98 / Basics of Environmental Science
are the principal source, or by stripping the phosphate from sewage before it is discharged. This is
possible, with 90–95 per cent efficiency (MASON, 1991, p. 131). but there have been cases of a
reduction in phosphate input being followed by the release of phosphate from sediment by mechanisms
which are not well understood. In extreme cases it may be feasible to remove the sediment itself by
dredging. Where land drainage is the main source of sediment and nutrient, reducing soil erosion
may be effective. If oligotrophic water is available, using it to recharge a eutrophic lake may bring
benefits. Beyond such measures as these, remediation usually involves manipulating the plant and
animal populations. Obviously, no two water bodies are precisely similar and remedial measures
must be appropriate to the particular conditions encountered.
Figure 3.4 The life cycle of a lake. A, Oligotrophic. Little bottom sediment;
water nutrient-poor; plants grow on banks only. B, Mesotrophic. Mud
accumulating on the bottom; plants rooted in mud extending into the lake;
moderate nutrient supply. C, Eutrophic. Deep bottom sediment; plants
rooted in mud far into the lake; water very rich in nutrients; depth of lake
decreasing owing to accumulation of sediment and evapotranspiration