Lakes in warm 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

In southern Europe most deep lakes are artificial (reservoirs), while shallow lakes are more common, though less frequently occurring than in the formerly glaciated areas of central and northern Europe. Mediterranean lakes are ice-free throughout the year, and are thus monomictic, with long summer stratification and a single circulation period in winter.

Human impact

Besides the common problem of eutrophication, Mediterranean lakes have suffered from water abstraction for multiple water uses (mainly irrigation) and from the impact of pesticides. As a consequence the water volume of most Mediterranean lakes has been reduced, and some smaller lakes have been disappeared completely.

Climate change impact

Increased demand for freshwater and higher transpiration will increase these problems; Climate Change will further reduce the water volume thus making lakes more susceptible to eutrophication and all forms of pollution.