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

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 15N 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.