From Peatlands to the Sea: aggregation, adsorption, and persistence of dissolved organic matter in coastal systems

The ocean plays a fundamental role in the global carbon cycle, acting as a major long-term sink for atmospheric carbon dioxide (CO₂). Among the various carbon reservoirs, dissolved organic matter (DOM) is a substantial and dynamic component that links terrestrial and marine carbon fluxes. However, the processes controlling the transformation of DOM into particulate organic matter (POM), which facilitates long-term carbon sequestration via sedimentation, remain poorly understood. This is particularly true for transitional environments along the terrestrial-marine continuum, such as coastal systems and
continental shelves, where riverine, autochthonous, and marine DOM sources intersect and interact under highly dynamic environmental conditions.
This dissertation investigates the mechanisms governing DOM-to-POM transformation, focusing on aggregation and adsorption processes in the North Sea and adjacent terrestrial systems. The work addresses critical knowledge gaps regarding the interplay between DOM composition, environmental drivers, and the role of mineral particles in modulating these transformation processes. Through a combination of laboratory experiments and targeted analyses, this work provides new insights into the reactivity and fate of DOM in coastal environments, contributing to a better understanding of the biogeochemical cycling of
carbon in the ocean.
The dissertation consists of three scientific manuscripts, each examining specific aspects of DOM-POM transformation along the land-sea continuum:

Manuscript 1 – Solid-phase extracted DOM defies aggregation and adsorption

This study assesses whether solid-phase extracted DOM (SPE-DOM), a representative fraction of semi-labile and refractory DOM, undergoes aggregation or adsorption under controlled laboratory conditions. Aggregation and adsorption experiments were conducted with SPE-DOM isolated from seawater, testing a range of physical and parameters such as turbulence and particle presence. The results demonstrate that SPE-DOM exhibits a remarkably low potential for aggregation and adsorption. Neither the addition of mineral particles nor variations in environmental conditions led to significant removal of SPE-DOM from the dissolved phase. These findings
suggest that SPE-DOM, due to its specific molecular composition and chemical characteristics, is inherently resistant to conversion into POM. This challenges the prevailing assumption that all DOM fractions are equally available for aggregation and subsequent export to the ocean interior. The study highlights the need to differentiate between DOM fractions based on their reactivity and structural properties when assessing their role in carbon cycling and sequestration.

Manuscript 2 – Flocculation and Adsorption of Terrestrial DOM: Transport Dynamics
from Northern German Rivers to the Southern North Sea

This manuscript investigates how flocculation and adsorption processes regulate the transformation and transport of terrestrially derived DOM from northern German rivers into the southern North Sea. Riverine DOM represents a major input of organic carbon to coastal systems, yet its fate and transformation pathways remain poorly constrained. DOM originating from river water of Weser, Elbe and Ems were analyzed alongside controlled laboratory experiments to quantify DOM removal via flocculation and adsorption. The study revealed that terrestrial DOM is highly susceptible to adsorption in the estuarine mixing zone, particularly under increasing salinity. Mineral particles of terrestrial origin, such as clays and silts, significantly enhance DOM removal through adsorption. The results demonstrate a strong link between the physicochemical conditions of the estuary and the efficiency of DOM-to-POM transformation. These findings imply that estuarine and coastal systems act as effective biogeochemical filters, regulating the export of terrestrial DOM to the open ocean. The study emphasizes the importance of considering local hydrodynamic and sedimentary conditions when evaluating coastal carbon budgets.

Manuscript 3 – From Peatlands to the Ocean: The Role of Adsorption in DOM Cycling
Across Aquatic Systems

Expanding the scope beyond estuarine environments, this manuscript broadens the spatial and ecological scope by examining adsorption processes along a complete land-sea continuum, from peatlands in northern Germany through river systems to the southern North Sea. The study focuses on the interaction between DOM of different origins and mineral particles, assessing how adsorption modulates DOM transport, transformation, and potential sequestration. Adsorption experiments were conducted using three distinct DOM sources—fen-derived DOM, riverine DOM, and marine DOM—alongside two mineral types (kaolinite and calcium carbonate). The results reveal that the adsorption potential of DOM is strongly source-dependent. Terrestrial DOM, rich in aromatic and carboxyl-rich compounds, exhibited the highest adsorption affinity, particularly onto clay minerals. Marine DOM, in contrast, showed lower adsorption rates, reflecting differences in molecular composition and
charge properties. Moreover, temporal aspects of adsorption were evaluated, indicating that aging and structural rearrangements of adsorbed DOM may enhance long-term carbon stabilization. This manuscript demonstrates that adsorption processes act as a key mechanism for DOM transformation and carbon retention along the terrestrial-marine gradient. It provides new evidence that terrestrial DOM is selectively removed and transformed in transitional environments, contributing to carbon sequestration in sediments and potentially altering carbon fluxes at regional and global scales.

Conclusion

This dissertation advances our understanding of DOM-POM transformation processes and highlights the critical role of adsorption in regulating coastal carbon dynamics. By integrating experimental and conceptual approaches, it provides a comprehensive assessment of how DOM composition, environmental conditions, and mineral interactions shape carbon fluxes along the terrestrial-marine gradient. These findings contribute to broader efforts to quantify carbon sequestration and assess the resilience of coastal ecosystems in the face of global climate change.