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

Jones, Kevin LU orcid (2025)
Abstract
Freshwater ecosystems in boreal and temperate regions are undergoing rapid change as increasing
inputs of terrestrially derived dissolved organic matter (DOM) and nutrient enrichment alter ecosystem
functioning. These processes, known as brownification and eutrophication, vary across space and time
but can profoundly reshape carbon cycling, microbial activity, and water quality. This thesis investigates
how seasonal variability, nutrient availability, and wetland design interact to influence DOM
transformations 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... (More)
Freshwater ecosystems in boreal and temperate regions are undergoing rapid change as increasing
inputs of terrestrially derived dissolved organic matter (DOM) and nutrient enrichment alter ecosystem
functioning. These processes, known as brownification and eutrophication, vary across space and time
but can profoundly reshape carbon cycling, microbial activity, and water quality. This thesis investigates
how seasonal variability, nutrient availability, and wetland design interact to influence DOM
transformations 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 carbon
use toward respiration and CO₂ release rather than biomass production. Sunlight also plays a critical
role: photochemical processes fragment aromatic DOM into smaller, more labile compounds, particularly
in shallow, well-lit waters where exposure is prolonged. The relative importance of microbial versus
photochemical pathways shifts with season, with warmer nutrient-rich periods amplifying microbial
respiration and clearer shallow systems enhancing photodegradation. Constructed wetlands provide an
opportunity to harness these processes for management. Results show that both water depth and
seasonal light regimes determine whether DOM removal is dominated by microbial reworking or
photochemical breakdown. Shallow wetlands promote rapid photodegradation, while deeper systems
sustain 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 effects
of light, nutrients, hydrology, and wetland design. By linking natural seasonal variability with targeted
wetland manipulations, it provides new insights into how freshwater carbon cycling may respond to
environmental change and highlights how strategically designed wetlands can mitigate brownification
across seasons. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Doctor Graeber, Daniel, Department Aquatic Ecosystem Analysis and Management, Helmholtz Centre for Environmental Research
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Dissolved organic matter, brownification, eutrophication, aquatic biogeochemistry, microbial carbon cycling, photodegradation, constructed wetlands, seasonal variability
pages
68 pages
publisher
Lund University (Media-Tryck)
defense location
Blå Hallen, Ekologihuset. Join via zoom: https://lu-se.zoom.us/j/62442637290?pwd=bpITxMINCVTpfgo278tzZAR2jBaMKf.1
defense date
2025-11-14 09:00:00
ISBN
978-91-8104-702-8
978-91-8104-701-1
language
English
LU publication?
yes
id
a44203e7-ef17-49c3-a0b3-25c347779c76
date added to LUP
2025-10-14 10:36:59
date last changed
2025-10-18 03:47:37
@phdthesis{a44203e7-ef17-49c3-a0b3-25c347779c76,
  abstract     = {{Freshwater ecosystems in boreal and temperate regions are undergoing rapid change as increasing<br/>inputs of terrestrially derived dissolved organic matter (DOM) and nutrient enrichment alter ecosystem<br/>functioning. These processes, known as brownification and eutrophication, vary across space and time<br/>but can profoundly reshape carbon cycling, microbial activity, and water quality. This thesis investigates<br/>how seasonal variability, nutrient availability, and wetland design interact to influence DOM<br/>transformations in freshwater and coastal systems.<br/>Field studies and experimental manipulations demonstrate that pulses of terrestrial DOM during high-<br/>flow events and seasonal transitions strongly affect DOM composition and microbial bioavailability.<br/>Nutrient enrichment stimulates microbial uptake but lowers bacterial growth efficiency, shifting carbon<br/>use toward respiration and CO₂ release rather than biomass production. Sunlight also plays a critical<br/>role: photochemical processes fragment aromatic DOM into smaller, more labile compounds, particularly<br/>in shallow, well-lit waters where exposure is prolonged. The relative importance of microbial versus<br/>photochemical pathways shifts with season, with warmer nutrient-rich periods amplifying microbial<br/>respiration and clearer shallow systems enhancing photodegradation. Constructed wetlands provide an<br/>opportunity to harness these processes for management. Results show that both water depth and<br/>seasonal light regimes determine whether DOM removal is dominated by microbial reworking or<br/>photochemical breakdown. Shallow wetlands promote rapid photodegradation, while deeper systems<br/>sustain microbial transformations that further process DOM before it leaves the wetland.<br/>Overall, this thesis demonstrates that the fate of DOM in freshwaters is shaped by the combined effects<br/>of light, nutrients, hydrology, and wetland design. By linking natural seasonal variability with targeted<br/>wetland manipulations, it provides new insights into how freshwater carbon cycling may respond to<br/>environmental change and highlights how strategically designed wetlands can mitigate brownification<br/>across seasons.}},
  author       = {{Jones, Kevin}},
  isbn         = {{978-91-8104-702-8}},
  keywords     = {{Dissolved organic matter; brownification; eutrophication; aquatic biogeochemistry; microbial carbon cycling; photodegradation; constructed wetlands; seasonal variability}},
  language     = {{eng}},
  month        = {{10}},
  publisher    = {{Lund University (Media-Tryck)}},
  school       = {{Lund University}},
  title        = {{Dissolved, Transformed, Retained : Carbon Cycling in Freshwater and Coastal Systems}},
  url          = {{https://lup.lub.lu.se/search/files/230091860/Thesis_Kevin_Jones_LUCRIS.pdf}},
  year         = {{2025}},
}