Water is a great conductor of sound, but not of light. Most sunlight that hits a lake or ocean is absorbed within the first 20 – 50 m depth. Light intensity varies with time of day and season and its rate of absorption depends on the amount of dissolved particles (usually organic) in the water column.
Sunlight contains many different colors (which are separated in a rainbow). Water mostly reflects, rather than absorbs, blue light waves, which is why the sky and water bodies are blue!
LIGHT IS IMPORTANT FOR PLANTS AND ANIMALS
Plants require light for photosynthesis. Loss of light in deep or mucky water limits the depth to which many plants can grow. Often the majority of plants in lakes and ponds are found close to the shore. Some plants have adaptations to float on the surface of the water and are able to receive the direct sunlight. Some types of cyanobacteria, photosynthetic bacteria, are able to change their position in the water to receive more sunlight.
Many aquatic animals also need to see to find catch, find mates, or avoid predators. Increases in turbidity caused by suspended sediments or plant material restrict this ability to see under water. Some fish, such as catfish, have evolved other mechanisms for surviving in dark or turbid waters. In lakes, many zooplankton (tiny animals that live suspended in the water,) and bottom dwelling macroinvertebrates take advantage of the more abundant food at the surface while also avoiding being eaten by fish by “migrating” down into darker waters during the day and moving up to the surface waters at night.
HOW IS LIGHT MEASURED?
There are multiple ways to measure light. The easiest way is using a Secchi disk. Learn how to make your own Secchi disk in the Utah Lakewatch Kids Book.
Utah Water Watch - Utah's citizen monitoring
Secchi Dip-In - nationwide citizen monitoring
Utah Lake Watch Reports - find Secchi data through 2011 for Utah