Seasonal variation in light, mixing depth and primary productivity in temperate northern hemisphere waters

Lauren Sahl and Jim McKenna,
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In this exercise students work with light, temperature, and phytoplankton biomass proxy (chlorophyll a concentration) data to;
  • Become more skilled in reading and interpreting semi log graphs, temperature profiles, and time series plots.
  • Practice unit conversions.
  • Gain an understanding of k, the attenuation coefficient for nondirectional light.
  • See how the depth of the photic zone and the surface mixed layer varies seasonally at temperate latitudes and how this relates to seasonal phytoplankton productivity dynamics.

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This exercise is used in an undergraduate Introduction to Oceanography course for non-majors. For many students it is their first college level science course.

Skills and concepts that students must have mastered

Students have been introduced to the concept of the surface mixed layer prior to completing this exercise.

How the activity is situated in the course

This exercise follows examination of the structure of the water column. It occurs prior to discussion of life in the ocean.


Content/concepts goals for this activity

  • Reinforce concept of water column structure.
  • Link water column structure and photic zone depth to primary productivity.

Higher order thinking skills goals for this activity

  • Students analyze data by plotting time series of different variables and then interpret the plots.
  • Students hypothesize about complex relationships between biology and the physical/chemical environment in the ocean.

Other skills goals for this activity

Students practice working with a semilog graph.

Students calculate a conversion factor to do a unit conversion. This can later be applied when manipulating data in spreadsheets.

Description and Teaching Materials

Light in the ocean - student handout (Microsoft Word 2007 (.docx) 283kB Jan24 14)
Light in the ocean - instructor solutions (Microsoft Word 2007 (.docx) 267kB Jan24 14)

Teaching Notes and Tips

  • Many students do not know how to read semilog graphs. To bring everyone up to speed count, with the class, through several cycles.
  • This exercise helps students see how data are used to understand a scientific process.
  • You may mention that students will derive the light intensity equation when/if they take differential equations.
Students are asked to hypothesize about complex relationships between biology (productivity) and physical/chemical oceanographic processes prior to discussing these relationships in class. The exercise is a way to get them thinking independently and critically about these relationships. They may not form the correct hypotheses but the objective is to get them thinking about these relationships and increasing awareness and understanding of these interactions when they are eventually discussed in class. This process may require additional guidance from the instructor to help students develop confidence in developing their own informed hypotheses.


Students are tested on the content of this exercise.

References and Resources

Knauss, J.A. 1978. Introduction to Physical Oceanography. Prentice-Hall, Inc. Englewood Cliffs, N.J.

Open University Course Team. 1995. Seawater: Its Composition, Properties and Behavior. 2nd ed. Jointly published by The Open University and Pergamon, an imprint of Elsevier Science Ltd., Oxford.

Sverdrup, H.U., Johnson, M.W. and Fleming R.H. The Oceans, Their Physics, Chemistry and General Biology. 1942. Prentice-Hall, Inc., Englewood Cliffs, N.J.