Description: Macroalgae are major primary producers and ecosystem engineers along rocky shores of the Arctic. With Svalbard being a hotspot of global warming, macroalgal species distribution and biomass is undergoing rapid changes, presumably affecting all associated life forms. The overall retreat of the yearly ice cover is only the centerpiece in a complex interplay of environmental drivers noticeably altering light availability, disturbance regimes and nutrient supply for Arctic seaweeds. Between 1996/1998 and 2012/2013 ecosystem changes were observed at our study site Hansneset in Kongsfjorden, as algal biomass in the littoral zone doubled and the macroalgal biomass peak, as well as the lower distribution limit of most dominating kelp species, shifted upwards by several meters. In summer 2021, we seek to complement these datasets. Our interdisciplinary group will repeat the quantitative monitoring study to examine how the community pattern of seaweeds and their associated fauna has changed since the last expedition. In detail, we will quantify macroalgal biomass and biodiversity, the age structure and fertility of kelp species, and the biomass and biodiversity of associated animals along a depth gradient between 0 and 15 m. Incorporated into the European Horizon 2020 project FACE-IT and two associated Svalbard Science Forum field grants, our superior aim is to analyse the response of benthic macroalgal assemblages to observed cryosphere changes in a broad context. In this poster, we will share the first results from this year’s field samplings and show how abundances, biomass and species diversity of macroalgae have changed over the last decades at our investigation site and what this means in general for the development of macroalgae on Arctic coasts.

Description: Marine forests and kelps as their foundation species are threatened by ocean warming especially at the warm distributional edges. Previously identified genetic divergence and ecotypic differentiation within kelp species may allow to produce more resilient lineages by intraspecific outbreeding among populations. In a mechanistic investigation of heat stress, heterosis (hybrid vigour), and underlying gene expression patterns, we assessed the thermal performance of inbred (selfings) and outbred (reciprocal crosses) sporophytes of the N-Atlantic kelp Laminaria digitata among clonal isolates from two divergent populations; one from the temperate North Sea (Helgoland) and one from the Arctic (Spitsbergen). First, we investigated the upper thermal tolerance of microscopic sporophytes in a 14-day experiment applying sublethal to lethal 20–23°C. The upper survival temperature of microscopic sporophytes was lower for the inbred Arctic selfing (21°C) than for the temperate selfing and the reciprocal crosses (22°C). Only in the temperate selfing, 4.5% of sporophytes survived 23°C. We then subjected 4–7 cm long sporophytes to a control temperature (10°C), moderate (19°C) and sublethal to lethal heat stress (20.5°C) for 18 days to assess gene expression in addition to physiological parameters. Growth and optimum quantum yield decreased similarly in the reciprocal crosses and the temperate selfing at 19 and 20.5°C, while inbred Arctic sporophytes died within seven days at both 19 and 20.5°C. In response to 20.5°C, 252 genes were constitutively regulated across all surviving lineages, which we use to describe metabolic regulation patterns in response to heat stress in kelp. At sublethal 20.5°C, ca. 150 genes were differentially expressed by either crossed lineage in comparison to the temperate selfing, indicating that they maintained a growth response similar to the temperate selfing with differential metabolic regulation during sublethal heat stress. Subtle differences in physiology and the differential expression of nine genes between the reciprocal crosses at 20.5°C indicate that female and male gametophytes may contribute differently to offspring traits. We consider potential inbreeding depression in the Spitsbergen selfing and quantify the better performance of both crosses using heterosis-related parameters. We discuss the potential and risks of outbreeding to produce more resilient crops for mariculture and marine forest restoration.

Description: The geographical distribution of organisms, such as the foun- dation kelp species Saccharina latissima, is mainly driven by temperature. Globally increasing sea surface temperature and further intensification of marine heatwaves have already resulted in local extinction of kelp populations worldwide. In the present study, we investigated temporal variation in the thermal susceptibility of S. latissima by assessing stress responses of field sporophytes sampled from Helgoland (Ger- man Bight) in June 2018, August 2018 and August 2019 in heatwave scenarios. We analyzed survival, growth, maximum quantum yield of photosystem II (Fv/Fm) and pigment compo- sition. Survival decreased with increasing environmental and experimental temperatures. Growth revealed seasonal pat- terns, being higher in June than in August, whereas Fv/Fm decreased with increasing temperature, independent of the sampling time. We found an increase in the concentration of light harvesting pigments and in the de-epoxidation state of the xanthophyll cycle with higher treatment temperature. This pattern was even more pronounced at higher environmental temperature prior to the experiment (June 2018〈August 2019 〈 August 2018). Our results show that the thermal tol- erance of S. latissima towards heatwaves in summer is signifi- cantly affected by the environmental history it previously experienced.

Description: 〈jats:p〉Kelps in the Arctic region are facing challenging natural conditions. They experience over 120 days of darkness during the polar night surviving on storage compounds without conducting photosynthesis. Furthermore, the Arctic is experiencing continuous warming as a consequence of climate change. Such temperature increase may enhance the metabolic activity of kelps, using up storage compounds faster. As the survival strategy of kelps during darkness in the warming Arctic is poorly understood, we studied the physiological and transcriptomic responses of 〈jats:italic〉Saccharina latissima〈/jats:italic〉, one of the most common kelp species in the Arctic, after a 2-week dark exposure at two temperatures (0 and 4°C) versus the same temperatures under low light conditions. Growth rates were decreased in darkness but remained stable at two temperatures. Pigments had higher values in darkness and at 4°C. Darkness had a greater impact on the transcriptomic performance of 〈jats:italic〉S. latissima〈/jats:italic〉 than increased temperature according to the high numbers of differentially expressed genes between dark and light treatments. Darkness generally repressed the expression of genes coding for glycolysis and metabolite biosynthesis, as well as some energy-demanding processes, such as synthesis of photosynthetic components and transporters. Moreover, increased temperature enhanced these repressions, while the expression of some genes encoding components of the lipid and laminaran catabolism, glyoxylate cycle and signaling were enhanced in darkness. Our study helps to understand the survival strategy of kelp in the early polar night and its potential resilience to the warming Arctic.〈/jats:p〉