On 06 February 2025, Luisa Düsedau successfully defended her PhD thesis, entitled “Assessment of Macroalgal Biodiversity and Habitat Change in the European Arctic“. Luisa worked within the FACE-IT project at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Germany). In her studies, Luisa used a combination of in situ field data and molecular biodiversity assessments to characterize the macroalgal communities of two Arctic fjord systems at different stages of deglaciation, namely the high-Arctic Kongsfjorden on Svalbard and the low-Arctic Porsangerfjorden in northern Norway. A more extensive summary can be found below.

Congratulations, Luisa!

Luisa was supervised by Inka Bartsch (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Germany)

The committee members of the defense were:

• Martin Diekmann (University of Bremen, Germany)
• Kai Bischof (University of Bremen, Germany)
• Florian Weinberger (GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany)
• Inka Bartsch (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Germany)
• Sarina Niedzwiedz (University of Bremen, Germany)
• Chiara Sickert (University of Bremen, Germany)

Abstract

Global warming is a major driver for change across European Arctic fjord systems. Macroalgae are important foundation species along the rocky Arctic coastline and their community structure, horizontal and vertical distribution and species richness serves as a sensitive indicator of changing environmental conditions. In this doctoral thesis, I use a combination of in situ field data (quantitative and semi-quantitative sampling of species) and molecular biodiversity assessments (DNA barcoding and eDNA metabarcoding) to characterize the macroalgal communities of two Arctic fjord systems at different stages of deglaciation. High-Arctic Kongsfjorden on Svalbard is surrounded by melting glaciers and is currently under transition from Arctic to cold-temperate conditions as a consequence of atlantification. Low-Arctic Porsangerfjorden in northern Norway is already largely ice-free and, although it is located north of the Arctic Circle (66°N), cold-temperate conditions prevail. The aim of this study was to improve our knowledge on the current status of subtidal and intertidal macroalgal community structure and their biodiversity at these two key locations in the European Arctic.

In a time-for-time approach, I showed that over the course of 25 years the subtidal community structure and kelp demography at the study site Hansneset in high-Arctic Kongsfjorden has changed considerably. The kelp forest and consequently the overall macroalgal biomass maximum had shifted upwards by several meters to the shallow subtidal. Furthermore, the kelp forest structure and demography has shifted as ‘Digitate Kelps’ and Saccharina latissima retreated to the uppermost depth level and Alaria esculenta became the dominant and only kelp species still reaching down to deeper levels. These alterations in species dominance over time are associated with a functional shift of the kelp forest as I showed that each kelp exhibits a species-specific strategy in biomass accretion and thereby carbon allocation. The main driver shaping Arctic macroalgal communities currently appears to be the decrease in water transparency as a consequence of melting glaciers and resulting sediment plumes, which was confirmed by in situ decreasing PAR and increasing turbidity data.

In a space-for-time substitution, I revealed that at present the intertidal and subtidal macroalgal communities in high-Arctic Kongsfjorden differ to a great extent from those of low-Arctic Porsangerfjorden. Kongsfjorden displayed an emerging intertidal macroalgal community primarily dominated by Fucus distichus, whereas Porsangerfjorden featured assemblages dominated by six species of Fucales, characterized by significantly higher biomass. However, the macroalgal biomass at the subtidal fringe level was similar between the fjords, althrough the species contributions varied. In contrast to the shallow kelp forests dominated by Alaria esculenta observed in Kongsfjorden, Saccharina latissima was the dominant species in Porsangerfjorden, where the kelp forests extended to considerable depths. I conclude, that the presence of flourishing intertidal macroalgal communities in the Arctic is supported by retreating sea ice and warmer water temperatures, while the extension of subtidal kelp forests is largely dependent on the underwater light conditions and hindered by increased water turbidity. As Arctic warming continues and biogeographical regions shift, cold-temperate Atlantic species are expected to expand their distribution northward. Ultimately, this expansion is likely to transform the high-Arctic macroalgal community in Kongsfjorden into a composition similar to that found in Porsangerfjorden in the future.

The quantitative field surveys were complemented by a comprehensive molecular biodiversity assessment, which represent a novel macroalgal sequence database for the Arctic. A total of 604 new DNA barcodes were generated using the best practice collection scheme and will serve as reliable reference DNA barcodes for the assessment of future species inventories with molecular methods. I found that a large part of the molecular macroalgal biodiversity in both Kongsfjorden and Porsangerfjorden was not represented in previous species lists based purely on identification via morphological characters. Using DNA barcoding I could confirm 51 morphological species across both fjords, of which 20 were hitherto overlooked species or misidentified. Additionally, I identified 13 genetic macroalgae species with no assigned morphological species concept and seven previously unsequenced taxa, all of which require further taxonomic work. Ulvophyceae metabarcoding conducted on surface water samples additionally revealed that 18 more Ulvophyceae species might be present in the European Arctic which were missed by my DNA barcode survey. My results highlight the power of molecular biodiversity assessments and the need to connect them with classical taxonomy to utilize their full potential. We are currently noticeably underestimating the biodiversity of Arctic fjord systems due to species misidentifications and resulting incomplete biogeographic information. It is therefore of utmost importance to implement regular monitoring efforts in the European Arctic in order to truly assess the biodiversity present in these habitats especially impacted by climate change and to establish molecularly verified species lists to detect future shifts in species distribution ranges. This is might not only be important for detecting but also for conserving macroalgal species at a time when the socio-economic situation in the Arctic is changing and effective coastal zone management concepts based on solid biodiversity data may be needed to preserve local heritage.

In conclusion, my findings clearly demonstrated substantial differences in macroalgal communities between two fjord systems in the European Arctic located at different latitudes, and facing contrasting temperature regimes and stages of cryosphere loss. I set newly collected quantitative and molecular datasets in perspective to historical observations which allowed for well-founded predictions about the future of macroalgae in warming high-arctic fjords after deglaciation. Furthermore, my research has established a comprehensive baseline for the current community composition and biodiversity of intertidal and subtidal macroalgae in the European Arctic. My doctoral thesis intends to facilitate future monitoring efforts on expected shifts in macroalgal community structure and biodiversity resulting from climate change-related changes in environmental conditions.

Photos: Hendrik Nitsch, Isabel Cardoso, Kai Bischof