Abstract
AbstractArctic waters are often enriched with terrestrial dissolved organic matter (DOM) characterized by having elevated visible wavelength fluorescence (commonly termed humic‐like). Here, we have identified the sources of fluorescent DOM (FDOM) in a high Arctic fjord (Young Sound, NE Greenland) influenced by glacial meltwater. The biological transformation of FDOM was further investigated using plankton community size‐fractionation experiments. The intensity of ultraviolet fluorescence (commonly termed amino acid‐like) was highly variable and positively correlated to bacterial production and mesozooplankton grazing. The overall distribution of visible FDOM in the fjord was hydrographically driven by the high‐signal intrusion of Arctic terrestrial DOM from shelf waters and dilution with glacial runoff in the surface waters. However, the high‐intensity visible FDOM that accumulated in subsurface waters in summer was not solely linked to allochthonous sources. Our data indicate that microbial activity, in particular, protist bacterivory, to be a source. A decrease in visible FDOM in subsurface waters was concurrent with an increase in bacterial abundance, indicating an active bacterial uptake or modification of this DOM fraction. This was confirmed by net‐loss of visible FDOM in experiments during summer when bacterial activity was high. The degradation of visible FDOM appeared to be associated with bacteria belonging to the order Alteromonadales mainly the genus Glaciecola and the SAR92 clade. The findings provide new insight into the character of Arctic terrestrial DOM and the biological production and degradation of both visible and UV wavelength organic matter in the coastal Arctic.
Highlights
Microorganisms produce, transform and consume dissolved organic matter (DOM) and as a result, all aquatic environments contain a complex mixture of organic molecules that vary greatly in chemical characteristics and size
The inner part of the fjord system was more stratified than the coastal shelf waters, but the wind mixing in October was enough to deepen the mixed layer depth (MLD) in the entire fjord (Fig. 2)
The low visible fluorescent DOM (FDOM) signal of the glacial runoff indicated that the rivers contributed to a seasonal dilution of DOM and FDOM in the surface waters, whereas the more saline coastal water had a higher content of visible FDOM
Summary
Microorganisms produce, transform and consume dissolved organic matter (DOM) and as a result, all aquatic environments contain a complex mixture of organic molecules that vary greatly in chemical characteristics and size. This results in a considerable analytical challenge for resolving the production, turnover and fate of carbon, nitrogen and phosphorus bound as organic matter. The most common approach to quantify DOM is to measure its carbon content (dissolved organic carbon, DOC), this provides no information on DOM characteristics, which impacts its fate Other measures such as bioavailability (BDOC) (Søndergaard and Middelboe 1995), and optical properties of the organic matter (Stedmon and Nelson 2015), offer insight into the DOM character. Arctic waters are characterised by having elevated visible fluorescence and this is linked to the high terrestrial input of organic matter (Amon et al 2003; Walker et al 2013), which have been used to trace large-scale water mass distribution of Arctic water
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