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Eutrophication
University of Liverpool (UK)
Dr. Adrian E. Williams B.Sc. (Hons.), M.Sc. (D.), M.I.F.M., C.Env.
Introduction
The Law of the Minimum (Odum, 1971), whilst simplistic, is an important ecological concept. It indicates that whichever essential factor needed for growth is in the least supply, relative to the requirements of production, that factor will limit production. In lakes the limiting factors for algae are usually phosphorus and light whilst for animals, food availability and temperature are often limiting (Harper, 1992). Once a population's growth rate increases there is a possibility that the population will come into competition for more or other resources with other populations or species. In a lake situation the increase in nutrient loading, in particular phosphorus and nitrogen, will tend to increase algal biomass and competition may result in the species contributing most to the biomass changing.
Indirectly other species, not immediately affected by an increase in nutrient loading, may themselves be affected by changes in the physico-chemical environment caused by changing algal or bacterial populations. With increasing algal biomass the light climate of a lake may change or increased bacterial respiratory uptake may reduce oxygen levels. Therefore eutrophication has implications for all trophic levels of a lake as the effects cascade through them.
After an initial increase in salts (typically phosphorus or nitrogen), the algal biomass increases (Lund, 1969). Species composition may change and the phytoplankton and epiphyton usually become dominated by diatoms, Cyanophyta and unicellular green algae (Brook, 1964). Submerged macrophytes may initially increase in biomass but typically macrophyte biomass will diminish (Harper, 1986; Phillips et al., 1978). In addition species intolerant to reduced oxygen, low light, high dissolved solids or interspecific competition may disappear and the clear water macrophyte-dominated lake can eventually switch to a turbid algal-dominated state (Moss, 1996; Scheffer 1998).
Eutrophication was once thought to be an inevitable and natural phenomenon (Lindeman, 1942; Deevey, 1942). However later investigations found that lakes may pass through periods where their waters become more oligotrophic (Haworth, 1969; Macan, 1970). Today eutrophication is usually considered to be anthropogenic in nature as there is a finite stock of nutrients within a catchment, although with climate change weathering and therefore nutrient loading can change (Pennington, 1981; Whiteside, 1983). Eutrophication has occurred globally and diachronically as can be seen from palaeolimnological records (Zullig, 1989; Fritz, 1989; Anderson, 1995). In nearly all cases the expansion of human colonisation has caused the onset of ecological change in lakes. The expansion of the Roman empire as well as the more recent settlement and growth of cities like Seattle have resulted in algal species changing, indicating that eutrophication has occurred (Jones et al., 1978; Stockner and Benson, 1967). Sawyer (1947) and Hasler (1947) were among the first scientists to see that industrialisation and urbanisation were having serious effects on lake ecosystems. Sawyer (1947) investigated a chain of lakes in Wisconsin, USA and found that sewage effluent from the town of Madison was the cause of algal blooms and resulting odours in Lakes Monona and Waubesa.
Of course lakes naturally high in nutrients do exist (McGowan et al., 1999; Moss et al., 1994). However, the increase in eutrophication due to industrial, urban and agricultural policies and phosphorus and nitrogen usage has wrought changes on lakes over the last few decades which have caused great concern. These concerns are based around problems of water supply, recreation, aesthetic value, public health and loss of biodiversity.
Eutrophication is one of the world's key problems facing freshwaters. Nutrient run-off and sedimentation resulting from intensification of agricultural land use are problems that are likely to increase, however the majority of current problems are caused by point sources from urban and industrial areas (Toerien, 1975). Carvalho and Moss (1995) and English Nature (1998) examined the eutrophic status and symptoms of eutrophication in one hundred and two SSSI freshwater sites. Seventy six cases showed symptoms of eutrophication. Whilst in 16% to 36% of cases showing eutrophication the prime cause was attributed to damage by benthivorous fish, in 43% of cases the prime cause of eutrophication was, as it was in 1947 for Lakes Monona and Waubesa, attributed to sewage effluent.
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Have a look at my CV (contains links to papers that I have published)