Seawater composition: an introduction

Joceline Boucher, Corning School of Oceanography, Maine Maritime Academy,

Author Profile
Initial Publication Date: May 22, 2013 | Reviewed: January 16, 2015


In this activity, students collaboratively "build" the hydrologic cycle and use it as a starting point for thinking about the composition of seawater.

Used this activity? Share your experiences and modifications



This activity is designed for students in an introductory oceanography course.

Skills and concepts that students must have mastered

Students are expected to recall information about the hydrologic cycle that they learned in K12.

How the activity is situated in the course

Just prior to a unit on seawater chemistry.


Content/concepts goals for this activity

Through the activity, students will review the hydrologic cycle and learn where the cycle transports solutes.

Higher order thinking skills goals for this activity

  • Applying existing knowledge (building the hydrologic cycle from memory)
  • Extending the conceptual framework (hydrologic cycle) to new processes (transport of solutes)

Other skills goals for this activity

Description and Teaching Materials

The activity is a two part worksheet. The first part asks students to list Earth's major reservoirs of water, rank the reservoirs by size, suggest transport processes between reservoirs, and think about transfer rates between reservoirs. This part takes about 10 minutes and is done individually. The instructor then calls on students to supply one answer at a time; this is most effective if the answers are recorded pictorially at the board by the instructor or by students. Then students similarly rank reservoir size and transfer rates, with the instructor supplying corrections and background information as needed. An important part of the activity is ensuring that students revise their worksheets based on the classwork.

In the second part, students determine which portions of the hydrologic cycle are likely to transport solutes, and begin to think about the types and relative amounts of substances that comprise seawater. Lastly students predict if the same substances will be found in fresh water, and if they are more concentrated there. This part of the activity also takes about 10 minutes.

Together, the activity and subsequent discussions require an entire 50 minute class period.

The activity helps introduce a unit on seawater chemistry.

Teaching Notes and Tips

I find that conceptually connecting the transfer rates of water molecules in the hydrologic cycle, which students learn in the materials above, to the residence time of solutes in seawater is a helpful extension of this activity. For example, students know that the cycling of water in the atmosphere is rapid based on their experience and the classwork. This atmospheric cycle can be formalized as a "residence time": (Size of reservoir)/(rate of input or output) = residence time. For the atmosphere, this is (13,000 km3)/(434,000 km3/year) = 0.030 years = 11 days; in this case the rate of input is the global rate of evaporation, which also equals the global rate of precipitation. This is basically the same calculation one uses to estimate residence times of solutes in seawater: (total mass of solute in ocean)/(rate of input or output of solute).


I do not grade the worksheet but students are responsible its content on their next exam.

References and Resources