ABSTRACT
The ecological influence of light is different on fauna and flora. While current and temperature are the two fundamental ecological factors in running waters, other factors may also play a role at variable degrees, namely, light, salts, and dissolved oxygen. Light and dissolved salts are important mainly for flora and dissolved oxygen for fauna. In plants, light is an energy factor that governs photosynthesis. In animals, it has only an indirect role, determining phases of activity, orientation, and seasonal rhythm. Phototactism leads to a daily rhythm of activity, which is expressed by a daily rhythm of drift. In summer, variations in light intensity play an essential role in the activity and peaks of drift. As with terrestrial flora, aquatic flora is a better ecological indicator than fauna because, besides the current and temperature, it reflects the penetration of the light and the qualities of the water and the substrate.
