Lakes are bodies of water deep enough to exclude plants from some part of the substrate ie plants cannot coat the whole of the lake bottom. Most have been formed by geological processes such as erosion, glaciation, faulting or subsidence. They are filled by rain water, inflow from river or stream or from underground through springs and seepages. Lakes can be found on all continents and can form at any altitude. Canada contains half of all of the lakes in the world.
Lakes can be classified on the basis of their nutrient levels. At one end are oligotrophic lakes which are nutrient poor and at the other end are eutrophic lakes which are nutrient rich. One of the major threats to this type of habitat is the input of nutrients, primarily by mans activities, which leads to an increase in plant growth and subsequently to decreases in the levels of available oxygen with consequences for the flora and fauna.