Clara Jule Marie Hoppe

The phytoplankton spring bloom constitutes the majority of annual Arctic primary production. In the Eurasian Arctic it is usually composed of various diatoms and the haptophyte Phaeocystis; recently, Phaeocystis-dominated blooms seem to be expanding. Low silicic acid (Si) availability has been hypothesised to promote Phaeocystis blooms, though the key drivers of Phaeocystis and diatom competition are not yet well understood. To investigate how Si availability controls bloom succession and biogeochemical fluxes we conducted a 3-week mesocosm experiment in Kongsfjorden, Svalbard in May 2024 on the spring phytoplankton community. The initial phytoplankton community was composed predominantly of Phaeocystis and the diatom Chaetoceros. Eight 450-L mesocosms received an Si gradient (0–40 µM) with nitrogen and phosphorus additions every 48 hours and we tracked nutrient uptake rates, community composition, elemental cycling and export. The nutrient additions stimulated phytoplankton growth across all Si levels. Phase I (Days 0–14) was dominated by a Phaeocystis bloom, terminating synchronously due to nitrogen and phosphorus depletion despite variable Si availability. In phase II (Days 16–22) Chaetoceros emerged as the dominant species with clear Si gradient effects: Chaetoceros spp. sustained exponential growth (µ = 0.40 d^-1) and achieved higher abundances in >3 µM Si treatments, while collapsing in Si-depleted mesocosms. Overall, particulate organic carbon export fluxes were low, but were strongly correlated with biogenic silica export fluxes, and increased exponentially in phase II of the experiment reflecting increasingly diatom-dominated sinking material. Our results demonstrate that Si availability does not govern initial Phaeocystis–diatom competition, but is the primary determinant of diatom bloom magnitude. The nutrient storage capacity of Chaetoceros confers resilience under transient depletion, while micrograzing and viral infection further mediate community successions. This study underscores the importance of integrating cell-specific and ecosystem-level measurements over extended timescales when predicting climate-driven shifts in Arctic primary production and carbon export.

The phytoplankton spring bloom constitutes the majority of annual Arctic primary production. In the Eurasian Arctic it is usually composed of various diatoms and the haptophyte Phaeocystis; recently, Phaeocystis-dominated blooms seem to be expanding. Low silicic acid (Si) availability has been hypothesised to promote Phaeocystis blooms, though the key drivers of Phaeocystis and diatom competition are not yet well understood. To investigate how Si availability controls bloom succession and biogeochemical fluxes we conducted a 3-week mesocosm experiment in Kongsfjorden, Svalbard in May 2024 on the spring phytoplankton community. The initial phytoplankton community was composed predominantly of Phaeocystis and the diatom Chaetoceros. Eight 450-L mesocosms received an Si gradient (0–40 µM) with nitrogen and phosphorus additions every 48 hours and we tracked nutrient uptake rates, community composition, elemental cycling and export. The nutrient additions stimulated phytoplankton growth across all Si levels. Phase I (Days 0–14) was dominated by a Phaeocystis bloom, terminating synchronously due to nitrogen and phosphorus depletion despite variable Si availability. In phase II (Days 16–22) Chaetoceros emerged as the dominant species with clear Si gradient effects: Chaetoceros spp. sustained exponential growth (µ = 0.40 d^-1) and achieved higher abundances in >3 µM Si treatments, while collapsing in Si-depleted mesocosms. Overall, particulate organic carbon export fluxes were low, but were strongly correlated with biogenic silica export fluxes, and increased exponentially in phase II of the experiment reflecting increasingly diatom-dominated sinking material. Our results demonstrate that Si availability does not govern initial Phaeocystis–diatom competition, but is the primary determinant of diatom bloom magnitude. The nutrient storage capacity of Chaetoceros confers resilience under transient depletion, while micrograzing and viral infection further mediate community successions. This study underscores the importance of integrating cell-specific and ecosystem-level measurements over extended timescales when predicting climate-driven shifts in Arctic primary production and carbon export.

The phytoplankton spring bloom constitutes the majority of annual Arctic primary production. In the Eurasian Arctic it is usually composed of various diatoms and the haptophyte Phaeocystis; recently, Phaeocystis-dominated blooms seem to be expanding. Low silicic acid (Si) availability has been hypothesised to promote Phaeocystis blooms, though the key drivers of Phaeocystis and diatom competition are not yet well understood. To investigate how Si availability controls bloom succession and biogeochemical fluxes we conducted a 3-week mesocosm experiment in Kongsfjorden, Svalbard in May 2024 on the spring phytoplankton community. The initial phytoplankton community was composed predominantly of Phaeocystis and the diatom Chaetoceros. Eight 450-L mesocosms received an Si gradient (0–40 µM) with nitrogen and phosphorus additions every 48 hours and we tracked nutrient uptake rates, community composition, elemental cycling and export. The nutrient additions stimulated phytoplankton growth across all Si levels. Phase I (Days 0–14) was dominated by a Phaeocystis bloom, terminating synchronously due to nitrogen and phosphorus depletion despite variable Si availability. In phase II (Days 16–22) Chaetoceros emerged as the dominant species with clear Si gradient effects: Chaetoceros spp. sustained exponential growth (µ = 0.40 d^-1) and achieved higher abundances in >3 µM Si treatments, while collapsing in Si-depleted mesocosms. Overall, particulate organic carbon export fluxes were low, but were strongly correlated with biogenic silica export fluxes, and increased exponentially in phase II of the experiment reflecting increasingly diatom-dominated sinking material. Our results demonstrate that Si availability does not govern initial Phaeocystis–diatom competition, but is the primary determinant of diatom bloom magnitude. The nutrient storage capacity of Chaetoceros confers resilience under transient depletion, while micrograzing and viral infection further mediate community successions. This study underscores the importance of integrating cell-specific and ecosystem-level measurements over extended timescales when predicting climate-driven shifts in Arctic primary production and carbon export.

The phytoplankton spring bloom constitutes the majority of annual Arctic primary production. In the Eurasian Arctic it is usually composed of various diatoms and the haptophyte Phaeocystis; recently, Phaeocystis-dominated blooms seem to be expanding. Low silicic acid (Si) availability has been hypothesised to promote Phaeocystis blooms, though the key drivers of Phaeocystis and diatom competition are not yet well understood. To investigate how Si availability controls bloom succession and biogeochemical fluxes we conducted a 3-week mesocosm experiment in Kongsfjorden, Svalbard in May 2024 on the spring phytoplankton community. The initial phytoplankton community was composed predominantly of Phaeocystis and the diatom Chaetoceros. Eight 450-L mesocosms received an Si gradient (0–40 µM) with nitrogen and phosphorus additions every 48 hours and we tracked nutrient uptake rates, community composition, elemental cycling and export. The nutrient additions stimulated phytoplankton growth across all Si levels. Phase I (Days 0–14) was dominated by a Phaeocystis bloom, terminating synchronously due to nitrogen and phosphorus depletion despite variable Si availability. In phase II (Days 16–22) Chaetoceros emerged as the dominant species with clear Si gradient effects: Chaetoceros spp. sustained exponential growth (µ = 0.40 d^-1) and achieved higher abundances in >3 µM Si treatments, while collapsing in Si-depleted mesocosms. Overall, particulate organic carbon export fluxes were low, but were strongly correlated with biogenic silica export fluxes, and increased exponentially in phase II of the experiment reflecting increasingly diatom-dominated sinking material. Our results demonstrate that Si availability does not govern initial Phaeocystis–diatom competition, but is the primary determinant of diatom bloom magnitude. The nutrient storage capacity of Chaetoceros confers resilience under transient depletion, while micrograzing and viral infection further mediate community successions. This study underscores the importance of integrating cell-specific and ecosystem-level measurements over extended timescales when predicting climate-driven shifts in Arctic primary production and carbon export.

The phytoplankton spring bloom constitutes the majority of annual Arctic primary production. In the Eurasian Arctic it is usually composed of various diatoms and the haptophyte Phaeocystis; recently, Phaeocystis-dominated blooms seem to be expanding. Low silicic acid (Si) availability has been hypothesised to promote Phaeocystis blooms, though the key drivers of Phaeocystis and diatom competition are not yet well understood. To investigate how Si availability controls bloom succession and biogeochemical fluxes we conducted a 3-week mesocosm experiment in Kongsfjorden, Svalbard in May 2024 on the spring phytoplankton community. The initial phytoplankton community was composed predominantly of Phaeocystis and the diatom Chaetoceros. Eight 450-L mesocosms received an Si gradient (0–40 µM) with nitrogen and phosphorus additions every 48 hours and we tracked nutrient uptake rates, community composition, elemental cycling and export. The nutrient additions stimulated phytoplankton growth across all Si levels. Phase I (Days 0–14) was dominated by a Phaeocystis bloom, terminating synchronously due to nitrogen and phosphorus depletion despite variable Si availability. In phase II (Days 16–22) Chaetoceros emerged as the dominant species with clear Si gradient effects: Chaetoceros spp. sustained exponential growth (µ = 0.40 d-1) and achieved higher abundances in >3 µM Si treatments, while collapsing in Si-depleted mesocosms. Overall, particulate organic carbon export fluxes were low, but were strongly correlated with biogenic silica export fluxes, and increased exponentially in phase II of the experiment reflecting increasingly diatom-dominated sinking material. Our results demonstrate that Si availability does not govern initial Phaeocystis–diatom competition, but is the primary determinant of diatom bloom magnitude. The nutrient storage capacity of Chaetoceros confers resilience under transient depletion, while micrograzing and viral infection further mediate community successions. This study underscores the importance of integrating cell-specific and ecosystem-level measurements over extended timescales when predicting climate-driven shifts in Arctic primary production and carbon export.

Identifying sources of ice nucleating particles (INPs) in the central Arctic is important for understanding controls on the phase of Arctic mixed‐phase clouds (AMPCs). We show meltwater samples collected from the Arctic pack ice in July 2020 contain biological INPs active at relatively warm temperatures ( T ≥ −10°C). We attribute the meltwater INPs to organisms and processes unique to a meltwater habitat. Concentrations of biological INPs active at T ≥ −10°C on filters deployed downwind of meltwater sites showed an enhancement associated with surface proximity. We hypothesize that time over the melt‐pond‐covered pack ice may have influenced the higher concentrations of biological INPs on the aerosol filters. More work to resolve emission mechanisms from melt ponds is necessary for understanding the extent of this potential source, which may increase importance as melt seasons extend spatiotemporally.

There is no light for photosynthesis by phytoplankton and sea-ice algae during the polar night, but microbial grazers remain active through the dark winter months in the Arctic Ocean. Where the energy to sustain these organisms comes from is unknown. Here we observed active tintinnid ciliates during the polar night, heterotrophic protists known to feed on phytoplankton and smaller heterotrophic protists. Our calculations indicate that the pelagic microbial loop transferring energy from bacterial production through microbial grazers was not sufficient to sustain the observed tintinnid biomass. However, the sea ice contained frozen-in particulate organic carbon produced during the previous growth seasons. We show that enough food particles can be released by mechanical break-up during sea-ice ridging, which together with bacterial production, sustained the observed tintinnid biomass in the water column. This is an important but overlooked mechanism for winter survival of plankton in the Arctic Ocean.

Carbohydrates are important components of marine organic aerosol particles and may influence Arctic cloud formation and properties, yet their sources and atmospheric fate remain poorly understood. We present the first year-round measurements of combined and dissolved carbohydrates (CCHOaer; DFCHOaer) in aerosol particles collected throughout the annual cycle of the MOSAiC expedition in 2019-2020. CCHOaer were detected in all seasons (0.5-17 ng m⁻³), and contributed between 0.03 and 2.2% (mean 0.3%) to the particulate mass. Their molecular composition was relatively stable and dominated by glucose, xylose, and galactose, with additional presence of uronic acids in summer. Both, CCHOaer and DFCHOaer showed pronounced summer maxima and seasonal variability that partially aligned with chlorophyll-a, nanophytoplankton, and heterotrophic microorganisms, indicating enhanced biological contributions after sea ice melt. The summer increase in DFCHOaer also coincided with warmer temperatures and higher humidity. In winter, the presence of carbohydrates may be sustained by microbial degradation or viral lysis of organic material in under-ice environments. CCHOaer and DFCHOaer concentrations showed no direct correlation to wind speed or air mass origins instead displaying a high variability in summer. The seasonal behavior of CCHOaer in Arctic aerosol particles differed from primary marine tracers like sodium that was associated with direct oceanic sources in summer and blowing snow in winter. This contrast suggests that carbohydrates, while possibly originated from marine biological sources, undergo significant atmospheric modification that overlay direct source signatures. Strong correlations between CCHOaer and low-molecular-weight organic acids further point to photochemical oxidation as an additional driver of secondary carbohydrate processing. CCHOaer displayed seasonal trends similar to warm-temperature ice-nucleating particles and hyper-fluorescent aerosol particles, supporting their role within a broader Arctic bioaerosol particle population. Overall, our results indicate that marine ecosystems provide a continuous source of atmospheric carbohydrates, but their composition is strongly modified by both biotic and abiotic processes, particularly in summer.Acknowledgement: This work was supported by the DFG funded Transregio-project TRR 172 “Arctic Amplification (AC)3“.

Although sea-ice ridges are prominent features of the Arctic Ocean, very little is known about their role as habitats and in biogeochemical cycles. Here, we show that ridges provide complex sea-ice habitats which host unique and diverse biological communities. Seasonally, ridges appear to transition from a biological repository in winter to biological hotspots in summer, surpassing algal biomass in level ice and surface waters by up to eight-fold. In summer, ridges can contain up to 80% of the total area integrated sea-ice algal biomass, emphasizing their importance in the Arctic sea-ice ecosystem. However, environmental shifts, such as meltwater infiltration and freezing inside the ridge in late summer, alter microbial communities from being predominantly autotrophic to heterotrophic. Our work provides evidence of contrasting roles of sea-ice ridges for Arctic carbon cycling in summer and shows that the habitats in the ridge interior harbor unique microbial communities, adding complexity to Arctic biodiversity.

The consequences of sea-ice decline for zooplankton survival in the Central Arctic Ocean (CAO) remain poorly understood, limiting predictions of ecosystem resilience. During the MOSAiC expedition (November 2019 - September 2020), we conducted the first year-round sampling of under-ice zooplankton (Ice-water interface and 10 m depth) along the Transpolar Drift using a remotely operated vehicle equipped with a zooplankton net (ROVnet). Multivariate analyses identified three spatio-seasonal community regimes: autumn and winter regimes in the Amundsen Basin, and a spring regime in both the Amundsen and Nansen Basin. Variations in taxon-specific abundances and biomass patterns indicate that the under-ice habitat shifts in function from a feeding and sheltering habitat in autumn and winter to a nursery habitat in spring. During the polar night, estimated particulate carbon supply, likely originating from sea ice, only marginally met community carbon demand. This deficit was partly offset by high trophic transfer efficiency inferred by normalized biomass size spectra (NBSS). This implies that the under-ice habitat enables zooplankton survival during the polar night at their very energetic limit, suggesting that changes in the duration and extent of sea-ice coverage will be critical for sustaining the future ecosystem functionality of the CAO.