facts about eutrophication help one to understand these processes.

    Western agricultural practices are highly efficient and very productive, but require that the soil is regularly fertilised to provide plants with the nutrients they need to maintain crop yields. The fertilizers provide the soil, and through it the crops, with additional phosphorus and nitrogen compounds needed to produce high yields. Inevitably, some of this material runs off the fields into watercourses as a result of rainfall and percolation through the soil. These watercourses feed into streams and rivers that ultimately feed into the sea. The sea is therefore the final sink for a significant fraction of the fertilisers which are applied to the land. However, these nutrients can also accumulate in rivers and lakes depending on the hydrodynamics of the specific environment.

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    The term eutrophication stems from the Greek eutrophos meaning "well-nourished." In modern oceanography, the term is used to describe the influx of nutrients into a body of coastal water; it is sometimes also referred to as “cultural eutrophication". Agricultural activities are not the only source of eutrophication: other sources include sewage discharges to sea and aquaculture (fish farming).

    At first sight, eutrophication may be imagined to be a beneficial process – the ingress of nutrients to the sea, but it is a major concern. The primary production of the oceans for microbiota in estuaries and the near-shore environment is limited by the amount of nitrogen in the water column, notably within the photic zone where sunlight can penetrate the water, enabling photosynthesis to take place. This means that because the microbiotic organisms need nitrogen to multiply, their populations are held in check by the limited quantity of nitrogen available. However, as a result of eutrophication in some regions, this limitation is no longer the dominant term in the population control and production of these microbiotic organisms (phytoplankton) explodes; a process known as a harmful algal bloom (HAB).

    Phytoplankton forms a key part of the marine food web, providing nutrient sources for higher life-forms which graze on it and are then, in their turn, predated by larger creatures.

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    Harmful Algal Blooms

    A harmful algal bloom can be formed when there is an explosive, rapid growth in the population of phytoplankton. The density of these floating colonies of microbiota is so high that the bloom forms a visible patch on the water body. The algae typically have a reddish brown colour which is why HABs are often referred to as “Red Tides".

    Since the growth of the algal community is so great, the amount of sunlight penetrating into the photic layer can be reduced, depriving organisms of the light that they need to survive. HABs can cause the death of other marine organisms (e.g. bottom-dwelling fish) through hypoxia; a depletion of the quantity of dissolved oxygen in the water because the fast growing phytoplankton community consumes oxygen dissolved in the water column.

    Certain species of phytoplankton release toxins that can be deadly to marine creatures and may bioaccumulate in some seafoods to levels which can be fatal to humans ingesting them (e.g. paralytic shellfish poisoning, due to saxitoxins).

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    Tackling The Problem

    With an ever-increasing demand for food to feed a hungry world, the problem of eutrophication will not be going away. Fertiliser use in China has risen from 5 million tonnes in 1970, to 20 million tonnes by 2005; representing 25% of current global use of fertilisers. The problem could be alleviated by ensuring that land is not over-fertilised and has the benefit of reducing the cost of fertilisation. However, easy to apply methodologies for ensuring that optimal dosing can be achieved (particularly in the third world) need to be developed. Fertilisers that release over time could be one solution, but these are more expensive to produce and therefore will be a problem for the hungry, developing world to afford.

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    1. Centers for Disease Control:
    2. Smithsonian National Museum of Natural History:
    3. The role of Eutrophication in the global proliferation of Harmful Algal Blooms; Gilbert et al:

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