Water Quality Lessons
Students will explore dissolving, saturation, and super saturation. They will then apply this concept to read and understand a dissolved oxygen graph. Finally, they will look at the acceptable parameters for oysters for dissolved oxygen.
What is saturation?
What does it mean for something to be dissolved?
How do I read a dissolved oxygen graph?
- What is the right amount of dissolved oxygen for my oysters?
Materials and Resources
- Dissolved oxygen measuring device
- Stirring rod
- Hot soda
- Cold Soda
Before you get started
Tips for Teachers
The lab compares the dissolving of a solid and a gas in water and the effect of raising the temperature. Though in this activity it is meant to lead to a discussion of Dissolved Oxygen, it can also lead to a discussion about salinity or other water quality parameters. The dissolved oxygen graph requires rulers. It is impossible to read or interpret without them.
Just like humans, all aquatic living creatures – from the fish and crabs that swim through its waters to the worms that bury themselves in its muddy bottom – need oxygen to survive.
Air is a gaseous solution of nitrogen, oxygen, and a bunch of other things in smaller amount. Humans use their lungs to inhale oxygen from the air (air is about 20% Oxygen). But worms, fish, crabs and other underwater animals use gills to get oxygen from the water. As water moves across an animal’s gills, oxygen is removed and passed into the blood.
Gills work better when there is more oxygen in the surrounding water. As dissolved oxygen levels decrease, it becomes harder for animals to get the oxygen they need to survive.
Scientists generally agree that creatures need dissolved oxygen concentrations of 5.0 mg/L (which is the same as a part per million) or more to live and thrive. However, the amount of oxygen an animal needs varies depending on how large or complex the animal is and where it lives.
Worms and clams that live on the muddy bottom – where oxygen levels are naturally low – only need dissolved oxygen concentrations of at least 1 mg/L.
Fish, crabs and oysters that live or feed along the bottom require dissolved oxygen concentrations of 3 mg/L or more.
Spawning migratory fish and their eggs and larvae need up to 6 mg/L during these sensitive life stages.
Areas with less than 0.2 mg/L of dissolved oxygen are called anoxic. Most animals cannot live in these areas, which is why they are often called “dead zones.”
Oxygen gets into the water when:
Oxygen from the atmosphere dissolves and mixes into the water’s surface
Algae and grasses release oxygen during photosynthesis
Water flows into the harbor from streams, rivers and the ocean. Ocean waters generally have more oxygen than fresh water due to the constant movement and stirring of the water. River waters are fast-moving, which helps oxygen from the air mix in a little more than still water.
Most areas in the harbor that have low dissolved oxygen levels are the result of a complex interaction of several natural and man-made factors. These include temperature, nutrient pollution, how water flows in the Bay, and the shape of the Bay's bottom.
Temperature limits the amount of oxygen that can dissolve in water. The waters can hold more oxygen during winter than during the hot summer months.
Too many nutrients in the water (called eutrophication) can fuel the growth of algae blooms. Oysters, menhaden and other filter feeders eat a portion of the excess algae, but much of it does not end up being consumed.
The leftover algae die and sink to the Bay’s bottom, where they are decomposed by bacteria. During this process, bacteria consume oxygen until there is little or none left in these bottom waters.
Another factor influencing dissolved oxygen levels is the division between water flowing in from the ocean and out of the freshwater rivers and streams. Water flowing from the ocean is generally salty and cooler, while river water is fresh and warmer. Because of these differences, river water weighs less than ocean water and actually floats on top of it. (Wind and other strong mixing forces may change this pattern.)
The boundary where the fresh water layer meets the saltier water layer below is called the pycnocline. The pycnocline acts as a physical barrier that prevents the two layers from mixing together. During the summer, when algae-consuming bacteria are most active, the pycnocline cuts off oxygen-deprived bottom waters from oxygen-rich surface waters. This can create large areas of low or no oxygen at the bottom of the Bay.
Shape of the harbor
The harbor bottom is not flat; rather, it has varying shallow and deep areas. In certain bowl-shaped areas of the bottom, the pycnocline can act like a “lid” that cuts off bottom waters from receiving any oxygen.
Heavily adapted from http://www.chesapeakebay.net/discover/bayecosystem/dissolvedoxygen
Have students pass around clear plastic cups with sugar dissolved and pure water. The cups should look identical. Ask them to compare them. Do they see a difference? Finally, have them taste each beaker. (Make sure that students wash their hands and then dip one finger in each cup).
When students have determined the difference, ask them where the sugar is. Introduce the idea that something can be “dissolved.”
Dissolve- to cause a solute to pass into solution, or to cause a substance (solid, liquid, or gas) to become a part of a liquid solution.
Explain that the sugar has been dissolved into the water, that is why it can’t be seen.
Define the words saturated and supersaturated.
Saturated-The point at which no more solute can be dissolved into the solution
Supersaturated- more solute is dissolved than would normally be possible at the current temperature. These solutions can be created by gradually changing the temperature.
Have the students make a hypothesis about whether they believe that heating would cause more or less
Solid to dissolve
Gas to dissolve
Once the students have made their hypothesis, have them complete the saturation lab activity.
In a class discussion, compare solids and gases dissolved into liquid.
Explain the idea of dissolved oxygen:
Dissolved Oxygen is the amount of gaseous oxygen (O2) dissolved in the water. Oxygen enters the water by direct absorption from the atmosphere, by rapid movement, or as a waste product of plant photosynthesis.
Explain that dissolved oxygen is very important for oyster growth (and all sea life).
In order to emphasize the dependency of Dissolved Oxygen on temperature, have students complete “Reading the DO Graph”
Discuss DO parameters for oysters. 5 mg/L is the minimum that oysters generally need to survive. Why do we take DO measurements in the field? Why is it important? How might the data change over time and why?