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

Dinitrogen (N-2) fixation is a major source of bioavailable nitrogen to oligotrophic ocean communities. Yet, we have limited understanding how ongoing climate change could alter N-2 fixation. Most of our understanding is based on short-term laboratory experiments conducted on individual N-2-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 N-2 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, pCO(2) = 352-1025 mu atm). We measured rates of N-2 and C fixation in both light and dark incubations over the 55-day study period. High pCO(2) negatively impacted light and dark N-2 fixation rates in the oligotrophic phase before simulated upwelling, while the effect reversed in the light N-2 fixation rates in the bloom decay phase after added nutrients were consumed. Dust deposition and simulated upwelling of nutrient-rich deep water increased N-2 fixation rates and nifH gene abundances of selected clades including the unicellular diazotrophic cyanobacterium clade UCYN-B. Elevated pCO(2) increased C fixation rates in the decay phase. We conclude that elevated pCO(2) and pulses of upwelling have pronounced effects on diazotrophy and primary producers, and upwelling and dust deposition modify the pCO(2) effect in natural assemblages.

Automated summary

The study found that climate change and simulated upwelling have significant effects on N-2 and carbon fixation rates in oligotrophic oceanic plankton communities. The impacts of climate change on diazotrophy and primary producers are modulated by upwelling and dust deposition in natural assemblages.