Seasonal Dynamics of Microbial Activity and Organic Matter in the Arctic Gateway

The Arctic Ocean is intrinsically tied to global processes and considerably affected by the rapidly changing physical environment. Consequences of global warming include increased ice-free periods, increased river run-off and most noticeable the loss of sea ice cover. Following the trend of the previous years, 2018 marked the sixth lowest sea ice summertime minimum (NSIDC, USA). Our aim was to assess the impact of short-term (e.g. seasonal) changes on DOM-microbe interactions by quantifying microbial processes and DOM composition on a molecular-level.

Within the scope of the MicroARC project, samples were obtained during summer and fall of 2018 in the upper water-column of the Fram Strait. Identical methods were applied to characterize DOM components like dissolved carbohydrate and amino acids, elemental concentrations (DOC, TDN, TDP), primary (phytoplankton) and bacterial production as well as gel particles (TEP, CSP) composition.

Seasonality impacted the biogeochemistry as well as primary and secondary production rates. Although DOC concentrations were similar throughout the seasons (summer: ~73 µmol L^-1 fall: ~72 µmol L^-1), the vertical distribution varied. DOC accumulated above the pycnocline in summer, while it accumulated with depth in fall. The bioavailability of DOM was assessed from labile organic compound concentrations. Both, dissolved carbohydrates (summer: ~803 µmol L^-1, fall: ~323 µmol L^-1) and amino acids (summer: 407 µmol L^-1, fall: 185 µmol L^-1) strongly decreased throughout the seasons, suggesting a decrease in DOM lability. Primary production decreased from ~4 µmol C L^-1 d^-1 in summer to ~0.9 µmol C L^-1 d^-1 in fall. Likewise, bacterial biomass production decreased from ~0.04 µmol C L^-1 d^-1 to ~0.01 µmol C L^-1 d^-1. Based on these results, we infer that a compositional change of DOM controls DOM-microbe interactions in the Fram Strait. Our dataset suggests that seasonal cycling of major biogeochemical compounds of DOM is directly related to microbial activity in the Arctic Ocean.