Most life in the oceans depends on sunlight that autotrophs are able to capture through pigments to photosynthesize and produce food. Different pigments absorb different wavelengths of light, allowing autotrophs to live at varying depths where certain wavelengths penetrate deepest. The rate of photosynthesis must exceed the rate of respiration for phytoplankton to survive, setting a compensation depth above which they can thrive. Coastal waters often have higher turbidity than the open ocean due to sediment and inputs that cloud the water and limit sunlight penetration.

1. Light in the Ocean

2. • Most life in oceans depends on
sunlight
• Autotrophs are producers who
capture light energy to make food

3. Pigments
• Capture light for plants
• Different plants may have different pigments
Ex: Chlorophyll, Xanthophyll, Carotenes, Phycobilins

4. Seaweeds with Different Pigments

5. Sun reaching the oceans’ surface may be:
1) Reflected back to atmosphere
– More when there are more waves
– Unusable by plants

6. 2) Absorbed – taken up by water, converted to heat
-most absorbed in first meter of depth (65%)
-<1 of light makes it past 100m
-So autotrophs must be near surface

7. Light
• Some wavelengths of
light are absorbed at
shallower depths than
others
• Open ocean: blue
penetrates deepest
• Plants living at different
depths have different
pigments to help them
absorb different
wavelengths of light

8. Compensation Depth
• Depth at which rate
of photosynthesis =
rate of respiration
• Above that depth
phytoplankton
make enough food
• Below that depth
phytoplankton
starve

9. Turbidity
• Cloudiness of the water
• Measured using a secchi disk
• Coastal waters = higher turbidity (lower secchi
disk depth)
– Q: Why? A: more organic matter, sediment,
plankton, sewage, etc.
– Water appears green/brown

11. Sources of Input that can cause Turbidity

12. Low Turbidity

13. High Turbidity