Description: The 77th cruise of the RV MARIA S. MERIAN contributed to various large national and international research and infrastructure projects (FRAM, ARCHES, INTAROS, ICOS, SIOS) as well as to the research programme PACES-II (Polar Regions and Coasts in the changing Earth System) of the Alfred-Wegener-Institute Helmholtz-Center for Polar and Marine Research (AWI). Investigations within Work Package 4 (Arctic sea ice and its interaction with ocean and ecosystems) of the PACES-II programme, aim at assessing and quantifying ecosystem changes from surface waters to the deep ocean in response to the retreating sea ice, and at exploring the most important (feedback) processes determining temporal and spatial variability. Contributions to the PACES-II Work Package 6 (Large scale variability and change in polar benthic biota and ecosystem functions) include the identification of spatial patterns and temporal trends in relevant benthic community functions, and the development of a comprehensive science community reference collection of observational data. Work carried out within WPs 4 and 6 will support the time-series studies at the LTER (Long-Term Ecological Research) observatory HAUSGARTEN (Fig. 1.1), where we document Global Change induced environmental variations on a polar deep-water ecosystem. This work is carried out in close co-operation between the HGF-MPG Joint Research Group on Deep-Sea Ecology and Technology and the PEBCAO Group (Phytoplankton Ecology and Biogeochemistry in the Changing Arctic Ocean) at AWI as well as the working group Microbial Geochemistry at the GEOMAR and the HGF Young Investigators Group SEAPUMP (Seasonal and regional food web interactions with the biological pump).

Description: Particle fluxes at the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic for the period December 2009 until May 2011 are discussed based on bathypelagic sediment trap time-series data collected at 1290 and 3439m water depth. The typically oligotrophic particle flux pattern with weak seasonality is modified by the appearance of a highly productive and low oxygen (minimum concentration below 2 μmol kg-1 at 40m depth) anticyclonic modewater eddy (ACME) in winter 2010. The eddy passage was accompanied by unusually high mass fluxes of up to 151 mgm-2 d-1, lasting from December 2009 to May 2010. Distinct biogenic silica (BSi) and organic carbon flux peaks of ~15 and 13.3 mgm-2 d-1, respectively, were observed in February–March 2010 when the eddy approached the CVOO. The flux of the lithogenic component, mostly mineral dust, was well correlated with that of organic carbon, in particular in the deep trap samples, suggesting a tight coupling. The lithogenic ballasting obviously resulted in high particle settling rates and, thus, a fast transfer of epi-/mesopelagic signatures to the bathypelagic traps. We suspect that the two- to three-fold increase in particle fluxes with depth as well as the tight coupling of mineral dust and organic carbon in the deep trap samples might be explained by particle focusing processes within the deeper part of the eddy. Molar C :N ratios of organic matter during the ACME passage were around 18 and 25 for the upper and lower trap samples, respectively. This suggests that some productivity under nutrient (nitrate) limitation occurred in the euphotic zone of the eddy in the beginning of 2010 or that a local nitrogen recycling took place. The d15N record showed a decrease from 5.21 to 3.11‰ from January to March 2010, while the organic carbon and nitrogen fluxes increased. The causes of enhanced sedimentation from the eddy in February/March 2010 remain elusive, but nutrient depletion and/or an increased availability of dust as a ballast mineral for organic-rich aggregates might have contributed. Rapid remineralisation of sinking organic-rich particles could have contributed to oxygen depletion at shallow depth. Although the eddy formed in the West African coastal area in summer 2009, no indications of coastal flux signatures (e.g. from diatoms) were found in the sediment trap samples, confirming the assumption that the suboxia developed within the eddy en route. However, we could not detect biomarkers indicative of the presence of anammox (anaerobic ammonia oxidation) bacteria or green sulfur bacteria thriving in photic zone suboxia/hypoxia, i.e. ladderane fatty acids and isorenieratene derivatives, respectively. This could indicate that suboxic conditions in the eddy had recently developed and/or the respective bacterial stocks had not yet reached detection thresholds. Another explanation is that the fast-sinking organic-rich particles produced in the surface layer did not interact with bacteria from the suboxic zone below. Carbonate fluxes dropped from ~52 to 21.4 mgm-2 d-1 from January to February 2010, respectively, mainly due to reduced contribution of shallow-dwelling planktonic foraminifera and pteropods. The deep-dwelling foraminifera Globorotalia menardii, however, showed a major flux peak in February 2010, most probably due to the suboxia/hypoxia. The low oxygen conditions forced at least some zooplankton to reduce diel vertical migration. Reduced “flux feeding” by zooplankton in the epipelagic could have contributed to the enhanced fluxes of organic materials to the bathypelagic traps during the eddy passage. Further studies are required on eddy-induced particle production and preservation processes and particle focusing.

Description: eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~20 μM), low dissolved iron (DFe ~0.2 nM) and low silicate concentrations (below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no significant deeper export. showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution of copepod grazing to iron cycling we used two different approaches: first, we measured for the first time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet was ~5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed leachable iron (pH 2.0) in 48 h (LFe48h) that, for the conditions during LOHAFEX, was shown to be an excellent proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in the upper 80 m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids freshly formed after the second fertilization by the predominant flagellate community resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper 100–150 m, the accumulation of LFe48h in the upper 80 m after the second fertilization and provided the iron required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate depleted areas of the Southern Ocean.

Description: Marine microalgae sequester as much CO2 into carbohydrates as terrestrial plants. Polymeric carbohydrates (i.e., glycans) provide carbon for heterotrophic organisms and constitute a carbon sink in the global oceans. The quantitative contributions of different algal glycans to cycling and sequestration of carbon remain unknown, partly because of the analytical challenge to quantify glycans in complex biological matrices. Here, we quantified a glycan structural type using a recently developed biocatalytic strategy, which involves laminarinase enzymes that specifically cleave the algal glycan laminarin into readily analyzable fragments. We measured laminarin along transects in the Arctic, Atlantic, and Pacific oceans and during three time series in the North Sea. These data revealed a median of 26 ± 17% laminarin within the particulate organic carbon pool. The observed correlation between chlorophyll and laminarin suggests an annual production of algal laminarin of 12 ± 8 gigatons: that is, approximately three times the annual atmospheric carbon dioxide increase by fossil fuel burning. Moreover, our data revealed that laminarin accounted for up to 50% of organic carbon in sinking diatom-containing particles, thus substantially contributing to carbon export from surface waters. Spatially and temporally variable laminarin concentrations in the sunlit ocean are driven by light availability. Collectively, these observations highlight the prominent ecological role and biogeochemical function of laminarin in oceanic carbon export and energy flow to higher trophic levels.

Description: To date, observations on a single location indicate that cryogenic gypsum (Ca{SO4} *2H2O) may constitute an efficient but hitherto overlooked ballasting mineral enhancing the efficiency of the biological carbon pump in the Arctic Ocean. In June–July 2017 we sampled cryogenic gypsum under pack ice in the Nansen Basin north of Svalbard using a plankton net mounted on a remotely operated vehicle (ROVnet). Cryogenic gypsum crystals were present at all sampled stations, which suggested a persisting cryogenic gypsum release from melting sea ice throughout the investigated area. This was supported by a sea ice backtracking model, indicating that gypsum release was not related to a specific region of sea ice formation. The observed cryogenic gypsum crystals exhibited a large variability in morphology and size, with the largest crystals exceeding a length of 1 cm. Preservation, temperature and pressure laboratory studies revealed that gypsum dissolution rates accelerated with increasing temperature and pressure, ranging from 6%d-1 by mass in polar surface water (-0.5 °C) to 81%d-1 by mass in Atlantic Water (2.5°C at 65 bar). When testing the preservation of gypsum in formaldehyde-fixed samples, we observed immediate dissolution. Dissolution at warmer temperatures and through inappropriate preservation media may thus explain why cryogenic gypsum was not observed in scientific samples previously. Direct measurements of gypsum crystal sinking velocities ranged between 200 and 7000md-1, suggesting that gypsum-loaded marine aggregates could rapidly sink from the surface to abyssal depths, supporting the hypothesized potential of gypsum as a ballasting mineral in the Arctic Ocean.

Description: 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.

Description: Long‐term data characterizing the oceans' biological carbon pump are essential for understanding impacts of climate variability on marine ecosystems. The “Bakun upwelling intensification hypothesis” suggests intensified coastal upwelling due to a greater land‐sea temperature gradient influenced by global warming. We present long time series of bathypelagic (approximately 1,200–3,600 m) particle fluxes from a coastal (CBeu: 2003–2016) and an offshore (CBmeso: 1988–2016) sediment trap setting located in the Canary Current upwelling. Organic carbon (Corg) and biogenic opal (BSi, diatoms) fluxes were twofold to threefold higher at the coastal upwelling site compared to the offshore site, respectively, and showed higher seasonality with flux maxima in spring. A relationship between winter and spring BSi fluxes to the North Atlantic Oscillation index was best expressed at the offshore site CBmeso. Lithogenic (dust) fluxes regularly peaked in winter when frequent low‐altitude dust storms and deposition occurred, decreasing offshore by about threefold. We obtained a high temporal match of short‐term peaks of BSi and dust fluxes in winter to spring at the inner site CBeu. We found synchronous flux variations at both sites and an anomalous year 2005, characterized by high BSi and Corg fluxes under a low North Atlantic Oscillation. Corg and BSi fluxes revealed a decreasing trend from 2006 to 2016 at the coastal site CBeu, pointing to coastal upwelling relaxation during the last two decades. The permanent offshore upwelling zone of the deflected Canary Current represented by the flux record of CBmeso showed no signs of increasing upwelling as well which contradicts the Bakun hypothesis.

Description: Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes' Law estimating particle SV from density and size has been widely used. But while Stokes' Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes' models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes' Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes' assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes' models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.