ALBERT

Description: The Arctic Ocean is a region particularly prone to ongoing ocean acidification (OA) and climate-driven changes. The influence of these changes on Arctic phytoplankton assemblages, however, remains poorly understood. In order to understand how OA and enhanced irradiances (e.g., resulting from sea–ice retreat) will alter the species composition, primary production, and ecophysiology of Arctic phytoplankton, we conducted an incubation experiment with an assemblage from Baffin Bay (71°N, 68°W) under different carbonate chemistry and irradiance regimes. Seawater was collected from just below the deep Chl a maximum, and the resident pytoplankton were exposed to 380 and 1000 latm pCO2 at both 15 and 35% incident irradiance. On-deck incubations, in which temperatures were 6°C above in situ conditions, were monitored for phytoplankton growth, biomass stoichiometry, net primary production, photo-physiology, and taxonomic composition. During the 8-day experiment, taxonomic diversity decreased and the diatom Chaetoceros socialis became increasingly dominant irrespective of light or CO2 levels. We found no statistically significant effects from either higher CO2 or light on physiological properties of phytoplankton during the experiment. We did, however, observe an initial 2-day stress response in all treatments, and slight photo-physiological responses to higher CO2 and light during the first five days of the incubation. Our results thus indicate high resistance of Arctic phytoplankton to OA and enhanced irradiance levels, challenging the commonly predicted stimulatory effects of enhanced CO2 and light availability for primary production.

Description: We collected Arctic Ocean water column samples for methane (CH4) and nitrous oxide (N2O) analysis on three separate cruises in the summer and fall of 2015, covering a ~10,000 km transect from the Bering Sea to Baffin Bay. This provided a three-dimensional view of CH4 and N2O distributions across contrasting hydrographic environments, from the oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive shelves of the Bering and Chukchi Seas. Percent saturation relative to atmospheric equilibrium ranged from 30-800% for CH4 and 75-145% for N2O, with the highest concentrations of both gases occurring in the northern Chukchi Sea. Nitrogen cycling in the shelf sediments of the Bering and Chukchi Seas likely constituted the major source of N2O to the water column, and the resulting high N2O concentrations were transported across the Arctic Ocean in eastward-flowing water masses. Methane concentrations were more spatially heterogeneous, reflecting a variety of localized inputs, including likely sources from sedimentary methanogenesis and sea ice processes. Unlike N2O, CH4 was rapidly consumed through microbial oxidation in the water column, as shown by the 13C enrichment of CH4 with decreasing concentrations. For both CH4 and N2O, sea-air fluxes were close to neutral, indicating that our sampling region was neither a major source nor sink of these gases. Our results provide insight into the factors controlling the distribution of CH4 and N2O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed.

Description: The Arctic Ocean is one of the regions most prone to on-going ocean acidification (OA) and climate-driven changes, including increased sea surface temperature, sea-ice melt and altered mixing regimes. However, the influence of these changes on Arctic primary productivity, phytoplankton ecology and elemental cycles remains poorly understood. To date, the impact of various environmental stressors on phytoplankton have largely been assessed in isolation, and only limited process-understanding was gained. In order to understand how OA and enhanced irradiances (resulting from sea-ice retreat and increased mixed layer stratification) will alter the species composition, productivity and ecophysiology of Arctic phytoplankton, we conducted four incubation experiments with natural plankton assemblages from Davis Strait (63°N), Baffin Bay (71°N) and Kongsfjorden (Svalbard, 79°N). Phytoplankton assemblages were exposed to 400 and 1200 µatm pCO2 at both low and high irradiance levels over several weeks. These incubations were monitored and characterised in terms of phytoplankton growth, nutrient usage, biomass stoichiometry, net primary production (NPP), photophysiology and species composition. Preliminary results indicate that while the Subarctic Davis Strait assemblage exhibited light- and CO2-dependent growth rates and NPP, while there were no such differences between treatments in the Arctic assemblages (Baffin Bay and Svalbard). The observed similarities and differences in composition, productivity and physiology of phytoplankton assemblages grown under different climate scenarios will be discussed. Overall, our results indicate a high level of resilience of Arctic primary producers to climate-dependent environmental change.

Description: The Arctic Ocean is one of the regions most prone to on-going ocean acidification (OA) and climate-driven changes, including increased sea surface temperature, sea-ice melt and altered mixing regimes. However, the influence of these changes on Arctic primary productivity, phytoplankton ecology and elemental cycles remains poorly understood. To date, the impact of various environmental stressors on phytoplankton have largely been assessed in isolation, and only limited process-understanding was gained. In order to understand how OA and enhanced irradiances (resulting from sea-ice retreat and increased mixed layer stratification) will alter the species composition, productivity and ecophysiology of Arctic phytoplankton, we conducted two incubation experiments with natural plankton assemblages from Davis Strait (63°N) and Baffin Bay (71°N) during the Arctic-GEOTRACES summer 2015 campaign. Phytoplankton assemblages were exposed to 400 and 1200 µatm pCO2 at both 15% and 35% surface irradiance over two weeks. These incubations were monitored and characterised in terms of phytoplankton growth, nutrient usage, biomass stoichiometry, net primary production (NPP), photophysiology and species composition. Preliminary results indicate that the Subarctic Davis Strait assemblage exhibited light- and CO2-dependent growth rates and NPP, while there were no such differences between treatments in the Arctic Baffin Bay assemblage. The suite of physiological measurements conducted in this study will be exploited to provide a mechanistic understanding of the observed differences between phytoplankton assemblages. Results from our work will provide insight into the resilience of Arctic primary producers to climate-dependent environmental change.

Description: As phytoplankton provide the carbon and the energy for all higher trophic levels in the oceans, future changes in phytoplankton productivity and species composition will also impact the entire ecosystem. In view of their pivotal role in ecosystem functioning and biogeochemistry, the responses of diatoms to ocean acidification have gained increasing attention. Even though responses seem to species specific, diatoms have been regarded as potential winners of ocean acidification. In the recent years, also the effects of multiple stressors (i.e. the combination of changes in carbonate chemistry, temperature as well as light and nutrient availabilities) have been investigated. Under these more realistic environmental settings, the future fate of marine diatoms looks grimmer. During the talk, our current understanding of diatom responses to climate change will be presented. Special emphasis will be put on our recent findings from the polar environment.

Description: The Arctic Ocean is one of the regions most prone to on-going ocean acidification (OA) and climate-driven changes, including increased sea surface temperature, sea-ice melt and altered mixing regimes. However, the influence of these changes on Arctic primary productivity, phytoplankton ecology and elemental cycles remains poorly understood. To date, the impact of various environmental stressors on phytoplankton have largely been assessed in isolation, and only limited process-understanding was gained. In order to understand how OA and enhanced irradiances (resulting from sea-ice retreat and increased mixed layer stratification) will alter the species composition, productivity and ecophysiology of Arctic phytoplankton, we conducted two incubation experiments with natural plankton assemblages from Davis Strait (63°N) and Baffin Bay (71°N) during the Arctic-GEOTRACES summer 2015 campaign. Phytoplankton assemblages were exposed to 400 and 1200 µatm pCO2 at both 15% and 35% surface irradiance over two weeks. These incubations were monitored and characterised in terms of phytoplankton growth, nutrient usage, biomass stoichiometry, net primary production (NPP), photophysiology and species composition. Preliminary results indicate that the Subarctic Davis Strait assemblage exhibited light- and CO2-dependent growth rates and NPP, while there were no such differences between treatments in the Arctic Baffin Bay assemblage. The suite of measurements conducted in this study will be exploited to provide a mechanistic understanding of the observed similarities and differences between phytoplankton assemblages. Our results indicate a high level of resilience of Arctic primary producers to climate-dependent environmental change.