Exploring the Ocean: CTDs

Overview

Oceanography is the study of the ocean. It covers a wide range of topics from marine life to the geology of the sea floor to the physical properties of the ocean. Physical oceanographers study the physical components of the ocean including light, waves, tides, currents and the composition of sea water. The physical properties of the ocean can change with location and depth. Studying these properties can tell us a lot about the ocean, including information on current movement, nutrient content, heat transfer and how much life can be supported in an area. This lesson investigates how physical oceanographers study bodies of ocean water and their properties using CTDs. It allows students to analyze real data, practice reading graphs and graphing data themselves. Students will also create a salinity density column and deduce the source of unknown water samples using salinity and relative density.

Objectives

• Students will learn how oceanographers study properties of the ocean.
• Students will learn how to better analyze graphs and graph real-world data.
• Students will understand how temperature and salinity impact ocean water density.
• Students will create a salinity density column.
• Students will use reasoning to identify unknown water samples.

Lesson Preparation

Background Information:
Students should have an understanding of density and how materials with different density interact. Students should also have a general understanding of how temperature and pressure affect density.

Students should have a basic understanding of how to read a graph and what data are. It is helpful if students have been introduced to basic oceanography and ocean currents prior to this lab.

Material Preparation:
Prepare 5 batches of water of different salinities and different colors according to directions below. Place on stir plates if available. Label solutions A, B, C, D, E in non-sequential order.

Procedure

Part 1: Introduction and Data Activity
Go over the PowerPoint lecture with the students, introducing the importance of studying the properties of ocean water. Watch the video of CTD deployment to demonstrate how oceanographers study the ocean using CTDs. Review real-world data collected from CTD casts. As a group or individually, allow students to complete their worksheet 1. Discuss the data from the CTD and worksheet.

Part 2: Lab Activity
The lab technicians forgot to write down which water samples were collected where during the CTD! All you have is unknown water samples and the data from the CTD (Show data chart) to sort this out. With what you’ve learned, do you think you can determine which water sample came from where?

Allow students to brainstorm ways as a group. Matching temperature and pressure won’t work as the water is no longer at pressure and temperature has already changed since it has been brought up from depth. The best solution is to match relative salinities! If students can determine the relative salinities of the samples, they can compare it to the data from the CTD cast and determine where each sample was taken from.

Using the pipettes, students will carefully layer the water in their test tubes to determine the relative density of each solution and rank them from lowest salinity to highest salinity. After determining the relative salinities of the unknown samples, students will compare their results to the data from the CTD cast to determine the location the samples were taken and the “actual” salinities.

[NOTE: The real salinities of the samples will not correspond to the graph. Solutions are of exaggerated salinities for the purpose of this exercise.]

Extension

• Students can use provided data sets to create their own graphs in Excel or another graphing program.
• Students can use water that is warmed/chilled to create new, unique density columns and explore how temperature affects density.
• Students can further explore concepts of density with related lessons in the PolarTREC Learning Resources Collection
  • Ocean Currents and Salinity
  • That Sinking Feeling: Density Currents Lab
  • Sea Ice Impact and Density

Resources

• NOAA: What is a CTD?

• National Oceanography Centre: CTDs

• Window to the Universe CTDs

• NASA: Thermohaline Circulation

• NOAA: The Global Conveyor Belt

• Window to the Universe Circulation

• Window to the Universe Density

Assessment

Evaluation consists of post-lab discussion and feedback given on worksheets and graphs. Formal assessment may include teacher’s own test questions related to physical oceanography, CTDs and density.

Credits

This lesson was developed in a joint effort between educators and researchers during the PolarTREC 2015 East Antarctic Ice Stream Dynamics expedition.

Dominique Richardson, Cabrillo Marine Aquarium (dominique.richardson [at] gmail.com); Dr. Frank Nitsche, Lamont-Doherty Earth Observatory; Dr. David Porter, Lamont-Doherty Earth Observatory; Dr. Guy Williams, University of Tasmania; Dr. Alex Fraser, Hokkaido University; Dr. Raul Guerrero, National University of Mar del Plata.