Why is eutrophication a problem?

4 March 2011

Today I was asked “why is eutrophication a problem, why should we worry about it?”. The very nature of this query illustrates just how serious this particular problem is in South Africa.

Elsewhere on this blog I have dealt with what eutrophication is – in essence, dams or lakes with too much food in the form of nutrients that plants and algae need – resulting in the overproduction of organic matter.  Any organism that eats too much will suffer.  Take the simple example of people who consume food in excess – they become overweight, prone to obesity and other disorders, increasingly prone to illness and their general quality of life deteriorates.  Parallels to these problems may be drawn to the situation that arises in our dams.

In lakes and dams eutrophication leads to:

  • Excessive and often problematical growth of aquatic plants and/or algae, typically evident as algal scums or floating mats of plants such as water hyacinth.
  • Sustained eutrophication leads to algal dominance and, typically, dominance by noxious forms of algae (cyanobacteria). Between 35% and 45% of South Africa’s water resources are already impacted.
  • Excessive algal growth results in a combination of biophysical and biochemical changes to the lake environment.
  • Biophysical changes are most commonly seen as reduced input of light into the water, this caused by the dense algal blooms preventing light penetration.  In certain systems “blooms” occur as extensive mats of cyanobacteria covering the lake floor in the shallow (edges).  Without sufficient light, all photosynthetic organisms suffer or die.
  • Biochemical effects encompass alterations in the pH and oxygen levels of the lake or dam, the release of toxins or other harmful metabolites – which may have a direct or indirect effect on other organisms.  For example, reduced oxygen can result in fish kills. Sustained eutrophication results in ecosystem degradation (reduction of species, increased proportion of hardy species such as carp).
  • The presence of algal toxins pose direct threats to human and animal health, via consumption or exposure (e.g. swimming).  Cyanobacterial toxins may be taken up by fish, rendering their consumption a health risk.
  • High levels of algal biomass, i.e. organic matter, can increase the risk of carcinogenic compounds being produced when the water is treated with chlorine.
  • The recreational value of a lake or dam deteriorates (reduced water clarity, floating mats of plants which reduce access, presence of toxins).
  • Fish species shifts occur, from desirable to coarse (undesirable) species.
  • Cyanobacterial toxins include liver and nerve toxins, compounds that may cause or influence the development of Alzheimers disease, and compounds that may act as estrogens.  Most of these compounds are not removed by conventional water treatment.
  • Eutrophication in South Africa is caused, in the main, by inadequately-treated sewage effluents dumped into dams.  Thus eutrophication may be seen as a proxy for a host of other ills – chemicals and pharmaceuticals (e.g endocrine disrupting compounds) that are not removed by wastewater treatment.
  • Other sources of eutrophication-driving nutrients include industrial effluents, agriculture, homes, urban and road surfaces.
  • Eutrophication progressively increases the cost of drinking water treatment – by between four and ten times the cost of treating non-polluted sources.  It also augments the costs of treating water for special uses (food manufacturing, pharmaceutical manufacture, high technology manufacturing, and the like).
  • Eutrophication is the primary cause of water quality impairment worldwide.
  • The recovery of lakes from eutrophication is slow, at least 10 to 15 years after nutrient pollution is terminated before beneficial recovery will occur, longer in some cases where polluted sediments occur.
  • Managing eutrophication in developing countries is much more difficult than in developed countries.  China (e.g Lake Taihu) is a classic example of this with over 70% of Chinese lakes now eutrophic, some so much so that the water is unsuitable even for industrial purposes.

The nett effect of the above is a reduction in the quality of life (socio-economic degradation).  A study conducted during 2010 found that for every 1% decline in water quality, 200 000 jobs will be lost and a reduction in per capita disposable income of 5.7%.

I was also asked “will climate change affect eutrophication?”

The answer is a resounding yes.  The timing and quantities of nutrient availability will alter or increase and algal blooms will become more frequent and of longer duration.  This has been an open discussion amongst scientists and lake managers since the early 1990s (e.g. the work by James Bruce).  Advanced research on this topic is underway in Europe where the effects are already measureable.

One Response to Why is eutrophication a problem?

  1. Johanna says:

    I’m doing my research on the eutophication effect on microbial distribution on the Swakkoport Dam, spatially and seasonally. This has helped me a lot. Thank you

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