Lakes Flashcards
What is Schmidt stability
States level of resistance to mixing.
What is wedderburn number?
A number indicating the likelihood of wind-driven upwelling events (when hypolimnion comes to the surface) If Wedderburn number is < 1, thermocline is likely to tilt to the surface allowing the epilimnion to mix with the hypolimnion
What is lake number?
A number indicating the likelihood of wind-driven internal mixing events.
Which factors promote instability in the water column
-Large, deep lakes -deep metalimnion -low temperature differntial -long fetch
How and why do emergent plants provide oxygen to roots?
- Gas tubes in the stem take oxygenated gas from the upper part (exposed part) and pump it down to the roots. - Without that process they would not be able to maintain roots in reducing sediments at great depths.
Why are emergent plants vital in small lakes?
-Buffer role for lake systems -Modifying water as it moves from groundwater & land through to emergent vegetation, -slowing the flow of water & changing geochemistry, -Habitat for wildlife.
Describe the mixing stages of a monomictic lake (warm and cold)
- A monomictic lake mixes once a year.
- Warm monomictic lakes never freeze and are thermally stratified during summer. Winter, Autumn, Spring = Holomixis.
- Cold monomictic lakes freeze over during winter so there is no mixing. Spring, Summer, Autumn = Holomixis.
What is the Profundal Zone?
The Profundal zone is the deep, dark bottom part of a lake where there is no primary production therefore material falling down from the shallower depths are relied on as a source of energy.
What is the Pelagic Zone?
- The Pelagic Zone is the middle, open part of a lake.
- Phytoplanktonic processes dominate
- Phytoplanton, Zooplankton, Fish
Why is Stratification important?
- Seperates photosynthesis from the bottom of the lake
- Oxygen not replaced
- Material in hypolimnion decomposes
- Bacteria respires away oxygen
- Small, organic-rich hypolimnion can become anoxic
What are optically active materials?
Materials that absorb or scatter light in water
- water
- cdom
- sediments
- algae
What are the features of a glacial lake?
- Found in mountainous areas
- Ice gouged out basins form most of the south island lakes
- deep, steep-sided, morraine-dammed, elongated, large
- Lake Wanaka
What are the features of a Dune lake?
- Low nutrient
- small, shallow, oligotrophic
- formed behind wind-blown sand and sediments
Lake Ototoa
Features of a coastal barrier lake
- formed behind bars created across river mouths by large longshore drift
- shallow, fertile, young
Features of a Volcanic Lake?
- Created in the hollows of volcanic craters for most of the larger North island lakes
- Often steep sided, round, small catchments, very old
Lake Taupo, Lake Rotorua
Features of Artificial Lakes?
- Caused by human activity
- Usually hydro or mining
- Mainly in NZ reservoirs for potable water and hydro electricity generation
Example of a permanently mixed lake?
Lake Heron
What is a Polymictic Lake?
- Found mostly in the tropics
- Generally shallow and exposed to winds
No set pattern for stratification and mixing stages
Lake Rotorua
What is an Amictic lake?
Perenially stratified always covered by Ice and mixing can be years apart
Features of a Meromictic Lake?
- Permanently chemically stratified
- Has chemocline
- Permanently cmemically stratified
- Lake is so deep there is not enough energy to mix it from top to bottom
- Occasionly stirred up my major storms
- Sulphuric acid build ups in the hypolimnion can be lethal once released
Top 3 mixing regimes in New Zealand
- Monomictic
- Polymictic
- Dimictic
- Holomictic
What affects the ammount of 02 in hypolimnion at a given time?
- Size (big and deep - Shallow and small)
- Concentration at onset of stratification
- Rate of consumption (temp, amount of organic material)
The rate of depletion of 02 is a proxy for…
..the amount of organic material in the lake hypolimnion and sediment
If a lake is becoming more productive, oxygen depletion rate will increase
VHOD is the accronym for
Volumetric hypolimnion oxygen depletion rate
(mass of oxygen removed per mass of hypolimnetic volume per unit of time)
What is the main force that tends to act to destabilise the water column?
Heat
Flux
Wind
What is the name of process whereby nitrate is converted to nitrogen gas?
Denitrification
In what form does N & P enter the lake?
N goes in as dissolved matter
P goes in as particulate matter
Zg =
depth at which the centre of gravity is
