Sanne Mariël Moedt:
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Primary producers, such as phytoplankton and algal biofilms, form the base of Arctic freshwater ecosystems. They play a key role in carbon cycling and are food for zooplankton and aquatic invertebrates. As the Arctic is warming at a rate four times faster than the global average, it is important to understand the impact of this warming on phytoplankton and biofilms. Despite them being key components of Arctic freshwater ecosystems, our knowledge of how environmental conditions shape their biomass and the composition of autotrophic organisms involved is limited.
In this thesis, drivers of both phytoplankton and algal biofilms in lakes and streams across Greenland were identified. We studied long-term phytoplankton dynamics in High Arctic lakes in North-East Greenland and found that time of ice melt and nutrient concentrations in the water were important factors for phytoplankton biomass. Additionally, there was substantial year-to-year variation in phytoplankton biomass, but taxon richness and diatom biomass increased throughout the monitoring period. In South Greenland, we found that biofilm biomass in lakes was mostly associated with the normalized difference vegetation index (NDVI) and water conductivity, while in streams it was with water temperature, phosphate concentrations, and pH. Furthermore, we showed that conventional methods used for the analysis of algal biomass (spectrophotometry) and composition (highperformance liquid chromatography) provided different results than using a portable in situ chlorophyll-a fluorescence probe (bbe BenthoTorch). Therefore, the BenthoTorch should be used with caution, especially when analysing thick lake biofilms or if the goal is to draw comparisons with other studies that use a different method. Our study of biofilms in streams across three climate regions in Greenland showed that temperature and nutrient concentrations were key drivers of biofilm biomass. Other important factors were catchment slope and seasonality.
The combination of long-term monitoring data, data covering a large climate gradient, and state-ofthe-art methods resulted in an improved understanding of how phytoplankton and algal biofilms in Greenland are shaped by their environment. This allows us to better predict how future climate warming will affect ecosystem functioning in freshwater ecosystems in the Arctic.