Accurate knowledge about the quality of water bodies is crucial for natural resource management. A significant part of the biological water quality is related to the chlorophyll-a content of the phytoplankton. Their spatial distribution is very inhomogeneous in the water bodies, depending on several influencing factors such as nutrient availability, water temperature, wind and underwater light conditions. To study this complex system, a long-term observation and sampling experiment was carried out on Lake Naplás, Hungary. As the first step, a database was developed of the actual vertical distribution of algae together with the most important influencing factors: underwater light conditions measured from ultra-violet (UV) to infrared (IR), water temperature and a number of selected chemical parameters. During the observation period, 34 campaigns were carried out at three observation points, and more than 1020 samples and in situ measurement data were collected. The measured parameters included water temperature, UV radiation, underwater light conditions (350-800 nm), chemical parameters (Fe, NO₂^-, NO₃^- NH₄⁺, PO₄^3-, pH, hardness) and chlorophyll-a content. The results showed that the main influencing parameters of the vertical distribution of chlorophyll-a were the water temperature, UV radiation and the available light.