Physical Resources / 97If the water body is used for water abstraction, angling, or navigation, eutrophication is likely to
reduce its value. The cost of treating water to bring it to potable standard will increase, navigation
may be impeded by plants, and preferred species of fish may disappear. At high densities, some
algae and cyanobacteria produce potent toxins. The alga Prymnesium parvum is highly toxic to fish,
and toxins produced by such cyanobacteria as Microcystis, Aphanizomenon, and Anabaena attack
the liver and may be neurotoxic. In 1989 there were outbreaks of toxic cyanobacteria in some British
lakes and a number of dogs died after swimming in them and ingesting their water. Not surprisingly,
eutrophication also brings about marked changes in the populations of aquatic organisms. The water
supports fewer plant and animal species, but more individuals, the water becomes more turbid because
of the large amount of organic matter suspended in it, the water becomes increasingly anoxic, and
the rate of sedimentation increases.
A eutrophic lake is an old lake, and eutrophication is an ageing process. When it first forms, a lake
typically supports little plant life, but fish such as trout, which feed on insects caught at the surface,
may thrive. Its water is clear and well oxygenated, but very deficient in nutrients. There is little or no
sediment at the bottom and plants grow beside it, but well clear of the water. A lake in this condition
is said to be ‘oligotrophic’ (the Greek oligos means ‘small’ and trophe ‘nourishment).
Rivers flowing into the lake bring nutrient and particulate matter, and in time the lake becomes
‘mesotrophic’ (Greek mesos, ‘middle’). Its water is still clear enough for light to penetrate deeply, so
algae flourish, but without proliferating uncontrollably because they are grazed by a diverse population
of invertebrate and vertebrate animals, including fish. Sediment is accumulating on the bottom. This
provides anchorage and nutrient for rooted plants, which now extend from the banks and into the
lake margins, colonization by plants that have to reach the air being limited only by the depth of
water. The accumulation of sediment also raises the bottom, so the lake has become shallower. In a
eutrophic lake (Greek eu-, ‘well’) the sediment is deep and the lake shallow. Plants rooted in the
sediment extend far from the banks. The three drawings in Figure 3.4 illustrate this life cycle.
Life cycles, which paradoxically are linear so far as individuals are concerned, end in death, and the
life cycle of a lake is no exception. It is the fate of all lakes and ponds eventually to become dry land
or, if they occupy low-lying ground where the water table is at or very close to the surface, a bog,
marsh, or fen. Accumulating sediment makes the water shallower, but its colonization by plants also
removes water, by transpiration. Once plants are established across the whole area of a lake, its
demise is fairly rapid. Aquatic plants give way step by step to land plants that can tolerate waterlogging
around their roots, and then these are replaced by true dryland or wetland plants. As the sediment
dries and becomes soil, it is the acidity of the soil that determines whether the lake evolves into lime-
loving grassland and, over much of north-western Europe, from there to scrub followed by woodland
and forest, or to acid-loving heath. Figure 3.5 illustrates this development.
Such eutrophication is natural, but the life span of a lake should be measured in thousands of years.
Artificial eutrophication, caused by discharging sewage and other wastes into lakes, short-ens it
greatly. Untreated human sewage may have a BOD of 300 mg litre-1, paper-pulp effluent 25000 mg
litre-1, and silage effluent 50000 mg litre-1. Deoxygenation is by far the commonest type of freshwater
pollution. Bacteria decomposing the faeces from one human use 115 g of oxygen a day; this is
enough oxygen to saturate 10000 litres of water (MELLANBY, 1992, p. 88). Halting natural
eutrophication may be undesirable, even if it is practicable, but artificial eutrophication should be
prevented or, if it is too late for prevention, cured.
It can best be remedied, of course, by finding alternative means of waste disposal or at least by
reducing the nutrient content of the discharges, especially of phosphates, which are the limiting
nutrient in most waters. This can be done by reducing the phosphate content of detergents, which