Description: The Arctic Ocean is subject to severe environmental changes, including the massive decline in sea ice due to continuous warming in many regions. Along with these changes, the Arctic Ocean’s ecosystem is affected on various scales. The pelagic microbial food web of the Arctic is of particular interest, because it determines mass transfer to higher trophic levels. In this regard, variations in the size structure of the microbial community reflect changes in size-dependent bottom-up and top-down processes. Here we present analyses of microscopic data that resolve details on composition and cell size of unicellular plankton, based on samples collected between 2016 and 2018 in the Fram Strait. Using the Kernel Density Estimation method, we derived continuous size spectra (from 1 μm to ≈ 200 μm Equivalent Spherical Diameter, ESD) of cell abundance and biovolume. Specific size intervals (3–4, 8–10, 25–40, and 70–100μm ESD) indicate size-selective predation as well as omnivory. In-between size ranges include loopholes with elevated cell abundance. By considering remote sensing data we could discriminate between polar Arctic- and Atlantic water within the Fram Strait and could relate our size spectra to the seasonal change in chlorophyll-a concentration. Our size spectra disclose the decline in total biovolume from summer to autumn. In October the phytoplankton biovolume size-spectra reveal a clear relative shift towards larger cell sizes (〉 30 μm). Our analysis highlights details in size spectra that may help refining allometric relationships and predator-prey dependencies for size-based plankton ecosystem model applications.

Description: 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.

Description: The collection of zooplankton swimmers and sinkers in time-series sediment traps provides unique insight into year-round and interannual trends in zooplankton population dynamics. These samples are particularly valuable in remote and difficult to access areas such as the Arctic Ocean, where samples from the ice-covered season are rare. In the present study, we investigated zooplankton composition based on swimmers and sinkers collected by sediment traps at water depths of 180–280, 800–1320, and 2320–2550 m, over a period of 16 yr (2000–2016) at the Long-Term Ecological Research observatory HAUSGARTEN located in the eastern Fram Strait (79°N, 4°E). The time-series data showed seasonal and interannual trends within the dominant zooplankton groups including copepoda, foraminifera, ostracoda, amphipoda, pteropoda, and chaetognatha. Amphipoda and copepoda dominated the abundance of swimmers while pteropoda and foraminifera were the most important sinkers. Although the seasonal occurrence of these groups was relatively consistent between years, there were notable interannual variations in abundance, suggesting the influence of various environmental condi- tions such as sea-ice dynamic and lateral advection of water masses, for example, meltwater and Atlantic water. Statistical analyses revealed a correlation between the Arctic dipole climatic index and sea-ice dynamics (i.e., ice coverage and concentration), as well as the importance of the distance from the ice edge on swimmer composition patterns and carbon export.