| Summary: | The increasing pCO2 of the world’s oceans due to increasing atmospheric CO2 are predicted to affect the physical & biochemical parameters on different temporal and spatial scales. Key processes within the microbial driven nitrogen cycle could be drastically affected directly and/or in-directly influencing marine ecosystems and have knock-on effects eventually affecting climate systems. Two multi-disciplinary mesocosm experiments took place during the summer (June to July 2012) and winter (Febuary to March 2013) within the Ligurian Sea in the Mediterranean Western Basin. Each experiment consisted of 9 amended mesocosms at different pCO2 level (ambient to 1250 ppm) and three controls, sampled daily for ~20 days. Rates of nitrogen fixation, nitrate uptake and nitrification were measured using a 15N dilution technique. Results showed that a decrease in pH caused by increasing pCO2 did not limit nitrification or produce a change in NO3- uptake, as according to the current literature pool. During the summer experiment there was an increase in nitrogen fixation during the first ~10 days in P5 (~1000 μatm) and P6 (~1250 μatm) from background rates 2.31 ± 1.69 nmol L-1 d-1 to a maximum 23.3 nmol L-1 d-1. Nitrogen fixation rate only became elevated above the background rates when pH < 7.73 (max at 7.71), pCO2 > 999 μatm (max at 1056 μatm). The output of this study is that an increase in pCO2 will not affect this nitrogen cycle within the Mediterranean before the end of the century with increased nitrogen fixation rates only after ~2100. In collaboration with Plymouth Marine Laboratory, UK.
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