A Monitor's Guide to Water Quality

A.    As the surface waters of a lake warm in the summer, they become underlain by a colder, denser layer of water. Once this occurs, there is little mixing between the warm surface layer on the top and the cooler layer below.

B.    In the fall, surface waters begin to cool and become more dense and heavy. This causes the water to mix through the depth of the lake, referred to as “fall turnover”.

C.    As ice begins to form on the lake, the lake may stratify again. However, this time the surface waters (at or near freezing, 0°C) are underlain by slightly warmer water (about 4°C). This happens because water is most dense at 4°C.

D.    In the spring as the ice cover breaks up, the water temperature becomes more uniform from the surface to bottom. This causes another period of mixing referred to as “spring turnover”. Then as the water at the top begins to warm even more, these stratification and mixing cycles are repeated.

Smaller, shallower lakes and streams change temperature more quickly than large lakes and rivers since there is less water to be heated or cooled. This means low flow levels in a river can have an impact on water temperatures, especially during warm, dry periods. In some rivers and streams, upwelling groundwater provides areas of cooler water for fish.

Climate change may affect water temperatures and therefore the distribution and survival of many fish. If temperatures become too warm during the summer in lakes and rivers, some fish species that cannot live in warm waters may disappear or be replaced by fish that can live in warmer waters.

Water temperature is influenced by: water clarity (e.g., Secchi depth, water colour), water levels/flow

Water temperature influences : dissolved oxygen, productivity measures