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

Abstract

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

Contrasting seasonal patterns in particle aggregation and DOM transformation in a sub-Arctic fjord

“…S3), displaying no changes in density, turbidity and oxygen saturation with depth, and nutrients were redistributed, as demonstrated by elevated nutrient concentrations in the surface water. Despite low biological activity, bacteria continue to decompose the available algal remains during this period (Vonnahme et al, 2022). This is also supported by the high POC/PN and low Chl-a/Phaeopigment ratios of organic particles during winter, which demonstrate that they were in a regenerated state (Fig.…”

“…The spring bloom in Ramfjorden and other close fjords is initiated at the end of March/ beginning of April within a deeply mixed water column (Riebesell et al, 1995;Vonnahme et al, 2022;Walker et al, 2022); this study). The sharp increase of DOC, fluorescence, turbidity, TPM, POC, Chl-a, protist abundance, bacterial abundance and activity (HNA/LNA) as well as nano-and picophytoplankton abundances in April, along with a strong drawdown of nutrients demonstrates the onset of the spring bloom.…”

“…Hereafter, nutrient concentrations remained low (Nitrate < 1 µM, Phosphate < 0.3 µM) throughout the summer. In September, an autumn bloom can develop (Vonnahme et al, 2022) and high zooplankton biomass can occur (Coguiec et al, 2021). Presumably, an autumn bloom developed in September during our study as well, because POC concentrations, standing stocks of total Chl-a, contributions of large cells (Chl-a > 10 µm) and protist abundance were similar to concentrations in April and July (Fig.…”

“…Organic matter concentrations are at their lowest during this period and seemingly recalcitrant due to dissolved organic carbon accumulation (Hansell, 2013b). However, microbial degradation of organic matter can still take place (Vonnahme et al, 2022;Wietz et al, 2021).…”

“…In Autumn, DOM is converted to more recalcitrant compounds with lower hydrogen saturation (Osterholz et al, 2014) and there is a decrease in TEP (von Jackowski et al 2020). Autumn mixing in fjords can redistribute nutrients in the water column and fuel autumn blooms later in the year (Vonnahme et al, 2022). During winter, low light conditions limit primary production and the water column is subjected to an increase in vertical mixing.…”