1. The Life Cycle of a Lake

2. Water Chemistry Basics Water freezes at 0° Celsius (32° Fahrenheit) Most substances get more dense as they get colder and less dense as they get warmer Water is WEIRD! – Water is densest at 4° C (39° F) – This means it becomes less dense as it warms up above 4° C. It also becomes less dense as it gets colder from 4° C.

3. Lake Cycle In One Year Lakes in temperate regions go through an annual cycle. They will undergo times of stratification and other times of mixing/turnover. – Stratification = distinct layers of water at particular temperatures occur in the lake – Mixing/Turnover = water temperature in the lake is generally the same

4. Lake Cycle In One Year - Summer Summer – Sun heats up epilimnion (upper lake) to warm temperatures. – The densest water (4° C) always sits at lake bottom. – The water stratifies by temperature/density layers as you go to surface with warmer, less dense water on top. – Oxygen and nutrients stay unmixed in their layers

5. Lake Cycle In One Year - Fall Fall – The upper layer becomes cooler and drops to 4° C and gets mixed with lower depths as the wind blows. – Now the lake is generally the same temperature, so water does not stratify into layers. The water moves about freely from deep to shallow depths. This is called turnover. – Turnover means that nutrients and oxygen is “refreshed” in the lake, as it freely moves about from where it was in summer.

6. Lake Cycle In One Year - Winter Winter – The densest water (4° C) continues to sit at the lake bottom – The upper layers are colder as the air temperature is below freezing. – The lake stratifies in temperature layers again. This time, the warmest bands are at the lake bottom. The colder and less dense water layers are closer to the lake surface. – Ultimately, the upper surface could be at or below the freezing point of water. This is why the lake freezes at the top, instead of the bottom.

7. Lake Cycle In One Year - Spring Spring – The cold weather starts to warm up in the spring and the upper surfaces get warmer. – They start to match the same temperature as the lake bottom ( 4° C ). – Since the lake is generally the same temperature, the stratified layers of different water temperatures go away and the water moves about freely between different depths. – Another turnover cycle occurs.

8. Lake Cycle Over Millions+ Years The natural process in which lakes age is called eutrophication. This means that nutrients get added into them over time. Eutrophication is NATURAL! – Humans can accelerate the process by adding nutrients at a faster rate. This process is called cultural eutrophication. The 3 main stages of a lake’s life are: Oligotrophic, Mesotrophic, and Eutrophic Oligotrophic Mesotrophic Eutrophic

9. Oligotrophic Lake Little to no nutrients added to the water. Low algae/plant growth Lower levels of biodiversity (not as much plant growth to start food chain) Cold water temperatures High dissolved oxygen Sandy/rocky bottom

10. Mesotrophic Lake Moderate nutrients added to the water. Medium algae/plant growth Moderate levels of biodiversity Moderate water temperatures Midrange levels of dissolved oxygen Slightly mucky bottom

11. Eutrophic Lake High levels of nutrients added to the water. High algae/plant growth Highest levels of biodiversity – extensive plant life fuels extensive food webs Warm water temperatures Low dissolved oxygen Lake bottom with lots of organic muck

12. Life of a Lake Over Millions+ Years Summary – Eutrophication is the natural process of lake aging when nutrients gradually get added to the water. – Lakes go from oligotrophic to mesotrophic to eutrophic as more nutrients get added – Cultural eutrophication is when humans accelerate this process by adding nutrients into waterways via their activities.