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Urban runoff.

Doctoral defence of Xudan Zhu, MSc, 10.5.2024: The mechanisms behind aquatic browning in boreal catchments

The doctoral dissertation in the fields of Biology and Water research will be examined at the Faculty of Science, Forestry and Technology, Joensuu Campus and online.

What is the topic of your doctoral research? Why is it important to study the topic?

Recent decades have witnessed a notable increase in dissolved organic carbon (DOC) concentrations across many boreal catchments, called aquatic browning. This phenomenon has far-reaching ecological, social and global carbon cycle implications. This thesis improves our mechanistic understanding of short-term variations and long-term trends in surface water DOC. It highlights the significance of considering multifaceted, spatially structured, and non-stationary drivers into predicting future trends of browning.

What are the key findings or observations of your doctoral research?

My doctoral research revealed that terrestrial greening and altering hydrology following climate change may have a major impact on DOC concentration in aquatic ecosystems, as well as reaffirmed the importance of DOC fluxes in controlling ecosystem C budgets, which is generally disregarded. Moreover, it provided evidence that large variations in browning trends among northern streams can reflect the outcome of interactions among multiple factors, including recovery from sulfate deposition, climate-related factors, and catchment properties. 

Our results further suggest that recovery from sulfate rather than from acidification per se has been the main driver of DOC change, despite the low deposition history in this region. This research also explained why browning has weakened in the last decade: sulfate levels have plummeted, and other drivers were insufficient to sustain the ongoing long-term trend of DOC. However, given the unexplained variations in our model and the heterogeneity in the inherent DOC dynamics among catchments, a critical evaluation is needed when extrapolating our models to other regions or predicting the future trends.

How can the results of your doctoral research be utilised in practice?

This work established a link between terrestrial greening and aquatic browing, and also explained the reasons for the spatiotemporal heterogeneity of browning across boreal catchments. And these results could be applied across various fields: 1) to provide essential data for developing process-based models for predicting DOC dynamics. 2) to help assess the impact of climate change on C dynamics in freshwater systems, including alterations in C storage, export, and greenhouse gas emissions; 3) to forecast future changes in DOC export under different scenarios; 4) to help water treatment facilities optimize treatment processes to minimize disinfection by-products formation and ensure safe drinking water supply.

What are the key research methods and materials used in your doctoral research?

The process of my doctoral research included: paper review, fieldwork, lab work, data analysis, modelling, data visualization, writing and revision. The key research methods I used are wavelet coherence analysis, distributed-lag linear models, linear regression and Total differential equation.

The doctoral dissertation of Xudan Zhu, MSc, entitled The mechanisms behind aquatic browning in boreal catchments will be examined at the Faculty of Science, Forestry and Technology, Joensuu Campus and online. The opponent will be Professor William H. McDowell, University of New Hampshire, and the custos will be Professor Frank Berninger, University of Eastern Finland. Language of the public defence is English.

For more information, please contact:

Xudan Zhu,, tel. +358 408 279 940