Chemical and photophysiological impact of terrestrially‐derived dissolved organic matter on nitrate uptake in the coastal western Arctic

Abstract

The Arctic is warming at a rate nearly twice the global average, leading to thawing permafrost, increased coastal erosion, and enhanced delivery of riverine terrestrially-derived dissolved organic matter (tDOM) to coastal waters. This humic-rich tDOM has the ability to attenuate light required for photosynthesis and stimulate heterotrophic growth by supplying a source of labile organic carbon. Due to tDOM's high carbon to nitrogen (C : N) ratio, additional nitrogen is required for microorganisms to utilize this excess carbon for growth, thus exacerbating competition between autotrophs and heterotrophs for limiting nutrients and potentially reducing primary production. The effect of Arctic tDOM additions on nitrate uptake by two microplankton size fractions in the coastal Chukchi Sea was quantified using 15 N tracer methods. To assess the biogeochemical vs. spectral impacts of tDOM, the uptake incubations were amended with either tDOM or light attenuating films that mimic light absorption by the tDOM. Nitrate uptake and primary production rates in the larger, predominantly phytoplankton, size fraction generally decreased with increasing tDOM additions. The change in light attenuation alone accounted for a 50% reduction in nitrate uptake. Responses in the smaller size fraction varied seasonally with tDOM additions stimulating uptake in spring and suppressing it in summer. The largest variation in summer nitrate uptake can be explained by the shared effect of biogeochemistry and light attenuation. Therefore, large increases in tDOM delivery currently occurring and predicted to increase in the coastal Arctic, could reduce primary production, broadly impact nitrogen and carbon cycling, and affect higher trophic levels.

Microbial Community Response to Terrestrially Derived Dissolved Organic Matter in the Coastal Arctic

“…The light attenuation associated with tDOM may be more physiologically consequential in spring when light is already limiting productivity (Sipler et al, 2017). A companion study Sipler et al (2017) found that the addition of tDOM reduced C-fixation during a 24 h incubation.…”

“…The extreme physical and chemical differences observed between April and August likely contributed to the distinct variations between the April and August ambient microbial communities. Large seasonal changes in organic matter composition are observed throughout the Arctic (e.g., Crump et al, 2003 ; Connelly et al, 2015 ; Mann et al, 2016 ), and can influence the composition ( Crump et al, 2003 ; Judd et al, 2006 ) and physiology of the ambient microbial community ( Sipler et al, 2017 ). An evaluation of offshore sites found differences in community composition between seasons to be insignificant ( Kirchman et al, 2010 ), which when combined with our results, further supports the importance of terrestrial influence (tDOM) in shaping coastal microbial communities.…”

“…The light absorption by the tDOM ranges into the visible spectrum, which decreases light needed for photosynthesis ( Arrigo and Brown, 1996 ). The light attenuation associated with tDOM may be more physiologically consequential in spring when light is already limiting productivity ( Sipler et al, 2017 ). A companion study Sipler et al (2017) found that the addition of tDOM reduced C-fixation during a 24 h incubation.…”

Seasonal nitrogen uptake and regeneration in the western coastal Arctic

“…Our results highlight the importance of some fractions of the DON pool, which were likely produced in situ. In the future, the components and lability of Arctic DON is expected to change as more terrigenous material is transported from the tundra to the coastal ocean (Sipler et al ). This highly aromatic terrigenous material will likely be less labile than the low molecular weight compounds studied here, but as of this writing, little is known about the changing bioavailability of DOM entering the Arctic coastal zone.…”

Bacterial Response to Permafrost Derived Organic Matter Input in an Arctic Fjord

“…A higher bacterial activity due to the degradation of tDOM might cause a higher turnover within the microbial loop and therewith increased CO 2 production, but ultimately depends on the bacterial growth efficiency. Increased carbon availability might also enhance the competition between bacteria and phytoplankton for inorganic nutrients and indirectly disadvantage larger phytoplankton (Thingstad et al, 2008;Sipler et al, 2017b). Thus, high tDOM input may decrease primary production in coastal Arctic areas.…”