Text
Introduction
Submarine algal forests are present along 25% of the world’s coastline ranging from temperate to polar regions (Filbee-Dexter & Wernberg, 2018). In the past their northward extension into the Arctic was physically limited by sea ice and low light conditions whereas their distribution towards the equator is restricted by nutrient availability and warm temperatures (Steneck & Johnson, 2013). These forests are formed by large kelps of the brown algae order Laminariales, which are foundation species on rocky shores and serve as important ecosystem engineers and major primary producers (Filbee-Dexter et al., 2019). Worldwide kelp forests are particularly valuable ecosystems that host a high biodiversity of marine life and complex food webs (Krause-Jensen et al., 2012; Teagle et al., 2017).
Caused by ubiquitous global climate change, the Arctic ocean has warmed four times faster than the global average since 1979 (Rantanen et al., 2022). Sea ice loss due to increasing water temperatures has led to a prolongation of the open water period potentially improving growth conditions for Arctic kelps (Krause-Jensen et al., 2012; Sumata et al., 2023). The associated changes of favorable underwater light regime and reduced physical disturbance by ice scraping are predicted to open up new habitats enabling an expansion of kelp forests into the future Arctic (Assis et al., 2022; Krause-Jensen & Duarte, 2014). At the same time, glaciers around the globe are melting rapidly (Hugonnet et al., 2021) and increasing meltwater runoff creates strong turbidity and salinity gradients with potential negative impacts on coastal productivity (Sejr et al., 2022; Jerosch et al., 2019).
Kongsfjorden (western Spitzbergen, Svalbard archipelago) is an example of a well monitored Arctic fjord that experiences strong impacts of climate change and is regarded to serve as a marine high latitude model ecosystem (Bischof et al., 2019b). The hydrography of Kongsfjorden is influenced by cold Arctic water from the East Spitsbergen Coastal Current flowing around the Svalbard shelf and warm saline Atlantic water from the West Spitsbergen Current (Svendsen et al., 2002). To the west Kongsfjorden is open to the Arctic Ocean where the water masses entering through the southern part are mixed with fresh water from glacial meltwater and river runoff before flowing out along the northern coast (Kruss et al., 2017). Reflecting the overall regime shift in the Arctic Ocean (Sumata et al., 2023), Kongsfjorden crossed the tipping point away from cold Arctic winters with persistent thick sea ice coverage in winter 2006 when warm water masses started to enter the fjord all year round (Tverberg et al., 2019). A long time series indicates that the overall seawater temperature and the number of ice-free days along the northwest coast of Svalbard has increased gradually within 30 years since 1980 (Kortsch et al., 2012). Continuous oceanographic measurements suggest that Kongsfjorden has already transitioned to an Atlantic-type fjord as depth averaged temperatures in the inner fjord in summer have increased by 0.26°C/yr since 2010 (De Rovere et al., 2022).
Kongsfjorden is lined by several glaciers at different stages of glacial retreat with four main calving tidewater glaciers and several land-terminating glaciers including Brøggerbreen at the Bayelva river (Pavlov et al., 2019; Svendsen et al., 2002). When freshwater from glacial melt, snow and precipitation enters the fjord in summer, it carries suspended terrestrial particles that form large sediment plumes which alter the spectral composition of the underwater light regime available for macroalgal photosynthesis (Niedzwiedz & Bischof, 2023; Pavlov et al., 2019).
The shallow subtidal of the Kongsfjorden coastline down to 15m depth is densely covered with macroalgal meadows but even at ~70m depth deep-water red algae and crustose coralline algae occur (Schimani et al., 2022; Kruss et al., 2017). Along the fjord axis multiple kelp forests are present which are strongly impacted by alterations in their environment on the species as well as the community level (Schimani et al., 2022; Bischof et al., 2019a; Hop et al., 2016). In 1996/98 Hop et al. (2012) investigated for the first time the biodiversity and biomass distribution of macroalgae along a depth transect on the rocky shore of Hansneset. They reported a rich kelp forest that was dominated by ‘Digitate Kelps’ (Laminaria digitata /Hedophyllum nigripes ), Alaria esculenta and Saccharina latissima and overall documented 62 macroalgal species. In 2012-14 the hard bottom community was examined a second time and standards for future monitoring were established by Bartsch et al. (2016) and Paar et al. (2016). In the upper subtidal zone (2.5m) kelp biomass in 2012/13 was significantly 8.2 fold higher compared to 1996/98 and the study revealed that not only the overall biomass maximum but the entire kelp forest had shifted upwards (Bartsch et al., 2016). These changes in the kelp forest community structure were discussed as being likely a consequence of altering abiotic conditions caused by Arctic warming and hence may largely impact coastal ecosystem services (Filbee-Dexter et al., 2019; Bartsch et al., 2016). Furthermore, the responses of different species and life stages of kelps in multi-stressor experiments indicate that some kelp species may benefit from climate change while others will retreat (Niedzwiedz & Bischof, 2023; Diehl & Bischof, 2021; Franke et al., 2021; Roleda, 2016; Zacher et al., 2016).
Despite their ecological importance, Arctic kelp forests are largely understudied and the investigations in Kongsfjorden are a rare example of consistent quantitative monitoring that provide important data for predictions of the future Arctic (Bischof et al., 2019a; Filbee-Dexter et al., 2019; Wernberg et al., 2019). It was therefore the objective of the current study to examine biomass and community development in an Arctic kelp forest spanning over 25 years of Arctic warming. In 2021 we repeated the investigations from 1996/98 (Hop et al., 2012) and 2012-14 (Bartsch et al., 2016) at the Hansneset sampling site and complemented the existing time series (Figure 1). The observed changes in the macroalgal community and new investigations on ecological differences between kelp species provide important insights for the key question, how kelp forests are affected in an Arctic fjord system that is influenced by warming, sea ice retreat and glacial melt.