How is light intensity measured in aquatic ecosystems?

Light intensity in aquatic ecosystems is measured using a device called a Secchi disk or a light meter.

A Secchi disk is a simple, yet effective tool used by scientists to measure the transparency or turbidity of water, which indirectly indicates the amount of light penetrating the water body. The disk is a circular plate, typically 30 cm in diameter, divided into quadrants painted alternately black and white. It is attached to a long, marked rope and is lowered into the water until it is no longer visible. The depth at which it disappears, known as the Secchi depth, is recorded. This depth is inversely proportional to the turbidity of the water; the clearer the water, the deeper the disk can be seen, and thus the higher the light intensity.

A light meter, on the other hand, is a more direct method of measuring light intensity. It is an electronic device that measures the amount of light in a given area at a specific time. In aquatic ecosystems, underwater light meters, also known as submersible quantum sensors or PAR (Photosynthetically Active Radiation) sensors, are used. These devices measure the amount of light available for photosynthesis, which is the light in the 400 to 700 nanometre range. The readings are usually given in microeinsteins per square meter per second (µE/m²/s).

Both methods are crucial in studying aquatic ecosystems as light intensity affects various aspects of these ecosystems. For instance, it influences the distribution and abundance of aquatic plants and phytoplankton, which are the primary producers in these ecosystems. Light intensity also affects the behaviour and distribution of aquatic animals. For example, some species are adapted to low light conditions and can only be found in deep waters, while others require high light intensity and are found near the water surface. Therefore, measuring light intensity helps in understanding and predicting the dynamics of aquatic ecosystems.