In this investigation, you will explore thermal stratification and how it affects the placement of nutrients and dissolved oxygen. Lakes are different from streams and rivers because the water they contain is not quickly replaced by fresh water. In a lake, the flushing and changing of water can take anywhere from a year to 100 years, depending on the size of the lake and the watershed that flows into it. This makes lakes very susceptible to damage by pollution. Acid deposition is common in lakes and can result in acid shock if a lake has a low alkaline content or if the soils surrounding it have very little acid-neutralizing capacity. Acid shock can damage or kill aquatic life in the lake.
Lakes can be characterized in three ways. Lakes with large or excessive supplies of nutrients are called Eutrophic (well nourished). This type of lake is typically shallow and murky. Lakes with a small supply of nutrients are called Oligotrophic (poorly nourished). This type of lake is typically deep and clear with a blue or green color. Most lakes are somewhere in between, and are called Mesotrophic.
The density of water increases as the temperature decreases. When water reaches 4°C its density begins to decrease until it freezes. Because the density of water differs with temperature, lakes undergo a process known as thermal stratification. In summer, thermal stratification separates a lake into different regions at different depths. This prevents mixing of water and nutrients between the lake surface and the lake bottom. In fall, the water temperature decreases at the surface, and the cooler water sinks to the lake bottom. Because the water at the bottom of the lake is warmer than the sinking surface water, it begins to rise to the surface. This causes a mixing of the water which brings nutrients from the bottom to the surface, and dissolved oxygen in the surface waters to the bottom.
- Measure water temperature at different depths with a temperature probe.
- Use a Water Depth Sampler to collect water samples at different depths in a lake.
- Measure dissolved oxygen, pH, and total dissolved solids of the collected water samples.