The northern Adriatic is a highly structured and shallow ecosystem characterized by numerous spatial and temporal ecological gradients such as temperature, salinity and nutrient salts. Such conditions allow us to monitor the adaptations of phytoplankton organisms, especially diatoms, which largely dominate the phytoplankton community of the study area. The growth of phytoplankton, and thus diatoms in the northern Adriatic is controlled by available light, temperature, and nutrient availability (mainly phosphorus), which depends on the Po River. In order to understand the strategies that diatoms use when compeeting for resources, in this doctoral thesis, eight in vitro experiments were performed on nine different diatom species (Leptocylindrus aporus, Leptocylindrus hargravesii, Chaetoceros decipiens, Chaetoceros peruvianus, Skeletonema grevillei, Thalassionema frauenfeldii, Thalassionema cf. nitzschioides, Pseudo-nitzschia delicatissima and Pseudo-nitzschia calliantha). Experimental measurements included analyzes of changes in the growth rate of each species at different orthophosphate concentrations, phosphate uptake rate, data on activity, localization and activation of alkaline phosphatase (AP) enzymes, and morphological adaptations of each species to phosphorus stress. The objectives of this study are to characterize the adaptations of selected diatom species from the northern Adriatic to spatio-temporal changes in phosphorus concentrations in the ecosystem, to describe individual species strategies in phosphorus limitation conditions and use these strategies to compete for resources within natural phytoplankton communities. The data obtained by experimental measurements were compared with long-term changes in order to better understand the dynamics and ecology of a particular species and its adaptation to climate change in the North Adriatic. The obtained results show a great diversity of ecological niches in marine diatoms, which ultimately explains the great biodiversity of this area.