The Evolution of Lakes and Wetlands

Abstract

When dealing with lake restoration, we should bear in mind that inland water ecosystems are not static units but subject to continuous evolution. Palaeolimnological studies have helped us to understand the development of lakes and their catchments. In particular, the development of northern European lakes has been studied very thoroughly. It has been revealed that the initially high productivity of lakes (shown by sediment growth rate) was due to the leaching of nutrients from the nutrient-rich moraine after the last deglaciation. With time, however, lake productivity dropped as the supply of nutrients from the catchment area diminished. This reduction depended partly on the decreased leaching and partly on the development of terrestrial vegetation as it accumulated and recycled nutrients. In southern Sweden, the current sediment growth rate is about 0.2 mm per year in more-or-less-intact, shallow oligotrophic lakes, and about 0.5–1.0 mm per year in shallow eutrophic lakes. If a lake becomes polluted by the discharge of nutrient-rich sewage, the sediment growth rate can increase to about 10 mm per year. The ageing of lakes, and their potential terrestrialisation, depends on the balance between production and decomposition of organic matter. In northern latitudes, the break-down of organic matter in cold and oxygen-free sediment and peat is much slower than in warmer waters in the south where mineralisation processes take place at higher rates and over a longer period of the year. It is therefore much harder to prevent lakes in the north from being terrestrialised.