On 16 September 2025, Víctor González Triginer successfully defended his PhD thesis, entitled “Ecological transitions in Arctic marine ecosystems – Macroalgal settlement, pelagic density, and fish community shifts associated with glacier retreat and Atlantification“. Prior to the defense, Víctor presented a trial lecture with the title “Integrating Underwater Acoustics, Optics, and Artificial Intelligence for Advanced Marine Ecosystem Monitoring”.

Víctor worked within the FACE-IT project at The University Centre in Svalbard (UNIS, Norway). She was part of the PhD programme at the Department of Arctic and Marine Biology of the Faculty of Biosciences, Fisheries and Economics at The Arctic University of Norway in Tromsø. Read the thesis summary below.

Congratulations, Víctor!

Víctor was supervised by Børge Damsgård (The University Centre in Svalbard UNIS, Norway), Arunima Sen (The University Centre in Svalbard UNIS, Norway), Maxime Geoffroy (Memorial University of Newfoundland, Canada) and Jørgen Berge (The Arctic University of Norway, Tromsø, Norway).

The committee members of the defense were:

• Disputation chair: Stephen J. Coulson (The University Centre in Svalbard UNIS, Norway)
• First Opponent: Kanchana Bandara (Akvaplan-niva, Tromsø, Norway)
• Second Opponent: Simon Jungblut (University of Bremen, Germany)
• Chair of Assessment Committee: Edel O. Elvevoll (The Arctic University of Norway, Tromsø, Norway)

Abstract

The Arctic is warming at an unprecedented rate, driving severe changes in marine ecosystems. Glacier retreat, increased sedimentation rates, and reduced sea ice cover are reshaping Arctic fjords, where terrestrial, freshwater, and marine influences interact. Glacier fronts influence fjord biogeochemistry and marine ecosystems across multiple trophic levels, and localized effects of glacier retreat on coastal ecosystems remain poorly understood due to logistical challenges of studying these areas. Simultaneously, the northward influx of temperate species, a process known as borealization, is altering Arctic marine communities. This thesis documents the localized and regional processes associated with glacier retreat and borealization in the Arctic, while also developing and testing sustainable technologies for studying these remote environments.

Papers I and II explored the effects of tidewater glacier retreat, particularly the transition from marine- to land-terminating glaciers, on the underwater coastscape and marine ecosystems of a high Arctic fjord in Svalbard. Sampling stations were established along a gradient of glacier retreat: at a recently land-terminating glacier front, in a river bay with terrestrial input from inland glaciers, and at a site with minimal glacial influence. Using hydroacoustics and autonomous vehicles, Paper I investigated macroalgal coverage and oceanographic conditions along the fjord coastline. This study revealed high variability in macroalgal presence, with abundant coverage in areas with little land runoff and virtually no macroalgae in the river bays or at the marine-terminating glacier front. Notably, kelp was observed near the land-terminating glacier front, highlighting the potential for rapid macroalgal colonization on newly available substrates following glacier retreat.

Paper II extended this research to include pelagic organisms, investigating the density and vertical distribution of fish and zooplankton along the same gradient of glacier retreat. Pelagic density was lowest near the land-terminating glacier front and highest at the site with minimal glacial input, which also featured a rich benthic habitat dominated by kelp beds. These findings suggest that the loss of pelagic productivity associated with the disappearance of subglacial upwelling may be mitigated by benthic habitat expansion. However, macroalgal growth will depend on factors such as fjord topography, sedimentation rates, and substrate availability, which are likely to be affected by higher melting rates and increased turbidity in Arctic waters.

These localized shifts in Arctic fjords are linked to large-scale regional transitions, such as the influx of boreal species into higher latitudes. Paper III upscaled the scope of this thesis by examining spatial differences in fish community composition and pelagic density across Svalbard during the polar night, and assessing the effectiveness of this extreme period as a barrier for borealization. Acoustic-trawl surveys and oceanographic measurements revealed distinct spatial patterns, with western fjords exhibiting lower pelagic density but higher species diversity compared to eastern sites, which were dominated by schooling mesopelagic fish. Interestingly, borealization was equally pronounced in the eastern, more Arctic-influenced sites as in the western, Atlantic-influenced fjords, suggesting that the extreme light regime of the polar night does not act as a strong seasonal barrier to borealization. This study discusses the interplay between wide-scale borealization processes and habitat shifts in Arctic fjords, and explores the potential role of benthic habitat expansion in facilitating the expansion of boreal species into Arctic waters.

This thesis advances our understanding of the localized effects of glacier retreat on Arctic fjord ecosystems and the broader patterns of borealization in the region. By integrating hydroacoustics with autonomous vehicles, this research provides efficient and sustainable protocols for studying coastal areas and glacier fronts, offering valuable tools for monitoring and managing Arctic marine ecosystems in a rapidly changing world.

Photos: Tina Dahl (The University Centre in Svalbard UNIS, Norway) & Simon Jungblut (University of Bremen, Germany)