Higher sensitivity towards light stress and ocean acidification in an Arctic sea‐ice‐associated diatom compared to a pelagic diatom

Thalassiosira hyalina and Nitzschia frigida are important members of Arctic pelagic and sympagic (sea‐ice‐associated) diatom communities. We investigated the effects of light stress (shift from 20 to 380 µmol photons m−2 s−1, resembling upwelling or ice break‐up) under contemporary and future pCO2 (400 vs 1000 µatm). The responses in growth, elemental composition, pigmentation and photophysiology were followed over 120 h and are discussed together with underlying gene expression patterns. Stress response and subsequent re-acclimation were efficiently facilitated by T. hyalina, which showed only moderate changes in photophysiology and elemental composition, and thrived under high light after 120 h. In N. frigida, photochemical damage and oxidative stress appeared to outweigh cellular defenses, causing dysfunctional photophysiology and reduced growth. pCO2 alone did not specifically influence gene expression, but amplified the transcriptomic reactions to light stress, indicating that pCO2 affects metabolic equilibria rather than sensitive genes. Large differences in acclimation capacities towards high light and high pCO2 between T. hyalina and N. frigida indicate species‐specific mechanisms in coping with the two stressors, which may reflect their respective ecological niches. This could potentially alter the balance between sympagic and pelagic primary production in a future Arctic

Arctic sea ice algae differ markedly from phytoplankton in their ecophysiological characteristics

acceptedVersio

Seasonal plankton dynamics in Kongsfjorden during two years of contrasting environmental conditions

Seasonal plankton time-series data are presented from Kongsfjorden from two years with contrasting environmental conditions. Kongsfjorden (west coast of Spitsbergen – 79°N) integrates inputs from Atlantic and Arctic waters, and glacier run-off, and is thus a prime location to study impacts on ecosystem dynamics of key environmental drivers that are relevant across the Arctic. Despite extensive research in Kongsfjorden, seasonally-resolved data are scarce. From late April/early May to early September 2019 and 2020, we conducted pelagic sampling at a mid-fjord station at mostly weekly to bi-weekly resolution investigating the environmental drivers of phyto- and zooplankton community composition and phenology. During spring 2019, Atlantic water masses with temperatures > 1 °C were found throughout the upper 250 m of the water column, and little sea ice occurred in the fjord. Spring 2020, in turn, was characterized by the presence of local water masses with sub-zero temperatures and relatively extensive sea-ice cover. The most striking contrast between the two years was the difference in phytoplankton spring bloom composition. In 2019, the spring bloom was dominated by the colonial stage of the haptophyte Phaeocystis pouchetii and diatoms played a minor role, while the spring bloom in 2020 was dominated by diatoms of the genus Thalassiosira succeeded by P. pouchetii. Selective grazing by large copepods and water mass structure seem to have been the decisive factors explaining the marked difference in diatom spring bloom biomass between the years while similar spring abundances of P. pouchetii in both years indicated that this species was less impacted by those factors. Our data suggest that differences in spring bloom composition impacted trophic transfer and carbon export. Recruitment of the dominant copepods Calanus finmarchicus and C. glacialis, Cirripedia and euphausiid larvae as well as the export of carbon to the seabed was more efficient during the diatom-dominated compared to the P. pouchetii–dominated spring bloom. In summer, the plankton composition shifted towards a flagellate-dominated community characterized by mixo- and heterotrophic taxa adapted to a lower nutrient regime and strong top-down control by copepod grazers. However, residual silicic acid after the P. pouchetii–dominated spring bloom fueled a late summer diatom bloom in 2019. Our data provide a first glimpse into the environmental drivers of plankton phenology and underline that high-resolution monitoring over many annual cycles is required to resolve the ephemeral variations of plankton populations against the backdrop of climate change.publishedVersio

Ecophysiological Responses of Sea Ice Algae and Phytoplankton to a Changing Arctic

The ice-covered seas of the Arctic have two major types of primary producers; phytoplankton growing in open waters and sea ice algae growing within and on the underside of the sea ice. This thesis investigates the controlling role of light availability on Arctic pelagic and sympagic (i.e. ice-associated) algae, and how light-induced responses are modulated by NO3 and pCO2 levels. A combination of field sampling, in situ experimental studies, and laboratory experiments were performed in order to investigate photophysiological and biochemical characteristics of pelagic and sympagic algae and identify their respective responses to changes in their abiotic environment. The results revealed that in both pelagic and sympagic algae, a change in light availability exerted stronger control on photophysiological and biochemical characteristics than variations in NO3 and pCO2 levels. Pelagic algae have evolved pronounced mechanisms into being flexible with different irradiances they encounter in a wind-mixed pelagic environment. Even though the ambient light during the polar night was not enough to support any measurable net primary production, they maintained an active photosynthetic apparatus, which ensured a fast recovery and utilization of even very low constant irradiances upon re-illumination. Furthermore, they effectively exploited very low irradiances for carbon fixation, handled instantaneous light stress well, and exhibited high photoacclimative capacity towards increasing irradiances. In conclusion, these results imply a high capacity of pelagic algae to compensate for changes in the environment, which can be understood in light of environmental conditions they have adapted to. Sympagic algae also efficiently harvested low irradiances for light-dependent photosynthesis. However, they probably used more of the photosynthetic resources for tolerating extreme physico-chemical properties within sea ice, which resulted in lower rates of carbon assimilation compared to pelagic algae. Sympagic algae also showed higher sensitivity towards high light than pelagic algae, where the highest irradiances caused dysfunctional photophysiology and non-vital cells in the former. Moreover, they exhibited higher sensitivity towards a combination of multiple stressors. The Arctic ocean is changing fast in many respects, amongst which increased light regimes, stratification, and ocean pCO2 levels stand out as being most important for microalgal communities. The results of this study suggest that sea ice algae will struggle more with adapting to the expected environmental changes compared to phytoplankton. We therefore anticipate a change in sea ice-based vs. pelagic primary production with respect to timing and quantity in a future Arctic, with potentially cascading effects on downstream food webs. The clearly distinct responses of pelagic vs. sympagic algae to environmental differences also need to be incorporated into model-based scenarios of future Arctic algae blooms and considered when predicting implications for the entire ecosystem

How Lamina Age Influence Photosynthetic Response to Light Variations in Saccharina latissima

Fotosyntetisk rate, målt på grunnlag av in vivo klorofyll a fluorescens, som en funksjon av økende lamina alder ble undersøkt i Saccharina latissima fra Trondheimfjorden, Norge i vinter, sommer og høst 2013. Sesongens innflytelse på forholdet mellom alder på vevet og akklimatisering ble undersøkt. Okende alder av vev ble bare funnet å ha signifikant innflytelse på fotosyntetiske parametere i sommer (mai) i motsetning til de andre sesongene. Variasjon i fotosyntetiske parametere over hele lengden av lamina varierte mellom individer, både mellom og innen årstider. Resultater fra denne studien viser at fotosyntese i et enkelt individ av S. latissima er avhengig av hvordan lamina er plassert i vannsøylen både over bredden og lengden av lamina, og hvordan det tilgjengelige lyset belyser de forskjellige deler av lamina. Forskjellen mellom vev av ulik alder var større i store individer, samlet i en tett tareskog. I dette scenariet kan lamina bli eksponert av en rekke irradianser, på grunn av selvskygging og skygging fra andre individer. Fotosyntetiske rater var også avhengig av sesong som et resultat av varierende temperatur, næringsstoffer og lys klima. Betydelig lavere Pmax , Ek og α ble funnet i individer samlet om høsten i motsetning til individer samlet i vinter og sommer, noe som indikerer næringsmangel og lav lys akklimatisering . Lav lys akklimatiserte alger om høsten er antatt å være et resultat av en høy konsentrasjon av optisk aktive komponenter i vannet, og dermed gi en betydelig demping av lys i vannsøylen og lavere lys. Denne studien viser at målinger av fotosyntese fra meristem vev kan være forskjellig fra andre deler av lamina på grunn av forskjellig alder og fysiologi. Derfor er det ikke nok å måle fotosynteserate og produksjon på en del av S. latissima lamina og anta at disse er representative for hele lengden av lamina

Inorganic nutrients measured on water bottle samples from CTD Water-sampler system and ice cores during FAABulous project period (2015-2018)

The project "Future Algae Blooms" (FAABulous) was funded by the Norwegian Research Council under their OKOSYSTEM call in 2014, and runs from April 2015 until March 2020 (project nr. 243702). This project was intended to show how ongoing climate change will affect the development of algae blooms in sea ice and water in a future Arctic ocean. The final results will consist of an amalgamation of (i) extensive field studies in two Arctic fjord systems with contrasting environmental characteristics, (ii) experiments to study the combined effect of increased light and CO2 on natural algae communities and single species, and (iii) developing models that allow us to study the relative importance (and joint effects) of different stress factors and the effect of these three processes on Arctic algal blooms in sea ice and water, with a special focus on the control of the onset and development of a bloom. Inorganic nutrients were measured in water samples and ice cores throughout the project period from 2015 to 2017

Spatial and temporal variability of ice algal trophic markers—with recommendations about their application

Assessing the relative importance of sea ice algal-based production is often vital for studies about climate change impacts on Arctic marine ecosystems. Several types of lipid biomarkers and stable isotope ratios are widely used for tracing sea ic-associated (sympagic) vs. pelagic particulate organic matter (POM) in marine food webs. However, there has been limited understanding about the plasticity of these compounds in space and time, which constrains the robustness of some of those approaches. Furthermore, some of the markers are compromised by not being unambiguously specific for sea ice algae, whereas others might only be produced by a small sub-group of species. We analyzed fatty acids, highly branched isoprenoids (HBIs), stable isotope ratios of particulate organic carbon (POC) (δ13C), as well as δ13C of selected fatty acid markers during an Arctic sea ice algal bloom, focusing on spatial and temporal variability. We found remarkable differences between these approaches and show that inferences about bloom characteristics might even be contradictory between markers. The impact of environmental factors as causes of this considerable variability is highlighted and explained. We emphasize that awareness and, in some cases, caution is required when using lipid and stable isotope markers as tracers in food web studies and offer recommendations for the proper application of these valuable approaches

Fatty acid composition, ice chemistry and algae biomass during an Arctic sea ice algal bloom on landfast ice near the settlement of Svea, in Van Mijenfjorden, Spitsbergen

The relative importance of sea ice algal-based production is often vital for studies about climate change impacts on Arctic marine ecosystems. The relevance of sea ice algal production for different parts of the polar ecosystem, ranging from key pelagic grazers to mammals and assessing the overall strength of sympagic-pelagic or sympagic-benthic coupling has been extensively studied. The key interest in all these studies is quantifying the relative importance of biomass produced by sea ice algae (as opposed to phytoplankton) for higher trophic level production. Different types of trophic markers are widely applied to analyse food web structure, based on numerous assumptions of how sea ice algae differ biochemically from phytoplankton. Several types of lipid biomarkers and stable isotope ratios are widely used for tracing sea ice associated (sympagic) vs. pelagic particulate organic matter (POM) in marine food webs. Beside the typical proximate data (POC, Chl-a, DIC, Biomass), we analysed fatty acids, highly branched isoprenoids (HBIs), stable isotope ratios of particulate organic carbon (POC) (δ13C), as well as δ13C of selected marker fatty acids during an Arctic sea ice algal bloom, focusing on spatial and temporal variability. Sampling was conducted on landfast ice near the settlement of Svea, in Van Mijenfjorden, Spitsbergen from 7 April to 5 May. A sea ice observatory was installed close to the deepest part of the inner basin on 8th of March providing background data for the entire study period on sea ice thickness, snow cover, and transmittance. Sea ice algal development was followed by sampling different stations in the inner basin from early March to early May 2017, including a spatial distribution along a transect starting from a very shallow station close to the shore (IS) to the mid-fjord station VMF2. The aim of our study was to document the spatial and temporal variability of different lipid and stable isotope-based trophic markers during an ice algal spring bloom and relate it to environmental conditions, as well as taxonomic composition. We then use these data to compare and contrast the reliability of each approach to distinguish between sympagic and pelagic POM