Although the total volume of freshwater water in the world is dominated by a few large and deep lakes, most lakes are small and shallow. Shallow and deep lakes exhibit significant differences in trophic structure and dynamics as well as in sensitivity to threats such as that posed by increasing nutrient loading. An essential difference is that deep lakes often show thermal stratification in summer, which largely cuts off the upper water layers (epilimnion) from the colder deep water (hypolimnion), thereby preventing interaction with the sediment.
Reservoirs are human-made lakes, created for storage of water mainly for drinking water supply and hydropower generation. Except for small reservoirs in farmland areas, their catchments are typically large compared with ordinary lake catchments. Accordingly, nutrient and sediment loading to and storage in reservoirs are often larger than in natural lakes. Major differences exist in the biological community structure of lakes and reservoirs, which to a large extent reflect the greater temporal water level changes in reservoirs. Many reservoirs lack a littoral zone because they are typically constructed in places in narrow steep-sided gorges (e.g., the world's largest reservoir, 'Three Gorges Dam,' in China). This implies a limitation of shallow water areas. In addition, the fluctuating water levels of reservoirs impede the development of a plant-rich littoral zone. In natural lakes, in contrast, the littoral zone can have a huge impact on the entire lake ecosystem owing to its use as a refuge by large-bodied zooplankton and small fish and as spawning grounds for fish. Often reservoirs have lower fish recruitment success, which may lead to reduced density of planktivorous fish, enhanced zooplankton grazing on phytoplankton, and clearer water than suggested by the nutrient level (for instance as seen in the London Reservoirs). Fish production may also be lower unless fish stocking is regularly repeated.
Additional measures to sustain recovery after a loading reduction vary between deep and shallow lakes and between lakes and reservoirs. Here, we will treat lakes and reservoirs together and only highlight differences in approaches for the specific water types when relevant. We will focus on measures to alleviate eutrophication and not include restoration measures applied to combat, for instance, acidification.
Was this article helpful?