Warming effect on nitrogen fixation in Mediterranean macrophyte sediments

Abstract. The Mediterranean Sea is warming faster than the global Ocean, with important consequences for organisms and biogeochemical cycles. Warming is a major stressor for key marine benthic macrophytes. However, the effect of warming on marine N2 fixation remains unknown, despite the high productivity of macrophytes in oligotrophic waters is partially sustained by the input of new nitrogen (N) into the system by N2 fixation. Here, we assess the impact of warming on N2 fixation rates on three key marine macrophytes: Posidonia oceanica, Cymodocea nodosa, and Caulerpa prolifera. We experimentally measured N2 fixation rates in vegetated and bare sediments at temperatures encompassing current summer mean, projected summer mean and projected summer maximum seawater surface temperatures (SST) by the end of the century under a scenario of moderate greenhouse-gas emissions. We found that N2 fixation rates in vegetated sediments were 2.8-fold higher than in bare sediments at current summer mean SST, with no differences among macrophytes. Currently, the contribution of N2 fixation to macrophytes productivity could account for up to 7 %, 13.8 % and 1.8 % of N requirements for P. oceanica, C. nodosa, and C. prolifera, respectively. We show the temperature dependence of sediment N2 fixation rates. However, the thermal response differed for vegetated sediments, where rates showed an optimum at 31 °C followed by a sharp decrease at 33 °C, and bare sediments, where rates increased along the range of experimental temperatures tested here. The activation energy and Q10 were lower in vegetated sediments than in bare sediments. The projected warming is expected to increase the contribution of N2 fixation to Mediterranean macrophytes' productivity. Therefore, the thermal dependence of N2 fixation might have important consequences for primary production in coastal ecosystems in the context of warming.