FW Eco Lecture 8 Lake Heat

Why is heat important for lakes?

Heat controls mixing, organism distribution/physiology, oxygen and nutrient distribution, and thermal inertia.

What is the primary source of heat in lakes?

Solar heating.

What are other sources of heat in lakes?

Streams, air, groundwater, and subsurface geothermal inputs such as hot springs.

How does light change with depth in a lake?

Light decreases exponentially with depth.

Why doesn't lake temperature simply follow the pattern of decreasing light?

Although light is absorbed and converted to heat, other processes distribute heat throughout the water column.

How can temperature be measured at depth in a lake?

Thermometers, reversing thermometers, bathythermographs, temperature probes, and thermistors.

What does an isothermal temperature profile indicate?

The water column is able to mix from top to bottom.

Why is lake mixing important?

It distributes oxygen, nutrients, organisms, and other particles throughout the water column.

What does a non-isothermal temperature profile indicate?

The water column is stratified into distinct layers along a density gradient.

What is thermal stratification?

The formation of distinct water layers caused by differences in water density associated with temperature.

Why is warmer water found at the surface in summer?

Solar heating warms surface water, and warmer water is less dense than cooler water.

Why does ice float on lake water?

Hydrogen bonding causes water to become less dense as it freezes, allowing ice to float.

What are the three major thermal layers of a stratified lake?

Epilimnion, thermocline, and hypolimnion.

What is the epilimnion?

The warm, upper layer of a stratified lake.

What is the hypolimnion?

The deeper, colder layer beneath the thermocline.

What is the thermocline?

The depth where water temperature changes most rapidly with depth.

What controls thermocline depth?

Solar radiation and wind-driven mixing, including fetch.

What happens to a typical temperate lake through the year?

Spring turnover → summer stratification → fall turnover → winter inverse stratification.

What is spring turnover?

Mixing that occurs when the lake becomes approximately isothermal after winter and can mix from top to bottom.

What is fall turnover?

Mixing that occurs when cooling surface water reduces density differences and allows the lake to mix.

What is winter inverse stratification?

Cold water/ice occurs at the surface while relatively warmer, denser water remains below.

What is stability in a lake?

The degree to which lake stratification resists mixing by wind.

What determines lake stability?

The difference in density between water layers.

What is Schmidt stability?

The quantity of work required to mix the entire lake to a uniform temperature, measured in J/m².

What does greater Schmidt stability mean?

More wind energy is required to mix the lake.

How does temperature affect water density?

Water density changes with temperature, with density differences generally becoming greater per °C at warmer temperatures.

What is a mixing regime?

A classification of lakes based on how often they mix within a year.

What is a dimictic lake?

A lake with two mixing periods each year: spring and fall turnover.

What is the seasonal pattern of a dimictic lake?

Summer stratification → fall turnover → winter inverse stratification → spring turnover.

Where are dimictic lakes typical?

Northern U.S. and some reservoirs in southwestern Virginia; they must have winter ice cover.

What is a warm monomictic lake?

A lake that stratifies in summer and mixes continuously through fall, winter, and spring without ice cover.

Where are warm monomictic lakes typically found?

The southeastern U.S.

What is a cold monomictic lake?

A lake that stratifies under ice in winter and mixes during the warmer summer period.

Where are cold monomictic lakes found?

Very cold regions, such as northern Alaska.

What is an amictic lake?

A lake that never mixes, remains stratified, and is always covered with ice.

Where are amictic lakes found?

Extremely cold regions such as Antarctica.

What is an oligomictic lake?

A lake that is thermally stratified most of the year but cools enough for rare, short or partial mixing periods.

Where are oligomictic lakes typically found?

In the tropics, where there is no cold season and no cold hypolimnion.

What is an example of an oligomictic lake?

Lake Tanganyika.

What is a meromictic lake?

A lake that never completely mixes, typically because high salt concentrations create density gradients that prevent mixing.

Why can salt cause meromictic conditions?

Salt increases water density and can create a strong density gradient that prevents complete mixing.

How much does 1 g/L of salt increase water density?

About 0.0008 g/cm³.

How does salt-induced density change compare with temperature-induced density change?

1 g/L salt increases density about 0.0008 g/cm³, while the difference between 8°C and 9°C is only about 0.00007 g/cm³.

What does 1 g/L of salt equal in salinity?

1 ppt (part per thousand).

What is the approximate salinity of seawater?

About 34 ppt.

What is a polymictic lake?

A lake that mixes frequently throughout the year.

What factors can influence a lake's mixing regime?

Temperature, density, ice cover, latitude, depth, size, wind, and climate.

Why does lake size/depth matter for mixing?

They influence how easily wind energy can mix the water column and where stratification develops.

How is lake mixing related to latitude?

Different climates and temperatures at different latitudes produce different expected mixing regimes.

How are lake temperatures changing globally?

About 95% of lakes in the Global Lake Temperature Collaboration dataset are warming.

What is the Global Lake Temperature Collaboration (GLTC)?

A global effort studying changes in lake temperatures from 1985 to the present.

Which lakes are warming faster: lakes with or without seasonal ice?

Lakes with seasonal ice cover are warming faster.

How do lakes without seasonal ice generally respond to warming?

They tend to warm following air temperatures.

Why can lakes with seasonal ice warm especially rapidly?

Loss of ice exposes dark open water, which absorbs much more light than reflective ice.

How does decreasing ice cover create a warming feedback?

Less ice means less light reflected away, so more solar energy is absorbed by the lake, accelerating warming.

How is climate change expected to affect lake mixing regimes?

Warming can alter thermal stratification, stability, and the timing/frequency of mixing and may shift lakes into different mixing regimes.

Why are changes in lake mixing important?

Mixing affects the distribution of oxygen, nutrients, organisms, and other particles, influencing lake biota and chemistry.

What did the Lake Mendota climate model test?

How increased air temperatures affect lake thermal structure, mixing, and stratification.

What model was used to study Lake Mendota's response to climate change?

The GLM (General Lake Model).

What is the GLM used for?

Manipulating climate variables such as air temperature to explore effects on lake mixing and stratification.

What three Lake Mendota air-temperature simulations were compared?

2021 observations, 2021 +3°C, and 2021 +5°C.

What was held constant in the Lake Mendota warming simulations?

All other weather variables; only air temperature was increased.

What can thermal heat maps reveal about climate warming?

How water temperature and thermal structure change through time and depth.

What happens to Schmidt stability as lake warming increases?

Warming can increase stability by strengthening density differences between layers, making mixing more difficult.

Are warming effects on lake temperature and stability necessarily linear?

Not necessarily; feedbacks and changes in stratification/mixing can cause nonlinear responses.

What are the main take-home relationships for lake heat?

Temperature and density control heat storage, stability, and mixing regime, which affect lake biota and chemistry.

What are the key characteristics defining how a lake functions?

The frequency and timing of its mixing periods.