Lakes in cold 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

A long ice cover period determines the lake’s stratification. Usually it is characterised by a short summer stagnation phase, which limits the likelihood of oxygen restriction in the deep zones. Cold temperature and short vegetation period cause low primary production; the lakes are usually oligotrophic and characterised by cold-water species which occur in low abundances.

Human impact

The main human impact in the past has been acidification, due to atmospheric pollution, while eutrophication was a less widespread problem.

Climate change impact

Climate change impacts will be significant, and they will mainly act directly through temperature increase, which prolongs the ice-free period and thus the duration of summer stratification. This may enhance eutrophication and lead to oxygen depletion in deep zones during summer, thus eliminating the refuge for coldwater-adapted fish species.