von Jackowski, Anabel (2022) Seasonal Dynamics of Organic Matter Turnover in the Arctic Ocean. (PhD/ Doctoral thesis), Christian Albrechts Univerisität zu Kiel, Kiel, Germany, 157 pp.

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Abstract

The Arctic Ocean is extremely susceptible to climate change, which has led to warmer air temperatures, accelerated sea ice loss, and an intensified inflow of Atlantic water masses into the Arctic. Furthermore, the reduction in sea ice has been linked to a prolonged phytoplankton growth season. Fewer days of sea ice cover and thinner sea ice increases the light availability for ice algae and expands the habitat for pelagic phytoplankton. An extension of the growth season prolongs the release of phytoplankton-derived organic matter with consequences for the seasonal carbon cycle. However, the seasonal carbon stock and microbial processes are largely unexplored because many parameters require in situ sampling. A systematic strategy to enhance microbial observations is through time series stations. The long-term ecological research observatory HAUSGARTEN was established in 1999 and is the only pelagic microbial observatory in the Arctic to date. The observatory is sampled annually during the summer months, which limits its explanatory power to assess the seasonal carbon cycling and microbial dynamics. To close this gap, the goals of this doctoral project were to (a) investigate the seasonal variability of organic matter, (b) explore seasonal organic matter turnover and the microbial community, and (c) evaluate the inter-annual variability in the Fram Strait. Organic matter is the dominant seasonal control on microbial dynamics (Manuscript 1) and shifts in the microbial community (Manuscript 2) in the eastern Fram Strait between summer and fall 2018. Our efforts to extend the summertime sampling frequency at HAUSGARTEN unprecedentedly provide a baseline for dissolved organic carbon, dissolved combined carbohydrates, dissolved hydrolyzable amino acids, and gel particles during fall. This is the first study to highlight a decoupling in the major classes of gel particles, namely transparent exopolymer particles and Coomassie stainable particles. Discovering the uncoupled seasonal variability shows that different classes of particles have a unique significance in marine biogeochemical processes. Moreover, the availability of organic matter controlled bacterial production, microbial abundances, and the microbial community. Identifying the microbial community composition revealed a low diversity with a dominance of polysaccharide-degrading bacteria in summer, which shifted to wider diversity including microbes that assimilate refractory biopolymers in fall. Our aim to combine the available organic matter pool and taxonomy of microbes is a novel approach in order to understand microbial substrate regimes. To showcase the spatial and temporal variability of microbial organic matter turnover, we generated a high-resolution analysis for Fram Strait from 2009 to 2019 (Manuscript 3). Our study combines year-round remote sensing approaches with in situ biogeochemical and microbial parameters from HAUSGARTEN during summer, which revealed a fundamental link of microbial dynamics to the regional oceanography. The remotely sensed bloom dynamics contextualized the inter-annual in situ variability of organic matter bioavailability, bacterial production, and microbial abundances of the HAUSGARTEN time series dataset. In the northward flowing Atlantic waters, the slowly terminating pelagic phytoplankton bloom releases organic matter that stimulates rapid microbial turnover. In the southward flowing polar waters, the remote sensing analysis revealed a phytoplankton bloom peak around May, which is prior to our in situ sampling period, resulting in relatively low concentrations of organic matter. In addition to organic matter, the cold temperatures co-limit bacterial production and microbial abundances. Our approach to combine the remote sensing with microbial observations contextualizes the inter-annual variability observed at HAUSGARTEN. This thesis contributes to a better understanding of the seasonal biogeochemical and microbial coupling in the Arctic. Our collected data provides a baseline for microbial observations during fall, contributes to a better understanding of seasonal gel particle dynamics, and explores the spatio-temporal microbial substrate regimes. Applying a multi-disciplinary approach has advanced the understanding of seasonality in biogeochemical and microbial processes across the Fram Strait. This newly acquired knowledge highlights current seasonal dynamics in the Arctic microbial loop that are essential to accurately assess the transformations in a rapidly changing Arctic Ocean.

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