Nutrient bioavailability and microbial responses in fresh and coastal aquatic ecosystems

Nutrients are the lifeblood of aquatic ecosystems, but not all nutrients are equally accessible to the organisms that depend on them. While traditional monitoring often tracks total or inorganic concentrations of carbon, nitrogen, and phosphorus, these measures do not reflect what microbial communities can actually use. This thesis explores that disconnect by investigating the bioavailability of key nutrients and their relationship to microbial community structure and function across a land-sea continuum. To bridge this gap, I developed a standardized, high-throughput multi-element bioassay to estimate bioavailable carbon, nitrogen, and phosphorus in aquatic ecosystems. Combined with dissolved organic matter (DOM) quality analysis and microbial community sequencing, this approach offered a consistent framework to compare nutrient accessibility and microbial responses across diverse ecosystems. The results reveal that bioavailable nutrient fractions often diverge substantially from total concentrations — and that these differences matter. Microbial communities, particularly bacteria and eukaryotes, responded not only to nutrient levels, but also to nutrient form and stoichiometry. DOM composition further influenced nutrient availability, with fresh, microbial-produced and protein-like DOM closely linked to greater bioavailability, while aromatic, humic-rich DOM often constrained microbial access. Meanwhile, landscape features such as lake size, catchment vegetation, and water colour influenced both DOM characteristics and microbial community structure. Furthermore, functional analyses indicated that microbes adapt to nitrogen and phosphorus limitation by modulating nutrient acquisition pathways. Together, these findings challenge the assumption that more nutrients always mean more microbial activity. They underscore the importance of measuring not just what is present in the water, but what is actually accessible. By combining chemical and biological perspectives, this work highlights the need for bioavailability-based approaches to better understand nutrient cycling and manage aquatic ecosystems in a rapidly changing world.