Description: The kelp Alaria esculenta represents a key species in high Arctic marine fjord ecosystems. However, the European Arctic is currently experiencing extensive environmental change. Glacial fjord systems, such as Kongsfjorden (Spitsbergen, Svalbard), are subjected to rising temperature, increased freshwater inflow from glaciers and melting snow and a changing ultraviolet (UV) radiation regime related to stratospheric ozone depletion. Thus, in addition to natural seasonality, sessile organisms require acclimation in order to adapt to an environment in transition. We examined the physiological and ultrastructural responses of A. esculenta to the combined exposure to hyposalinity and UV radiation. Photosynthetic quantum yield slightly decreased during a low-salinity treatment of 7 d. Exposure to UV radiation also lowered quantum yield, but specimens previously treated with hyposalinity were significantly less susceptible to UV than nontreated individuals. Concomitant with a loss of chlorophyll during the hyposaline treatment, phlorotannin and antioxidant contents were maintained, and samples treated with low salinities exhibited higher UV-screening characteristics as demonstrated by significantly higher absorption ratios at 300/680 nm. Ultrastructural analyses revealed a treatment-dependent swelling of cell walls and accumulations of phlorotannin-containing vesicles. Our findings point to a strategy by which kelps apply a fast and cost-efficient redistribution of phlorotannins rather than increased synthesis as a general stress response to different environmental drivers in contrast to stress-specific responses. The notion that acclimation to one stressor (low salinity) reflects increased tolerance towards a second stressor (UV radiation) supports the concept of 'cross-acclimation' as established for higher plants but not yet for seaweeds.

Description: Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today's relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light, low mixing treatment) and a strong mixing scenario (high Fe and low light, strong mixing treatment). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (~75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (~65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (~80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

Description: Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today's relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light, low mixing treatment) and a strong mixing scenario (high Fe and low light, strong mixing treatment). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (~75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (~65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (~80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

Description: Polar Regions are facing rapid temperature increase. Combined with other factors temperature increase might have a strong impact on foundation species in Arctic shallow-water coastal ecosystems, such as the abundant kelp Saccharina latissima. We ran two short-term 2-factor experiments with field samples from Kongsfjorden (Svalbard) to reveal the impact of temperature increase in summer combined with hyposalinity (temperature × salinity) or nutrient enrichment (temperature × nutrients) and analyzed different biochemical and physiological parameters. The experiments were conducted with field samples at AWIPEV Station in Ny-Ålesund, Svalbard (Spitsbergen) in June/July 2019. As physiological parameter, size and the maximum photosynthetic quantum yield of photosystem II (Fv/Fm; Imaging-PAM, Walz GmbH Mess- und Regeltechnik, Effeltrich, Germany) were monitored every second day. For growth, the size of the algal discs was analyzed with ImageJ (Version 1.52a). For better comparison of the physiological parameters, Fv/Fm and growth the initial size of the different treatments was adjusted to 100% and size of each sample as % of initial was calculated. The C:N ratio, total nitrogen and total carbon content were analyzed with an elemental analyzer. Mannitol, as well as absolute pigment concentrations were analyzed using a HPLC. The de-expoxidation state of the xanthophyll cycle (DPS) and chlorophyll a : accessory pigment ratio calculated afterwards. Phlorotannins were analyzed using the photometric Folin-Ciocalteu method.

Description: Kelps act as ecosystem engineers and foundation species on many polar rocky shore coastlines. The main driver for their vertical and latitudinal distribution is the underwater light climate and temperature. Both are changing drastically in the Arctic in the course of global climate change. It was the aim of this study to analyse the effects of rising temperature and deteriorating underwater light climate on the potential habitat of kelps in the Arctic. The analyses of the underwater light climate in Arctic Kongsfjorden, Svalbard in July 2021. We divided Kongsfjorden in three areas, which are influenced by the run-off of sea-terminating glaciers (station A–J), the run-off of a land-terminating glacier (station K–O) and mostly clear water (control, station P–Q). In each area, we measured the spectrally resolved underwater light climate in the UV-B radiation (280-320 nm), UV-A radiation (320-400 nm) and photosynthetically active radiation (PAR, 400-700 nm) with a RAMSES-ACC-UV/VIS radiometer (TriOS Optical Sensor, Oldenburg, Germany) from 0–12.5 m. UV-B, UV-A and PAR were calculated by integrating the irradiance over the respective wavelengths.

Description: Kelps, perennial brown seaweeds of the order Laminariales, are foundational species in Arctic coastal ecosystems. Presently, their ability to persist under polar night conditions might be significantly affected by increasing winter temperatures. We assessed physiological parameters (photosynthesis, pigment content, respiration, carbohydrate storage) in two species of Arctic kelp, the boreal-temperate Saccharina latissima and the Arctic-endemic Laminaria solidungula, during the polar night 2016/17. Algae were sampled from Kongsfjorden, Svalbard, 78° 55' N, shortly before the onset of the dark period in October, and at the end of the polar night in early February. Analyses were conducted for different tissue sections along the phylloid (Meristem, Centre Region, Distal Region). Data suggest that kelp maintain their photosynthetic competence throughout the entire winter period, as indicated by PE-curve-parameters, and photosynthetic pigment contents. Overall laminarin content was reduced by 96 % in S. latissima, and by 90 % in L. solidungula during winter indicating that this storage glucan fuelled metabolic function during the polar night. Marked differences in laminarin content between the phylloid regions and across species indicated specific adaptive mechanisms between boreal-temperate and Arctic-endemic kelp. We suggest that laminarin turnover represents a sensitive parameter to assess kelp physiology under a changing temperature regime.