Reservoirs are increasing in number worldwide. The implementation of these infrastructures into the rivers are known to reduce water flow and increase the retention time which give the conditions to allow primary production and nutrients sedimentation in the reservoirs. As a consequence of these factors, nutrient concentrations, ratios, and fluxes of nitrogen (N), phosphorus (P), and silicon (Si) are altered downstream. The alteration of nutrient ratios can affect eutrophication processes by changing both the amount of nutrients delivered (by retaining or creating opportunity for fixation or internal nutrient loading) and by changing the limiting nutrient in downstream ecosystems. Hence, nutrient ratios are considered to be strong selective forces shaping phytoplankton communities.

The aim of this research is to better understand the effect of reservoirs on N:P:Si stoichiometry along a longitudinal gradient of river and the influence of changing N:P:Si ratios and residence time on phytoplankton communities. The Danube River Basin has been selected as a case study for this research, because the presence of high numbers of reservoirs in the upper part of the basin (one dam every 17 km in the upper 1000 km of the river). Additionally, there has been an observed decline in Si flux, which may have led to changes in phytoplankton community structure in the Black Sea, but for which the causes for this decline are not yet clear.

To achieve the aim of this research, first data analysis will develop from results of the monitoring programme of the International Commission for the Protection of the Danube River -ICPDR, in order to analyze the influence of land use/land cover (LULC) and seasonal variation in runoff on N:P:Si stoichiometry along the longitudinal gradient in the river. Also, water samples will be collected from different reservoirs located in the upper part of the basin, to see the effect of reservoirs on N:P:Si stoichiometry and phytoplankton biomass and community structure. Finally, a multi-factorial experimental design will be used to test the effects of nutrient ratios and different residence times on the relative abundance of phytoplankton biomass.