What causes eutrophication?

Introduction

The main causes of eutrophication are an increase in the concentration of nutrients in an ecosystem. A distinction is sometimes made between 'natural' and 'cultural' (anthropogenic) eutrophication processes.

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Natural runoff

Natural eutrophication has been occurring for millennia. It is the process of addition of nutrients to water bodies, including lakes, rivers, estuaries and oceans resulting in changes to the primary production and species composition of the community. This process occurs over extended periods of time that are typically geological time scales. Cultural eutrophication is the process that speeds up natural eutrophication because of human activity.

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Fertilizer runoff

These activities come from many diverse sources including agriculture, aquaculture, septic tanks, urban wastewater, urban stormwater runoff, industry, and fossil fuel combustion. Nutrients such as phosphates and nitrates enter aquatic ecosystems via the air, surface water, or groundwater.

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Untreated sewage inflow

Trophic status

The stage at which the process of eutrophication is at any given time in a particular water body is useful to describe the status of the water body. The following terms are used:

Oligotrophic

Low in nutrients and not productive in terms of aquatic animal and plant life.

Mesotrophic

Intermediate levels of nutrients, fairly productive in terms of aquatic animal and plant life and showing emerging signs of water quality problems

Eutrophic

Rich in nutrients, very productive in terms of aquatic animal and plant life and showing increasing signs of water quality problems

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Example of an eutrophic lake

Hypertrophic

Very high nutrient concentrations where plant growth is determined by physical factors. Water quality problems are serious and almost continuous.

References

Walmsley RD, 2000. A Review and Discussion Document. Perspectives on Eutrophication of Surface Waters: Policy/Research Needs in South Africa. Water Research Commission. Project K8/360