Long‐term nutrient load reductions and increasing lake <scp>TN</scp> : <scp>TP</scp> stoichiometry decrease phytoplankton biomass and diversity in a large shallow lake

Abstract Nutrient loading of freshwater and marine habitats has increased during the last century as a result anthropogenic activities. From 1980s onwards, following implementation new policy targeting eutrophication, total phosphorus (TP) nitrogen (TN) loads were reduced in many European waters. Often, however, decreases TP stronger compared to TN, leading TN : ratios. Our analysis shows that large shallow lake IJsselmeer (the Netherlands) experienced similar trend, whereas was by 50%, 89% between 1975 2018. Most this nutrient load reduction achieved before year 2000, changes concentrations became smaller afterwards, especially for further increase stoichiometric imbalance up yearly averaged (molar) 296 The observed nutrients accompanied decline phytoplankton biomass, slight declines genus evenness diversity. Although biomass likely resulted from overall decrease availabilities, diversity may have shift toward very high ratios indicate relatively low levels enhanced competition phosphorus. Overall, our findings demonstrate long‐term trends with decreased imbalance. Ultimately, such at food web base alter structure functioning entire aquatic food‐ IJsselmeer.