| dc.description.abstract | In recent decades, lakes have been observed to increase in color, also known as brownification. Increases in dissolved organic matter (DOM) concentrations in lakes, or more specifically chromophoric DOM (CDOM), caused this increase. In addition to terrestrial DOM, lakes have also been shown to release DOM from the sediment, mostly DOM that was previously bound to iron (Fe). However, only a few studies included non-terrestrial sources of DOM, and the general dynamics of DOM within lake ecosystems are still poorly understood.
This research aimed to examine the internal DOM dynamics in Lake Solbergvann, Norway, focusing on the role of stratification and prevalence of oxygen. The research objectives were to investigate (1) DOM cycling under different oxygen conditions (oxic layer, transition zone, anoxic layer) during a spring/summer period (May-September) in a lake that is subject to browning, (2) the seasonal variation in DOM molecular weight (MW) across different depth layers to provide a better understanding of environmental conditions that influence DOM dynamics. The results revealed complex DOM dynamics that do not always result in linear relationships. Nevertheless, an attempt was made to clarify these interactions to enhance the understanding of DOM dynamics.
In the oxic water layers, the DOM concentrations nearly doubled due to high runoff conditions, and the strong thermal stratification created barriers for oxygen diffusion and DOM transport to deeper layers. CDOM was strongly correlated to dissolved organic carbon (DOC). Minimal variability in DOM dynamics was observed. However, the overall MW of CDOM increased in this oxic layer due to increased DOM runoff from terrestrial sources. While maintaining stable CDOM and DOC concentrations, the transition zone – at the oxic-anoxic boundary – exhibited the greatest variability in the relationships across DOC and CDOM fractions. Mechanisms from oxic and anoxic waters concurrently govern the DOM dynamics in this zone (e.g., redox conditions, diffusion barriers, and decomposition), which likely resulted in increased diversity in their DOM dynamics. The anoxic water layers showed increasing concentrations of CDOM, DOC, Fe, manganese, and phosphorus despite major influxes from shallower waters. Stratification isolated these water layers, suggesting influences beyond dilution impacting these concentrations. The results indicate that Fe-bound DOM, identified as CDOM3 (3 – 10 kDa), was released from the sediment due to Fe reduction: This fraction made up a large proportion of the entire DOM pool, thus severely affecting DOM dynamics. Key factors restricting DOM release from the sediment were also identified; e.g., sulfate presence increases the competition for electron acceptors, thus reduces Fe reduction potential. Furthermore, pH governs various processes that impact DOM dynamics, but this study demonstrated that redox reacts may impact pH, but a low pH contribute to DOM release through enhanced Fe dissolution, thus creating feedback loops. However, more data is needed to draw strong statistically significant conclusions. | |
