Lakes in temperate ecoregions

– Water type description –

• Lakes in general

  Shallow lakes are characterized by submerged plant communities which are the main primary producers. Phytoplankton is also present but less important for primary production than in deep lakes due to grazing of zooplankters, which hide against fish predators between the macrophytes.

  Naturally characterised by clear water, these systems have frequently been altered into turbid states lacking macrophytes, caused by eutrophication and specific impacts that destroy the plants. Warming increases eutrophication by accelerating phytoplankton growth, nutrient release from the sediment and by supporting invasive plant species.

  Deep lakes are mainly found in mountainous regions and under natural conditions characterised by very low nutrient loads. Macrophytes are restricted to a narrow belt along the shores, and phytoplankton abundance is low. Even the deep zones are well oxygenated throughout the year and provide summer refuges for coldwater adapted fish. The main human impacts are acidification and – more widespread – eutrophication.

  Eutrophication causes enhanced primary production, algal blooms and oxygen depletion (particularly in the deep zones) which may affect almost all processes and species. The main physical effects of warming are an extended stratification phase and accelerated oxygen depletion of the deep zones.

The pristine status

Most lakes in temperate regions are dimictic, with two periods of stratification in summer and winter, although there are exceptions such as the Lake Constance, which lacks winter stratification. Depending on origin, depth, altitude, catchment size and geology there is a multitude of lake types with characteristic biota. Primary production of most shallow lakes is dominated by macrophytes, while phytoplankton is the main primary producer in most deep lakes.

Human impact

Eutrophication has been the main human impact for several decades, leading to algal blooms, oxygen depletion in the deep zones and fish kills. Though many of the extreme cases have been restored, the majority of lakes are still suffering from eutrophication, as well as from shoreline modification.

Climate change impact

Climate change is expected to enhance the symptoms of eutrophication, through increased primary production and less oxygen storage capacity of the water. Many dimictic lakes will become monomictic with a long stratification period in summer and a single circulation period in winter, which is not interrupted by ice cover.