How does sunlight change as it travels deeper through water?

As sunlight travels deeper into an inland water ecosystem, its intensity decreases exponentially. This reduction is primarily driven by the turbidity of the water, as light is absorbed by water molecules, dissolved substances, and suspended particulates, or scattered by these elements.12

The vertical distribution of light creates two distinct biological zones:

• Photic (or Euphotic) Zone: The upper layer of the water where sufficient light penetrates to support photosynthesis. In this zone, the production of biochemical energy by primary producers exceeds the energy consumed through respiration.31
• Aphotic (or Tropholytic) Zone: The deeper region where light levels are insufficient for photosynthesis. Here, the consumption of energy through respiration and decomposition exceeds its production.31

These zones are separated by a compensation depth, where the rate of photosynthetic production is exactly matched by the energy lost to respiration. This plane is not static; it can shift diurnally and seasonally based on changes in light penetration. Because light also serves as a source of heat, its vertical attenuation is a fundamental factor in the thermal stratification of lakes.134