Inter-annual Trends in High-Molecular-Weight Dissolved Organic Material and Microbial Abundances in the Fram Strait between 2009-2018

Abstract:

The extent to which climate change can impact biological processes in the Arctic Ocean is still poorly understood, but recent research suggests that it could alter the distribution, magnitude, and duration of phytoplankton blooms. Phytoplankton are the primary source of bioavailable dissolved organic matter (DOM) and changes in phytoplankton bloom dynamics might influence microbial processes. As such, we identified changes in inter-annual bloom dynamics and assessed whether changes in quality and quantity of DOM affected microbes.

Water column samples were collected within the upper 100m between June and August from 2009 to 2018 in the eastern Fram Strait, except for 2013. We analyzed samples for dissolved organic carbon, high-molecular-weight (HMW) dissolved combined carbohydrates (dCCHO) and dissolved hydrolysable amino acids (dHAA), bacterial abundances and production. Additionally, we used publicly available satellite data to determine the daily chlorophyll-a (chl-a) surface concentration and assess the temporal variability. The start and end of a bloom was set at a chl-a threshold of 0.85 mg/m^3. Years with multiple start and end phases where characterized as successional blooms, whereas, years with a single phase where characterized as extended blooms.

Our results show inter-annual variation in the quality and quantity of HMW-DOM compounds between 2009 and 2018, which could be associated with bloom dynamics. Concentrations of dCCHO were two-fold higher during successional blooms with enriched molar concentrations of arabinose, galacturonic acids, and rhamnose. In contrast, galactose, galactosamine, and glucosamine were linked to extended blooms and successional post-bloom stages. Similarly, more labile dHAA such as leucine, isoleucine, and phenylalanine were associated to bloom events whereas glycine and alanine were linked to post-bloom stages. HMW-DOM significantly correlated with bacterial abundances, suggesting narrow niche response to OM degradation. Our dataset suggests that an increasing frequency of extended blooms could affect the quality and quantity of HMW-DOM available for microbial remineralization. Thus, understanding the temporal variability of biomarkers may help predicting future carbon cycling in the rapidly changing Arctic.