ALBERT

Beschreibung: The Arctic Ocean is particularly sensitive to climate change. Its ecosystem structure and function are prone to be disturbed by fast warming and massive retreat of sea-ice, which in turn, might result in feedbacks on climate. Moreover, such drastic changes are expected to influence the meridional fluxes of heat, freshwater and biogeochemical tracers between subpolar areas and the Arctic. As the third most important greenhouse gas and major ozone-depleting substance in the stratosphere, nitrous oxide (N2O) is a crucial gas to study in order to assess the ocean’s role in the production and exchange of climate-relevant compounds to the atmosphere. Between 2018 and 2019 we conducted ship-based surveys to elucidate the source-sink dynamics of N2O in the subpolar-polar North Atlantic. Based on results from those campaigns, we show the distribution and spatial variability of surface N2O, which ranged from moderate supersaturation (positive sea-air fluxes) in ice-free subpolar areas to unusually strong undersaturation (negative sea-air fluxes) in partially or fully ice-covered areas. We also present a comprehensive overview of the water column distribution of N2O in the region, and by combining this data with hydrographic and chemical (O2 and inorganic nutrients) information, we trace back the origin of the dominant water masses so as to illustrate the connectivity between the Fram Strait and the Nordic Seas off southeast Greenland. This analysis is used to discuss how the meridional water mass exchange in the region influences the balance of local vs. remote N2O production and its spatial variability. Furthermore, we use the results from collocated molecular analyses (functional gene markers) to infer the occurrence and abundances of the main microbial communities responsible for the cycling of N2O. This contribution is relevant for assessments of expected changes in trace gas emissions with further climate-driven changes in the Arctic Ocean.

Beschreibung: We present results from a coordinated frontal survey in Fram Strait in summer 2016 using an autonomous underwater vehicle (AUV) combined with shipboard and zodiac-based hydrographic measurements. Based on satellite information, we identified a front between warm Atlantic Water and cold Polar Water. The AUV, equipped with oceanographic and biogeochemical sensors, profiled the upper 50 m along a 10 km-long cross-front oriented transect resulting in a high-resolution snapshot of the upper ocean. The transect was dominated by a 6 km-wide, 10 m-thick subsurface patch of high chlorophyll, located near the euphotic depth within a band of cold water. Nitrate was depleted in the surface, but abundant below the pycnocline. Potential vorticity and Richardson number estimates indicate conditions favorable for vertical mixing, which indicates that the high chlorophyll patch may have been sustained by upward nitrate fluxes. Our observations underline the complex hydrographic and biogeochemical structure in a region featuring fronts and meanders, and further underline the patchy and small-scale nature of subsurface phytoplankton blooms potentially fueled by submesoscale dynamics, which are easily missed by traditional surveys and satellite missions.

Beschreibung: Time-series observations provide an essential baseline to identify biological responses to environmental fluctuations, and to distinguish natural variability from human impact. Here, we describe for the first time year-round microbial and oceanographic dynamics in the partially ice-covered Fram Strait (Arctic Ocean) using autonomous samplers deployed as part of the infrastructure program FRAM, allowing unprecedented insights into the marine microbial ecology of the polar night. Bacterial communities showed a strong seasonal signal, especially in the West Spitsbergen Current. Here, distinct temporal succession of phytoplankton and bacterial clades occurred, including covariance of the magnetotactic bacterium Magnetospira with daylight hours. Summer featured weekly variability in blooming taxa, including the flavobacterial genera Formosa and Polaribacter succeeding peaks of the diatoms Thalassiosira and Grammonema. The Bacteroidetes peak in summer was followed by dominance of SAR11 in fall and Nitrosopumilus in winter, suggesting timely controlled ecological roles. Bacterial diversity was highest in winter, featuring elevated abundances of Planctomycetes and Nitrospinia as well as the heterotrophic eukaryote taxa Syndiniales and Radiolaria. Late winter was characterized by increasing proportions of e.g. Dadabacteria before the onset of the productive season. In the ice-covered East Greenland Current (Arctic Ocean outflow), bacterial community structure showed less seasonality. Here, bacterial diversity decreased in response to ice cover dynamics and nitrate availability, underlining that changing ice and light regimes likely impact plankton diversity and biogeochemical processes.

Beschreibung: The biological carbon pump removes CO2 from the atmosphere via phytoplankton growth in the sunlit stratified upper ocean. This is followed by a partial export to deeper water layers and finally deposition on the sea floor. In polar regions, sea ice affects stratification and light availability. However, it remains unclear to what extent sea ice cover affects the biological carbon pump. Additionally, climate change is expected to increase the seasonal ice zone (i.e., the part of the ocean that is ice-covered for part of the year). Observational time series of the seasonal cycle contrasting production and export in ice-covered and ice-free conditions are still scarce. Here, we present multidisciplinary time series observations from the marginal ice zone in Fram Strait (located between Greenland and Svalbard) of all ocean compartments from the sea surface to the seafloor. Data are from two contrasting years when our measurement site was either partially ice-covered (2016-2017) or when the ice edge was located further to the north (2017-2018). We start by introducing the study site followed by a description of the atmospheric forcing and its effects on the upper ocean stratification and hydrography. In mostly ice-free conditions, the mixed layer went from deep to stratified in a single event, whereas the presence of sea ice resulted in a number of alternating events of shallow and deep mixed layers. These patterns changed phytoplankton and bacterial dynamics and upper ocean chemistry. The onset of the bloom was much more gradual in the presence of sea ice. We also explore the different trophic levels in the upper that contributed to the export recorded at the seafloor which fueled biological benthic activity a few weeks after the bloom. Our observations reveal an impact of the respective sea ice patterns on concomitant species and biogeochemical reactions in the upper water column and for the export of organic matter to the deep sea in the Fram Strait.

Beschreibung: The ocean is currently a significant net sink for anthropogenically remobilised CO2, taking up around 24% of global emissions. Numerical models predict a diversity of responses of the ocean carbon sink to increased at- mospheric concentrations in a warmer world. Here, we tested the hypothesis that increased atmospheric forcing is causing a change in the ocean carbon sink using a high frequency observational dataset derived from un- derway pCO2 (carbon dioxide partial pressure) instruments on ships of opportunity (SOO) and a fixed-point mooring between 2002 and 2016. We calculated an average carbon flux of 0.013 Pg yr−1 into the ocean at the Porcupine Abyssal Plain (PAP) site, consistent with past estimates. In spite of the increase in atmospheric pCO2, monthly average seawater pCO2 did not show a statistically significant increasing trend, but a higher annual variability, likely due to the decreasing buffer capacity of the system. The increasing pCO2 led to an increasing trend in the estimated CO2 flux into the ocean of 0.19 ± 0.03 mmol m−2 day−1 per year across the entire 15 year time series, making the study area a stronger carbon sink. Seawater pCO2 variability is mostly influenced by temperature, alkalinity and dissolved inorganic carbon (DIC) changes, with 77% of the annual seawater pCO2 changes explained by these terms. DIC is in turn influenced by gas exchange and biological production. In an average year, the DIC drawdown by biological production, as determined from nitrate uptake, was higher than the DIC increase due to atmospheric CO2 dissolution into the surface ocean. This effect was enhanced in years with high nutrient input or shallow mixed layers. Using the rate of change of DIC and nitrate, we observed Redfieldian carbon consumption during the spring bloom at a C:N ratio of 6.2 ± 1.6. A comparison between SOO and PAP sustained observatory data revealed a strong agreement for pCO2 and DIC. This work demonstrates that the study area has continued to absorb atmospheric CO2 in recent years with this sink enhancing over time. Furthermore, the change in pCO2 per unit nitrate became larger as surface buffer capacity changed.

Beschreibung: Amino acids (AA) and carbohydrates (CHO) are important components of the marine organic carbon cycle. Produced mainly by phytoplankton as part of the particulate organic carbon (POC) fraction, these compounds can be released into the outer medium where they become part of the dissolved organic carbon (DOC) pool and are rapidly taken up by heterotrophs (e.g., bacteria). We investigated the quantity and quality of POC and DOC, AA and CHO composition in both pools in three different water masses in the Fram Strait (Arctic Ocean) in summer 2017. Polar Waters and Atlantic Waters showed similar concentrations of particulate and dissolved AA and CHO, despite Polar Waters showing the highest DOC concentrations. In Mixed Waters, where the two water masses mix with each other and with melting sea ice, the concentrations of particulate and dissolved AA and CHO were highest. AA and CHO composition differed substantially between the particulate and dissolved fractions. The particulate fraction (〉0.7 μm) was enriched in essential AA and the CHO galactose, xylose/mannose, and muramic acid. In the dissolved fraction non-essential AA, several neutral CHO, and acidic and amino CHO were enriched. We further investigated different size fractions of the particulate matter using a separate size fractionation approach (0.2–0.7 μm, 0.7–10 μm and 〉10 μm). The chemical composition of the 0.2–0.7 μm size-fraction had a higher contribution of non-essential AA and acidic and amino sugars, setting them apart from the 0.7–10 μm and 〉10 μm fractions, which showed the same composition. We suggest that the relative differences observed between different size fractions and DOC with regards to AA and CHO composition can be used to evaluate the state of organic matter processing and evaluate the contribution of autotrophic phytoplankton or more heterotrophic biomass. In the future, changing conditions in the Central Arctic Ocean (Atlantification, warming, decreasing ice concentrations) may increase primary production and consequently degradation. The AA and CHO signatures left behind after production and/or degradation processes occurred, could be used as tracers after the fact to infer changes in microbial loop processes and food web interactions.