Light and freshwater discharge drive the biogeochemistry and microbial ecology in a sub-Arctic fjord over the Polar night

The polar night has recently received increased attention as a surprisingly active biological season. Yet, microbial ecology is vastly understudied field. To identify the physical and biogeochemical parameters driving activity over dark season, we studied sub-Arctic fjord system in northern Norway from autumn to early spring with detailed monthly sampling. We focused on impact of mixing, terrestrial organic matter input light ecosystem dynamics. Our study highlights strong differences key drivers between spring, autumn, winter. bloom started March fully mixed water column, opposing traditional critical depth hypothesis. Incident solar radiation was driver maximum Chlorophyll reached April. onset phytoplankton controlled by vertical causing nutrient upwelling dilution zooplankton grazers, which had their highest biomass during this time. According dilution-recoupling hypothesis grazer reduced grazing stress allowed fall formation. Mixing at that time initiated winds stratification consequence freezing temperatures lower freshwater runoff. During light-limited night, primary production extremely low but bacteria continued growing decaying algae, exudates also allochthonous matter. A melting event January could have land, supporting mid-winter bacterial bloom. In conclusion, biogeochemistry not only driven availability, strongly affected variability reshwater discharge carbon input. With climate change will increase Arctic, likely importance dynamics described study.