MAP Technical Reports Series No. 106 UNEP

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Visibly, eutrophication and its side-effects cause discolouration of waters, reduced
transparency, unsightliness and disturbance to bathers thus impairing recreation. Dense
macrophyte and macro-algae agglomerations chop channels, lagoons and estuaries impairing
fishery and navigation, and reducing flow and the holding capacity of freshwater reservoirs, etc.
When aging, the decaying organic material consumes, and in serious cases depletes
the oxygen reserve of the water causing an array of secondary problems such as fish mortality,
formation of corrosive and other undesirable substances such as CO 2, CH 4, H 2S, NH 3,
organoleptic (taste and odour producing) substances, organic acids, toxins, etc.
Sticking algal material and high pH can cause dermatitis and conjunctivitis, and
ingestion of algae can cause diarrhoea in sensitive individuals. The development of toxin
producing algae in the marine environment, when accumulated in fish, particularly shellfish, is
a threat to human health. In fresh waters, toxic blue-greens (cyanobacteria) are a cause of
livestock mortality.
2.3 Anthropogenic versus natural eutrophication
Eutrophication as a water quality problem differs from those listed above mainly in the
increased difficulty to distinguish the process of eutrophication caused by man from processes
and phenomena that may occur also naturally. This difficulty has unavoidably led to some
controversial positions within the scientific community about the reality and importance of
eutrophication as a water quality problem. Lakes and enclosed marine systems may, or may
not become naturally eutrophied by processes of aging. Certain aquatic systems lie in
catchment systems, or exhibit morphometric and hydraulic conditions that favour elevated
trophic conditions. Climatic variations that cause alterations in the vegetation and soil conditions
of the respective catchment basins may lead to alterations in trophic conditions of bodies of
water. In the marine environment changes in current and upwelling patterns due to climatic
variations may also cause changes in the local and regional trophic state of waters. However,
such variations may act in either direction, i.e. leading either to eutrophication, or to
oligotrophication.
Still, it is to be noted that the time scale at which the natural processes operate, is
substantially different from that of man induced processes. The time scale of natural processes
is at least in the order of centuries, and mostly longer, while anthropogenic eutrophication occurs
at time scales of decades and less. Even with this qualification, the distinction between natural
and anthropogenic causes of eutrophication is not always clear cut.
2.4 Delineation of the problem of eutrophication versus other forms of pollution
Eutrophication is but one form of water quality problems. Other water quality problems
result from pollution of waters with heavy metals, from inorganic, organic and microbiological
pollutants in urban, agricultural and industrial sewage; from agricultural run-offs that also contain
herbicides and pesticides, from industrial effluents and discharges and other losses such as oil
and petroleum residues, radioactivity and heat in thermal effluents; from acid precipitation, and
not least from simple litter (Waldichuk, 1987; Coté, 1989).
Pollution of this sort, though not being its main cause, may directly or indirectly enhance
or counteract eutrophication. All the respective interactions and interconnections are not fully
understood yet, but should not be ignored as a potential though minor cause of eutrophication.