The water column structure refers to the vertical distribution of water layers in oceans, lakes, and rivers. It is typically divided into zones based on temperature, salinity, and density. The upper layer, known as the epilimnion, is warmer and well-mixed, while the deeper layer, called the hypolimnion, is colder and denser. Between these layers lies the thermocline, where temperature changes rapidly with depth. In marine environments, the water column also includes the photic zone, where sunlight penetrates, allowing photosynthesis. Below this zone, the aphotic zone exists, where light is absent, affecting the types of organisms that can thrive at different depths.