Nature's Density Column

Nature creates its own density column in marine systems, which is extremely important for the triggering of the spring bloom of phytoplankton. Phytoplankton are an important food source for all organisms- from microscopic zooplankton to large marine mammals such as walrus and whales. The different salinities (and therefore, different densities) of water help to stabilize the water column (by limiting mixing) and keep the phytoplankton near the light, which is one factor necessary for the spring bloom to occur, especially in areas like the Arctic. Students will build their own density columns with different substances and answer questions in a worksheet to reinforce concepts learned.

Objectives

The goal of this lesson is to convey a part of my research to the students, and I want to tie in what they are currently studying to my research. I also want them to see that nature can create its own variations in density. My research is centered in the Bering Sea, and I wanted to have the chance to show students some of what we studied while we were in the Arctic. Students should walk away with the understanding that density columns exist in nature.

Lesson Preparation

Set up projector for PowerPoint presentation and prepare lab materials and worksheets for students. The students can be grouped into teams of four to complete the lesson.

Background information:

Density columns can form in water that has different salinities. These kinds of density columns are extremely important in nature- in the Arctic the ice melts and forms a freshwater layer on top of the saltier ocean water. Inside this ice are phytoplankton that melt out. The water is in two layers (freshwater on top of salt water). The phytoplankton stay in the freshwater layer because it cannot easily mix with the saltier layer below it. This keeps them at the surface where they are exposed to lots of sunlight, which allows them to expand their numbers. This is important because they are an important food source for other organisms- most of the marine ecosystem depends on the phytoplankton in some way for food.

Procedure

1. Elicitation questions:

   • Can water exist at different densities? (prior knowledge that water has a density of 1.0g/mL)

   • Which do you think is more dense, fresh or salt water? Can you think of a way to test which one is more dense?

2. After discussing the above questions with the students, have them create their own density columns using salt and fresh water (in the form of ice). They will do this in groups of four. The salt water will already be prepared (and dyed) for them. Colored ice will be available. Students will put together the column and observe what happens as the ice cube melts (observations will be recorded in a data table).

3. As a class, discuss their observations and what they learned about the density of different types of water. Talk about some real life examples of natural density columns and why they are important- using specific examples from the Bering Sea. Specific topics covered could be:

   1. The importance of density in the stabilization of the water column

   2. The presence of phytoplankton in the bottom of the sea ice- which are released into the freshwater layer as the ice melts. These phytoplankton help to start the spring bloom in the water column.

   3. A stable water column with a freshwater layer keeps the phytoplankton at the top, where they are exposed to more sunlight than if they were in deeper parts of the water column. Since phytoplankton are microscopic plants, being exposed to sunlight helps them to grow in number.

   4. Phytoplankton go to feed all other organisms in the marine environment. Using a PowerPoint presentation, students will view graphs of the water column as it goes through different stages of ice cover, and see how as the layers form, the chlorophyll increases in the less dense freshwater layer.

During the PowerPoint, students will be in charge of filling out a worksheet. At the end, students will fill out a couple of short evaluation questions that address some of the concepts covered in the lesson, and bring them back to the discussion that we had at the beginning of the lesson

In the event that your lesson runs short of the allotted time, you can extend the discussion of density in nature and talk about other examples besides the Bering Sea where they might see a density column form. If your lesson is running long, you can shorten the discussion about real world density columns.

Extension

ELL/SPED Modifications: Pictures and diagrams will be used along with discussion, and students will work in groups. The evaluation/lab sheet can be modified to meet the needs of other classrooms if necessary.

Assessment

All work will be completed in class. There will be a worksheet for the students to fill out about the lesson and assess their understanding of the concepts covered.

Credits

Emily Davenport, emilysdavenport [at] gmail.com