Freshwater systems are essential for human well-being, but are under pressure worldwide due to nutrient pollution resulting from the excessive use of nitrogen- and phosphorus-containing fertilisers. This leads to eutrophication, resulting in, amongst other things, algal blooms, oxygen depletion and loss of biodiversity. As the effects of nutrients depend on environmental factors such as initial concentrations, temperature and salinity, in this thesis I investigated how aquatic plants and invertebrates respond to nutrient enrichment under different conditions. Based on data from several experimental studies, it emerged that low nitrogen concentrations often have a positive effect on species abundance, whilst higher concentrations, on the other hand, cause negative effects. Phosphorus had predominantly positive effects. I used these results to make global projections of the future decline of freshwater communities under various scenarios for 2070. The greatest losses of biodiversity were expected in parts of Africa, whilst recovery appears possible in (parts of) Europe, North America and East Asia. Finally, I developed a model for mosquitoes, a group of species that often responds differently to nutrients than other invertebrates. My model showed that higher temperatures, increased nitrogen concentrations and moderate salinity levels can significantly accelerate the development of the common mosquito. My results highlight the importance of effective nutrient management to limit biodiversity loss and health risks.
Floris Neijnens is an environmental scientist with a broad background in ecology, sustainability and environmental modelling. After completing a Bachelor’s degree in Liberal Arts and Sciences at University College Utrecht, with a focus on cell biology, earth and environmental sciences, he continued his studies with a Master’s in Sustainable Development at Utrecht University. During this Master’s programme, he specialised in Environmental Change, Ecosystems and Complex Systems. For his MSc thesis, he conducted research into early warning signs of salt marsh collapse due to sea-level rise, carried out at the Royal Netherlands Institute for Sea Research (NIOZ). He then began his PhD at the Department of Environmental Science at Radboud University, in collaboration with Deltares. His PhD research focused on the effects of nutrient pollution on freshwater organisms, with an emphasis on the ecological integrity of freshwater systems and the interaction between nutrients and other environmental factors.