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

Abstract

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 (CO 2 ) 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 CO 2 to yield a range of treatments (partial pressure of CO 2 , p CO 2 = 352–1025 μatm). We measured rates of N 2 and C fixation in both light and dark incubations over the 55‐day study period. High p CO 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 p CO 2 increased C fixation rates in the decay phase. We conclude that elevated p CO 2 and pulses of upwelling have pronounced effects on diazotrophy and primary producers, and upwelling and dust deposition modify the p CO 2 effect in natural assemblages.

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