L4 - Lake Physics Flashcards
(13 cards)
Water mostly occur in 3 states.
When heated up molecules vibrate and reform
Lots of energy required to go from solid to liquid and liquid to gas
- Slow to Freeze
- Slow to evaporate
Key Terms
‘Residence time’ & ‘Retention’
A lake is only a semi-closed (not fully) body of water as the boundary is porous. Arrows indicate how materials can flow into the lake one end and out the other, it can exchange through its surface with the atmosphere and can exchange with the sediments.
Residence time (same as turnover time) = volume of lake divided by volume of water flowing in every year. Usually in units of time and refers to residency time of the water. Water once it has flown into a lake, will eventually leave the lake via an outflow or through evaporation, otherwise it would just fill up and spill over.
Two fundamental states of lakes
Fully mixed or stratified
Stratified:
Warm water less dense than cold hence why it is stratified.
Can occur due to a lack of energy/water movement.
Fully mixed:
Constant currents churning up entire lake = no stratification.
- Primarily driven by solar radiation at the surface and being absorbed by the waters surface.
- Mixed through water collumn by heat and convection.
- Stratification occurs when more heat enters the lake than can be removed by convection currents etc that mix it down into the water collumn.
What goes In and out?
IN: SASPS
OUT: BASES
‘Work required to mix a lake’
Schmidt stability
‘Resistance to mixing’
Wedderburn number
‘Likelihood of wind-driven upwelling events - when hypolimnion comes to the surface’
Lake number
Likelihood of wind-driven internal mixing events
“Ratio of mean depth to centre of gravity is a major part of schmidt stability”
Large, deep lakes
Deep metalimnion
Low temperature differential
Long fetch
Strong winds
When wind stress is applied to a stratified lake, it pulls water at the surface and piles it at the down wind end.
Gravity takes over and replaces the water sucked from upwind end and it sucks up the hypolimnion towards the surface and tilt the thermocline.
Why is stratification important?
Epilimnion
The main reason we care is when there is a stratified system, the epilimnion is in contact with the atmosphere and exchanging gasses and attaining or losing oxygen.
Hypolimnion
Hypolimnion is sealed off to the atmosphere because of the epilimnion and hypolimnion layers above it. Consumption of oxygen is through decomposition of organic material (mostly sedimentation). Production is only by photosynthesis (usually negligable)
Scenario
A lake is starting to stratify at the end of winter. Solar energy entering the lake is begining to outdo the wind mixing. When it first stratifies the Hypolimnion is probably going to be quite oxygenated as it has been mixing all winter, oxygen constantly being replaced throughout meaning the water O2 concentration is at saturation point at the onset of stratification aka at equilibrium with the air. Within that Hypolimnion we are always consumin oxygen because of the decomposition of organic material, this material can come from anywhere in the lake such as the Epilimnion in the upper parts of the lake where there is plenty of sunshine for plants to grow, once they die though they will eventually drift down to the bottom of the lake where they will decompose and take up O2 (ORGANIC MATERIAL OXYGEN SINK)
“The only way to produce O2 in the hypolimnion is through photosynthesis, this is negligible as there is poor light penetration at those depths. So lots of oxygen is being consumed and none replaced, the rate of depletion of oxygen relfects the amount of organic material in the lake (sediments) and the amount of oxygen that was present in the lake to begin with. Organic rich sediments can make lakes completely inoxic”
Top graph represents temperature.
You can see red and yellow lines which are shallow water.
Blue lines are deep water.
When the red and yellow lines seperate from the blue, they represent temporary stratification.
The bottom plot displays the O2 concentration in the eplilimnion and hypolimnion. The data correspons to the top graph and you can see that when the red, yellow and blue lines become seperated (stratified), the 02 depletes accordingly.
STRATIFICATION = OXYGEN DEPLETION
Describe this chart
Top left: lake rotoiti
Bottom right: lake Rotorua
(connected via Ohau channel, water flows from rotoiti to rotorua)
In winter lake Rotorua is much colder that lake rotoiti.
Rotorua gets colder because it has a massive surface area to lose heat from.
In winter time, cold water flowing down the Ohau channel into lake rotoiti is dense, what this means is when it flows down the channel in winter it sinks below the warmer water of lake rotorua, this is known as underflow.
In the summer, the water coming from lake rotorua and into the channel is warm (less dense) and sits on the surface of lake rotoiti.
THE ISSUE
Lake Rotorua carries nutrients and oxygen demand to the bottom of lake rotoiti contributing to its eutrophication so because lake rotoiti is productive and nutrient rich, its able to export lots of this organic material to lake rotoiti and if they plunge to the bottom of the lake where we are increasing the nutrient loading and adding to the eutrophication.
THE SOLUTION
A wall was built diverting flow from the Ohau channel (from rotorua) straight down the Kaituna river as to decrease the increased eutrophication occuring in lake rotoiti. Expensive but an expensive strategy to protect lake rotoiti.
