Issue 5: Eutrophication

General definitions

Eutrophication as a phenomenon is described in different ways by limnologists and marine scientists.

Most limnologists consider eutrophication as an increase in the rate of supply of organic matter to an ecosystem.

For marine scientists, eutrophication (GESAMP, 1990) is “used simply to mean ‘enhanced nourishment’ and refers to the stimulation of aquatic plant growth by mineral nutrients, particularly the combined forms of phosphorus or nitrogen”.

Why is there a difference in approach?

Though widely associated with pollution, eutrophication is also a natural process in lakes as they gradually accumulate carbon, nitrogen and phosphorus. Lake ecosystems can recycle some of the nitrogen and phosphorus compounds deposited in the lake bed and this promotes more production of organic material. The organic material builds up on the lakebed and contributes to the lake’s own demise. Bogs are shallow lakes that have filled with detritus (often peat) in this manner. The process may take hundreds or thousands of years and lead to new land formation when the bogs themselves are colonised by trees. Eutrophication of a lake may therefore be considered as an increase in organic matter in the system. This may be a result of direct river or sewage discharges but is most commonly associated with the increase in plant production caused by an increased supply of nitrogen and phosphorus compounds, essential nutrients for plant growth. The problem is that human intervention is accelerating this natural process, leading ultimately to the accelerated decline of lakes.

In the marine environment, eutrophication has only been recognised in the past three decades. It was first identified in coastal lagoons of the US eastern seaboard through the presence of unusually intense blooms of phytoplankton. Since eutrophication has been documented in coastal seas throughout the world – this is not to say that all coastal seas are eutrophied. In all cases, eutrophication is seen as a non-natural phenomenon associated with an increase of essential nutrients, nitrogen and phosphorus compounds, utilised for phytoplankton growth.

Eutrophication can be a naturally occurring phenomenon but there is concern over the number of rivers, lakes and coastal waters in which it is developing or being aggravated through human inputs.

See Lake classification for more details.

GIWA definition

Eutrophication refers to artificially enhanced primary productivity in receiving water basins related to the increased availability or supply of nutrients.

Environmental impacts

1. Redox changes

2. Increased algal blooms

3. Changes in plankton community structure

4. Changes in macrophyte community structure

5. Loss of habitat (e.g. coral reefs)

6. Change in composition of feral fisheries and loss in case of anoxia

Indicators

The following are environmental impact indicators for eutrophication.

Select the environmental impact indicators that are relevant from the list below and input data into the appropriate indicator sheets.

Map and satellite derived

Biological

Parameters

Pollution

Hydrological

Proxy indicator of eutrophication