Impact of increasing carbon dioxide on dinitrogen and carbon fixation rates under oligotrophic conditions and simulated upwelling

<p>Dinitrogen (N₂) fixation is a major source of bioavailable nitrogen to oligotrophic ocean communities. Yet, we have limited understanding how ongoing climate change could alter N₂ fixation. Most of our understanding is based on short-term laboratory experiments conducted on individual N₂-fixing species whereas community-level approaches are rare. In this longer-term in situ mesocosm study, we aimed to improve our understanding on the role of rising atmospheric carbon dioxide (CO₂) and simulated deep water upwelling on N₂ and carbon (C) fixation rates in a natural oligotrophic plankton community. We deployed nine mesocosms in the subtropical North Atlantic Ocean and enriched seven of these with CO₂ to yield a range of treatments (partial pressure of CO₂, <i>p</i>CO₂ = 352–1025 μatm). We measured rates of N₂ and C fixation in both light and dark incubations over the 55-day study period. High <i>p</i>CO₂ negatively impacted light and dark N₂ fixation rates in the oligotrophic phase before simulated upwelling, while the effect reversed in the light N₂ fixation rates in the bloom decay phase after added nutrients were consumed. Dust deposition and simulated upwelling of nutrient-rich deep water increased N₂ fixation rates and <i>nifH</i> gene abundances of selected clades including the unicellular diazotrophic cyanobacterium clade UCYN-B. Elevated <i>p</i>CO₂ increased C fixation rates in the decay phase. We conclude that elevated <i>p</i>CO₂ and pulses of upwelling have pronounced effects on diazotrophy and primary producers, and upwelling and dust deposition modify the <i>p</i>CO₂ effect in natural assemblages.</p>