Lab 5 – Investigating Density and Stratification in the Ocean
Instructor Guide

The density of seawater plays a vital role in driving ocean circulation and determining primary production. In this lab your students will explore how salinity and temperature affect the density of seawater and how density can change with season and location.

In oceanography, density is used to characterize and follow water masses in studying ocean circulation. Density is a measure of the compactness of material, or how much mass (stuff) is “packed” into a given volume (space). It is measured as the mass per unit volume. The majority of the ocean has a density between 1020 and 1030 kg/m3. The density of seawater is not measured directly; instead, it is calculated from measurements of temperature, salinity, and pressure.

Most of the variability in seawater density is due to changes in salinity and temperature. A change in salinity is from changing the mass of dissolved salts in a given volume of water. As the salinity of seawater increases, the density increases. A change in temperature of seawater results in a change of volume for a given mass of water. An increase in the temperature of seawater causes the volume of a water parcel to increase and its density to decrease. The temperature and salinity of seawater can change dramatically with depth, or be pretty stable, depending on many different factors.

Temperature of surface areas vary with latitude with the warmest surface waters found at low latitudes. With very few exceptions, the temperature of seawater decreases with depth. A typical seawater temperature-versus-depth profiles consists of three temperature layers. The figure below shows a typical temperature profile for the temperate ocean.

Typical temperature profile for temperate ocean. Depth on y-axis, temperature on x-axis. Mixed later and thermocline labeled.

Public domain via Wikimedia Commons

Since most of the energy to heat the ocean comes from incoming solar radiation, only a thin layer at the surface of the ocean is heated directly. Winds mix the heat to deeper depths and the depth of the mixed layer depends on the season and the strength of winds. The mixed layer overlies the thermocline, where temperature decreases rapidly with depth. Beneath the thermocline, temperature is homogenous and cold.

 

Salinity of surface seawater also varies as a function of latitude, but unlike temperature, the vertical profile of salinity can increase or decrease and change with latitude and season. The figure below shows changes in seawater salinity with depth for equatorial, subtropical and polar latitudes. In all three profiles, there is a surface mixed layer of relatively constant salinity. Beneath the mixed layer at high latitudes, salinity increases with depth and low latitudes, salinity decreases with depth.

 

This figure shows how ocean water salinity varies with depth at different latitudes. Increase depth on the y-axis and increasing salinity on the x-axis.

From: Applications and Investigations in Earth Science (8th Edition)

This lab is divided into four scaffolded activities that should be completed in order, although the instructor can stop the lab after any particular activity if the necessary learning outcomes have been met. The activities are intended for undergraduate students in Introductory Oceanography courses (for either marine science majors or non-science majors).

Approximate time involved:

  • Each of the 4 activities may take 30-90 minutes for students to explore, discuss and complete the assigned questions.
  • Instructors should plan to implement one activity per 50 or 60 minute lecture course, or
  • Activities could be assigned in a 3-hour computer lab, or
  • Activities could be assigned as a weekly homework assignment.

Learning Outcomes:

  • LO1. Students will demonstrate basic data literacy in graph interpretation by identifying changes in temperature and salinity with depth.
  • LO2. Students will explain how T and S relate to density stratification and stability of water masses in the ocean.
  • LO3. Students will describe the development of a seasonal pycnocline and explain the differences between temperate and polar latitudes.
  • LO4. Students will predict what has a more controlling effect on density – temperature or salinity.
Learning outcomeActivity 1Activity 2Activity 3Activity 4
Outcome 1AppliedAppliedAppliedApplied
Outcome 2AppliedAppliedAppliedApplied
Outcome 3n/aIntroducedAppliedn/a
Outcome 4n/aAppliedn/aApplied

Materials needed:

  • No materials needed
  • All figures are included

What students should know before this activity:

  • Data literacy: Read graph axes, determining depth ranges from profiles, outliers, averages, compare and interpret patterns.
  • Content knowledge:
    • Students should have already been introduced to the concept of density and the factors that determine seawater density.
    • Students should have been introduced to the factors that determine seawater temperature and salinity.
    • Students should be able to identify the geographic location(s) of the sampling location(s).
    • Students should be able to describe trends of a measured variable with water depth.
    • Students should be able to read data points from a water column profile.
    • Students should be able to plot data points on an x-y line graph.

What instructors should know before this activity:

  • Density is defined as mass per unit volume of a substance. Water density is usually measured in grams per cubic centimeter.
  • The three primary factors that determine seawater density are pressure, temperature and salinity.
    • The pressure effect is usually small since the water molecule is nearly incompressible.
    • Water density is very sensitive to temperature change. There is a inverse relationship between seawater density and temperature. As seawater temperature increases, density decreases. This results from the volume change in water as it is heated and cooled.
    • When salts are dissolved in water, the density of the water increases as salts have a greater density than water.
  • Temperature of the surface ocean is primarily determined by solar heating. Wind mixing distributes heat to deeper depths.
  • Stratification describes the layering of water properties relative to depth. While density increases with depth, it does not necessarily do that at a constant rate. Layers where properties are changing rapidly with depth are called “clines”, so where temperature changes quickly is the thermocline, where salinity changes fast is the halocline, and where density changes rapidly is the pycnocline. Oftentimes, there are regions where there is no change with depth, and these are called mixed layers. In a stable water column, the density increases with depth. When stable, it takes a lot of energy to mix water between any two layers. If a change in temperature or salinity occurs that results in a layer of dense water being above less dense water, the water column is unstable. The denser water then sinks until it reaches a depth that is of the same density (called an isopycnal). An unstable water column in polar regions is the main driver of thermohaline circulation, which affects climate. Overturn in the water column caused by variations in density can affect timing, magnitude and location of biological productivity.
  • There are many different combinations of temperature and salinity that produce the same seawater density. This can be illustrated in a temperature-salinity or T-S diagram. Oceanographers use T-S diagrams to identify vertical structure of the water column and water masses.

Optional pre-lab activities: Textbook readings (Chemistry of Seawater & Ocean Structure and Circulation)

Teaching Notes

  • Procedure: These data lab activities should be introduced after students have covered introductory concepts on temperature, salinity, and density. Explain that this exercise looks at changes in temperature and salinity over extended time periods and in different locations. Emphasize that changes can occur quickly. Also note that pressure (depth) is not addressed in this lab because T and S have more of a controlling effect on density due to the incompressibility of water.
  • Commonly encountered issues/questions/misconceptions:
    • Misconceptions for static water column figures:
      • Water column always looks this way at the location/latitude
      • At all locations in the ocean, temperature decreases and salinity increases with increasing water depth.
      • Water at depth ‘came from’ the surface at the same location. Water at the surface sunk to depth at the same location.
    • Emphasize that all answers should have the proper units.
  • Adaptations for different course levels and duration of activity (lab vs. lecture period): Activity is stand alone and may be used as a lab activity, in-class exercise, or homework exercise.
    • Extensions: Activity can be used in an upper-level course as a refresher on density stratification in the ocean.
  • Reflection prompts:
    • Did the labs you just completed help you answer the questions above about salinity, density and stratification?
    • What parts of the activities you just completed on these topics (salinity, density and stratification) were the most useful in helping you learn about these concepts?

Answer key

  • Sample answers will be supplied to instructors upon request