Sunlight penetrates the topmost ocean surface, below that it is always dark. The sunlit top ocean layer is called the photic zone and its depth ranges from a few to hundreds of meters. The bottom of the photic zone is the depth at which the light intensity is about 1% of its intensity at the ocean surface. At any location, the photic zone depth is not constant because of variations in water clarity and daily-to-seasonal changes in the angle of incoming sunlight.
Ocean color is linked to the optical properties of water and how light is absorbed and scattered by dissolved material and suspended particles, including phytoplankton cells. In clear tropical oceans, sunlight penetration is very deep and waters appear indigo blue in color primarily because of the optical properties of water itself. Conversely, the photic zone is shallow where biological productivity is high; in such turbid waters, phytoplankton cells absorb sunlight for photosynthesis while scattering green and yellow light. Ocean color satellites are optimized to detect the range of colors reflected from the ocean surface. Data from these sensors are used to monitor the concentration of chlorophyll -- an indication of phytoplankton concentration -- in the upper ocean.
  • Observe reflection, scattering and absorption of light
  • Discuss how ocean color depends on scattering and absorption of sunlight
  • Consider how sunlight's penetration affects photosynthesis
  • Understand the differences between transparent, translucent and opaque liquids
4 - 6 teaspoons each of two colors of liquid dye ('A' and 'B'; preferably dark blue and dark green); 16 oz. liquid antacid (magnesium hydroxide + aluminum hydroxide), Four 20 oz. clear plastic drink bottles with twist-on tops (labels removed), Two identical shallow baking dishes, 24 pennies (approx.), Duct or masking tape, Liquids measuring cup, Teaspoon, Flashlight, Disposable gloves (optional)
To introduce this activity, you may wish discuss the concept of the photic zone and its importance to life on Earth. Primary producers that use sunlight to photosynthesize -- and all members of the marine food web -- depend on this sunlit layer. This activity illustrates how solar radiation is absorbed and scattered where phytoplankton concentrate and why -- in those turbid areas -- sunlight only penetrates to a shallow depth.
  1. Prior to class, put 2 - 3 teaspoons of Dye A into two 20 oz. bottles and the same amount of Dye B into the other two bottles. Twist caps on tightl and label them 'A' and 'B'.
  2. During class, select four students to help with the demonstration. Ask them to cover the top half of each bottle with tape (i.e., from the middle of the bottle to just below its cap). Have them slowly pour 12 oz. (1.5 cups) of water into each bottle and twist on cap tightly. Gently swirl the dark liquid within the bottle without inverting the bottle. Ask the class to describe the liquid inside each bottle. (The concentrated dyes will appear black in color.) Can they guess what these liquids might be? (You may find that preparing the liquids and bottles ahead of time is more effective at keeping students 'in the dark' about the bottles' contents. )
  3. Next, have one of the students with a bottle marked 'A' add 1 cup of liquid antacid to his or her bottle. The other student should slowly add 1 cup of water to his or her bottle 'A'. Those with bottles marked 'B' should do the same. Observe the results.
  4. Ask the students to explain why they are unable to detect the liquids' color without antacid. (Antacid acts as a scattering agent, without it light is fully absorbed and not reflected back to the eye.)
  5. Have two students (who understand that they might get temporary stains on their fingers) to stack 12 pennies in each of two shallow baking pans. They should agree as to whether they wish to work with dye 'A' or 'B'. Have them slowly and carefully pour the bottle without the antacid in one pan and the bottle with antacid in the other pan. (Stacked pennies should protrude above the liquid... if not, add more to each stack and keep track of the total.) Have students shine a flashlight on the liquid without antacid, removing pennies until the submerged ones cannot be seen. How many are left? Just before repeating this process using the liquid with antacid, have students predict whether they'll remove more or fewer pennies and why.
  6. Shine a flashlight at various angles to explore how light reflects off the surface of the liquid in each pan. (Using a white sheet of paper to 'catch' the reflected light may be helpful.) Does this illustrate how sunlight reflects differently off clear and turbid ocean waters?
  • Discuss how your results relate to sunlight penetration, scattering and reflection in the ocean.
  • Add a 'control' bottle that has 12 oz. of water and 8 oz. of liquid antacid. Perform Step 4 with this control mixture in one pan and pure water in the other. How does this show you are observing an optical -- not chemical -- phenomenon? Then discuss differences between transparent, translucent and opaque liquids.
Bigelow Laboratory for Ocean Sciences, Copyright 2000