Dissolved, Transformed, Retained : Carbon Cycling in Freshwater and Coastal Systems

Freshwater ecosystems in boreal and temperate regions are undergoing rapid change as increasinginputs of terrestrially derived dissolved organic matter (DOM) and nutrient enrichment alter ecosystemfunctioning. These processes, known as brownification and eutrophication, vary across space and timebut can profoundly reshape carbon cycling, microbial activity, and water quality. This thesis investigateshow seasonal variability, nutrient availability, and wetland design interact to influence DOMtransformations in freshwater and coastal systems.Field studies and experimental manipulations demonstrate that pulses of terrestrial DOM during high-flow events and seasonal transitions strongly affect DOM composition and microbial bioavailability.Nutrient enrichment stimulates microbial uptake but lowers bacterial growth efficiency, shifting carbonuse toward respiration and CO₂ release rather than biomass production. Sunlight also plays a criticalrole: photochemical processes fragment aromatic DOM into smaller, more labile compounds, particularlyin shallow, well-lit waters where exposure is prolonged. The relative importance of microbial versusphotochemical pathways shifts with season, with warmer nutrient-rich periods amplifying microbialrespiration and clearer shallow systems enhancing photodegradation. Constructed wetlands provide anopportunity to harness these processes for management. Results show that both water depth andseasonal light regimes determine whether DOM removal is dominated by microbial reworking orphotochemical breakdown. Shallow wetlands promote rapid photodegradation, while deeper systemssustain microbial transformations that further process DOM before it leaves the wetland.Overall, this thesis demonstrates that the fate of DOM in freshwaters is shaped by the combined effectsof light, nutrients, hydrology, and wetland design. By linking natural seasonal variability with targetedwetland manipulations, it provides new insights into how freshwater carbon cycling may respond toenvironmental change and highlights how strategically designed wetlands can mitigate brownificationacross seasons.