Southern Ocean phytoplankton under multiple stressors: The modulation of Ocean Acidification effects by iron and light

The Southern Ocean (SO) contributes significantly to the sequestration of anthropogenic CO2 and is furthermore especially prone to Ocean Acidification (OA). The aim of this thesis was to investigate how key environmental factors influence the manifestation of OA effects on phytoplankton physiology and ecology. Publication I describes systematically occurring inconsistencies between pCO2 values calculated from different pairs of input parameters and their implications for the OA research field. Publication II presents the results of an experiment, in which interactive effects of OA and iron availability on SO primary production and species composition were observed. In publication III, dynamic light was found to strongly alter the effects of OA on the diatom Chaetoceros debilis. The aim of publication IV was to understand how iron, light and other factors control natural SO phytoplankton blooms. In conclusion, there is no universal phytoplankton response to OA. The effects of OA will always be modulated by the respective set of environmental conditions prevailing in the ecosystem of interest, which themselves may be subject to change

The Arctic picoeukaryote Micromonas pusilla benefits synergistically from warming and ocean acidification

In the Arctic Ocean, climate change effects such as warming and ocean acidification (OA) are manifesting faster than in other regions. Yet, we are lacking a mechanistic understanding of the interactive effects of these drivers on Arctic primary producers. In the current study, one of the most abundant species of the Arctic Ocean, the prasinophyte Micromonas pusilla, was exposed to a range of different pCO2 levels at two temperatures representing realistic current and future scenarios for nutrient-replete conditions. We observed that warming and OA synergistically increased growth rates at intermediate to high pCO2 levels. Furthermore, elevated temperatures shifted the pCO2 optimum of biomass production to higher levels. Based on changes in cellular composition and photophysiology, we hypothesise that the observed synergies can be explained by beneficial effects of warming on carbon fixation in combination with facilitated carbon acquisition under OA. Our findings help to understand the higher abundances of picoeukaryotes such as M. pusilla under OA, as has been observed in many mesocosm studies

Arctic sea ice algae differ markedly from phytoplankton in their ecophysiological characteristics

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Overview of the MOSAiC expedition:Ecosystem

The international and interdisciplinary sea-ice drift expedition “The Multidisciplinary drifting Observatory for the Study of Arctic Climate” (MOSAiC) was conducted from October 2019 to September 2020. The aim of MOSAiC was to study the interconnected physical, chemical, and biological characteristics and processes from the atmosphere to the deep sea of the central Arctic system. The ecosystem team addressed current knowledge gaps and explored unknown biological properties over a complete seasonal cycle focusing on three major research areas: biodiversity, biogeochemical cycles, and linkages to the environment. In addition to the measurements of core properties along a complete seasonal cycle, dedicated projects covered specific processes and habitats, or organisms on higher taxonomic or temporal resolution in specific time windows. A wide range of sampling instruments and approaches, including sea-ice coring, lead sampling with pumps, rosette-based water sampling, plankton nets, remotely operated vehicles, and acoustic buoys, was applied to address the science objectives. Further, a broad range of process-related measurements to address, for example, productivity patterns, seasonal migrations, and diversity shifts, were made both in situ and onboard RV Polarstern. This article provides a detailed overview of the sampling approaches used to address the three main science objectives. It highlights the core sampling program and provides examples of habitat- or process-specific sampling. The initial results presented include high biological activities in wintertime and the discovery of biological hotspots in underexplored habitats. The unique interconnectivity of the coordinated sampling efforts also revealed insights into cross-disciplinary interactions like the impact of biota on Arctic cloud formation. This overview further presents both lessons learned from conducting such a demanding field campaign and an outlook on spin-off projects to be conducted over the next years.</jats:p&gt

Photosynthetically active radiation during in-situ incubations in February 2018 in Kongsfjorden (79°N)

The AWI-funded AMUST project aims at understanding at current and future controls of Arctic spring blooms and concurrent effects on biogeochemistry by combining experimental work with long-term monitoring in Kongsfjorden in spring. This dataset encompasses ecophysiological data (Chl-a, POC, C:N, 14C-based Primary Production) from surface water samples collected at the Ny-Ålesund jetty in February 2018, as well as two datasets of 24h continuous light measurements during in-situ incubations 0.2m below the sea surface

Das Phytoplankton des Südpolarmeeres unter multiplen Stressoren: Die Modulation von Ozeanversauerungseffekten durch Eisen und Licht

The Southern Ocean (SO) contributes significantly to the sequestration of anthropogenic CO2 and is furthermore especially prone to Ocean Acidification (OA). The aim of this thesis was to investigate how key environmental factors influence the manifestation of OA effects on phytoplankton physiology and ecology. Publication I describes systematically occurring inconsistencies between pCO2 values calculated from different pairs of input parameters and their implications for the OA research field. Publication II presents the results of an experiment, in which interactive effects of OA and iron availability on SO primary production and species composition were observed. In publication III, dynamic light was found to strongly alter the effects of OA on the diatom Chaetoceros debilis. The aim of publication IV was to understand how iron, light and other factors control natural SO phytoplankton blooms. In conclusion, there is no universal phytoplankton response to OA. The effects of OA will always be modulated by the respective set of environmental conditions prevailing in the ecosystem of interest, which themselves may be subject to change

February phytoplankton ecophysiology in Kongsfjorden (79°N)

The AWI-funded AMUST project aims at understanding at current and future controls of Arctic spring blooms and concurrent effects on biogeochemistry by combining experimental work with long-term monitoring in Kongsfjorden in spring. This dataset encompasses ecophysiological data (Chl-a, POC, C:N, 14C-based Primary Production) from surface water samples collected at the Ny-Ålesund jetty in February 2018, as well as two datasets of 24h continuous light measurements during in-situ incubations 0.2m below the sea surface

Interactive effects of warming and ocean acidification on the Arctic picoeukaryote Micromonas pusilla

In the Arctic Ocean, climate change effects such as warming and ocean acidification (OA) are manifesting faster than in other regions. Yet, we are lacking a mechanistic understanding of the interactive effects of these drivers on Arctic primary producers. In the current study, one of the most abundant species of the Arctic Ocean, the prasinophyte Micromonas pusilla, was exposed to a range of different pCO2levels at two temperatures representing realistic scenarios for current and future conditions. We observed that warming and OA synergistically increased growth rates at intermediate to high pCO2 levels. Furthermore, elevated temperatures shifted the pCO2-optimum of biomass production to higher levels. Based on changes in cellular composition and photophysiology, we hypothesise that the observed synergies can be explained by beneficial effects of warming on carbon fixation in combination with facilitated carbon acquisition under OA. Our findings help to understand the higher abundances of picoeukaryotes such as M. pusilla under OA, as has been observed in many mesocosm studies

Implications of observed inconsistencies in carbonate chemistry measurements for ocean acidification studies

The growing field of ocean acidification research is concerned with the investigation of organisms’ responses to increasing pCO2 values. One important approach in this context is culture work using seawater with adjusted CO2 levels. As aqueous pCO2 is difficult to measure directly in small scale experiments, it is generally calculated from two other measured parameters of the carbonate system (often AT, CT or pH). Unfortunately, the overall uncertainties of measured and subsequently calculated values are often unknown. Especially under high pCO2, this can become a severe problem with respect to the interpretation of physiological and ecological data. In the few datasets from ocean acidification research where all three of these parameters were measured, pCO2 values calculated from AT and CT are typically about 30% lower (i.e. ~300 μatm at a target pCO2 of 1000 μatm) than those calculated from AT and pH or CT and pH. This study presents and discusses these discrepancies as well as likely consequences for the ocean acidification community. Until this problem is solved, one has to consider that calculated parameters of the carbonate system (e.g. pCO2, calcite saturation state) may not be comparable between studies, and that this may have important implications for the interpretation of CO2 perturbation experiments