In Finland, it seems lake surface temperature is rasing about half a degree per decade. Since most Finnish lakes are small and shallow their temperature increase is pronounced.

Different aquatic organisms have variable temperature preferences (as explained last week in Arctic Lakes)

Prolonged stratiication period

• Lake cycle naturally has a period of stratification in the springwhere different temperature layersform
• Increasing surface temperature makes the temperature gradient higher
• With climate change this period starts earlier
• This increased stratification period results in oxygen depletion at the surface

Turbulence

• Increasing wind speed (more storms) and dropping lake levels increases overall turbulence in many lakes
• Usually, we only think of this in large lakes where wind speeds get higher. However, in larger, deeper lakes, the energy has more room to diffuse
  • In a small lake, the same wind speed can cause higher turbulence in the lakes where there is less room to dissipate
• This tends to force the thermocline (temp gradient) deeper due to mixing. This results in further increasing surface temperature
• Some shallow lakes that were stratified earlier might become continuously mixed from the thermocline going deeper, which could result in ecosystem collapse.

Brownification

(Lake browning)
The water color of lakes and rivers is generally increasing due to dissolved organic carbon (DOC) flux increase (and dissolved iron)

• This is the most important predictor of water color in lakes and rivers
• Higher organic soil content results in higher DOC. Therefore Finland overall we have many brown lakes, but it is till increasing
• Highest DOC flux is during snowmelt
• Future prediction with climate change - the snowmelt will be less important and w will have fairly constant DOC flux, which will increase brownification

Anthropogenic factors increasing brownification

• Forestry, ditching, peat production all increase DOC inlakes
• Decrease in anthropogenic acidification (good thing, due to regulation) - when soils become less acidic, it increases solubility of organic compounds. When lakes were acidified it became very clear

Eutrophication

• Phosphorus loading naturally increass in spring runoff/snowmelt. As with DOC, with climate change the spring peak will decrease and phosphorus loading in winter will increase (less snow and more rain)

Effect of turbulence on food webs

• Zooplankton biomass by taxa changed a lot in experimental lakes with turbulence machines
  • Fish were able to decrease the zooplankton biomass in most turbulence conditions
  • When moderate turbulence was added, the predation rate of "ambush" style insect larvae increased by moving the zooplankton into their mouths
  • In very high turbulence, even some fish had trouble hunting due to reduced visibiltiy
• Overall, turbulence has unpredictable effects on aquatic food webs