Mikkel Madsen-Østerbye:
Sources and fate of dissolved organic matter in lakes

Date: 21-12-2018    Supervisor: Ole Pedersen & Kaj Sand-Jensen

Increasing input of terrestrially derived coloured dissolved organic matter (CDOM) to aquatic ecosystems has been observed throughout the Northern Hemisphere over the past decades. The accompanying increase in water colour has been termed browning or brownification. The impact of browning on lake ecosystems has been profound with documented consequences for light climate, food webs and biodiversity. Particularly submerged macrophytes are likely to suffer from reduced light availability, which influences the trophic structure and ecological quality of the ecosystem. Despite the negative implications of browning, few studies have addressed the degradation of CDOM in lake waters. In this thesis, I present new insights on CDOM transformation and light climate dynamics in lakes. By applying a new experimental setup mimicking the natural processes in surface waters, I show that CDOM degradation occurs at a constant rate resulting in a much higher CDOM removal than previously found. In closed laboratory systems, imitation of the seasonal solar irradiation resulted in the removal of 90 % of the CDOM within a year in the humic Lake Tvorup. This finding suggests that CDOM degradation alone could improve the light climate; however, input of new CDOM impeded this development and played a fundamental role in controlling the actual light climate. Furthermore, I showed that a change in catchment vegetation from coniferous forest to heathland could result in a four-fold improvement of the light climate, expanding the macrophyte colonisation area from 3 % to 35 % in Lake Tvorup.

Monitoring of the light climate over four years in the re-established Lake Fil showed a high complexity in the temporal dynamics of the light attenuation parameters. A decrease in CDOM- or particle-induced light attenuation could result in an increase in phytoplankton biomass and associated light attenuation. Despite unclear waters, low light penetration and low depth colonisation of macrophytes the open lake shores and extreme shallow waters supported a high species richness of macrophytes.

In conclusion, I showed that UV-mediated bacterial mineralisation of CDOM and DOC is linear with time when only a small proportion of the water is exposed to UV light. I also showed that in situ UV-mediated bacterial removal of CDOM is substantial, while continuous input of CDOM from the catchment will mask this removal. Finally, I showed that changes in land use from coniferous forest to heather could be a powerful management tool to protect, or even restore, the pristine isoetid vegetation of brownified Lobelia lakes.