Lec 10: Lakes II

Question 1

How long is lake residence time?

Answer 1

the average time that water spends in a lake

Residence times in lakes can be as short as days, but typically are longer, e.g. weeks, months up to >100 years

Question 2

Describe the 5 biological zones of a lake

Answer 2

1) Littoral: shoreline

2) Limnetic: Open area zone. includes photic and aphotic zones

3) Photic
- where sunlight reaches
- more oxygen because of increased contact with air
- fish and algae

4) Aphotic
- below thermocline
- less life because less oxygen
- no light

5) Benthic
- bottom of the pond
- grows as the pond ages

Question 3

At which temperature is fresh water’s density the highest?

Answer 3

3.98 C

Question 4

What is lake stratification?

Answer 4

Lake stratification is the layering of water due to density differences (salinity and temperature)

denser water is at 4C and salty

Question 5

What are the three layers of lake stratification?

Answer 5

• Epilimnion
  warm and rich in oxygen
• Metalimnion
  middle layer with steep temperature gradient = thermocline
• Hypolimnion
  bottom layer, cold, dark, oxygen depleted

Question 6

In which layer is the thermocline?

Answer 6

the metalimnion

Question 7

Which layer is the littoral zone in?

Answer 7

in the epilimnion

Question 8

What are the main types of lakes defined by mixing behavior

Answer 8

• Holomictic (mixing throughout entire lake)
• Meromictic (some layers never mix)
• Amictic (no mixing, due to continuous ice cover)

Question 9

Describe mixing at different seasons

Answer 9

spring
- everything mixes, everything at 4C
- oxygen levels same everywhere

summer
- everything below thermocline: 4C
- everything above: mixes with each other due to wind, everything at 20C
- oxygen decreases from epilimnion to hypolimnion

fall
- everything thing mixes, everything at 10C
- oxygen levels same everywhere

winter
- below thermocline: 4C
- above thermocline: 1C
- nothing mixes due to no wind cause of ice cover
- oxygen lowest in hypolimnion, highest in metalimnion, and medium in epilimnion

Question 10

How is oxygen introduced into lakes?

Answer 10

atmosphere and/or from photosynthesizing organisms (in photic zone)

Question 11

What properties affect lake mixing? (6)

Answer 11

► Change in temperature (climate zones)
► Wind (open space versus well shielded lakes)
► Size and depth of lake
► Bathymetry and shape of lake
► Inflow/outflow of lake (including groundwater flows)
► Ice cover