Abstract
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 fact that 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 the N2 fixation rates of 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 (25 and 27 ∘C), projected summer mean (29 and 31 ∘C), and projected summer maximum (33 ∘C) seawater surface temperatures (SSTs) 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 macrophyte 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, in which rates showed an optimum at 31 ∘C followed by a sharp decrease at 33 ∘C, and bare sediments, in which rates increased along the range of the experimental temperatures. The activation energy and Q10 were lower in vegetated than bare sediments, indicating the lower thermal sensitivity of vegetated sediments. The projected warming is expected to increase the contribution of N2 fixation to Mediterranean macrophyte productivity. Therefore, the thermal dependence of N2 fixation might have important consequences for primary production in coastal ecosystems in the context of warming.
Highlights
Global mean surface temperatures increased 0.85 ◦C from 1880 to 2012 and are projected to increase between 1 and 3.5 ◦C by the end of the 21st century relative to preindustrial times (IPCC, 2013)
N2 fixation rate was detected in C. prolifera (0.22 ± 0.05 nmol N g DW−1 h−1), whereas the minimum mean N2 fixation rate was measured in C. nodosa (0.15 ± 0.04 nmol N g DW−1 h−1)
Nitrogen fixation rates differed among the four different sediment types (χ32, 56 = 10.68, p = 0.005) when expressed by sediment dry weight
Summary
Global mean surface temperatures increased 0.85 ◦C from 1880 to 2012 and are projected to increase between 1 and 3.5 ◦C by the end of the 21st century relative to preindustrial times (IPCC, 2013). Oceans store most of the accumulated heat in the biosphere, warming at an average rate of 0.11 ◦C per decade at the surface (up to 75 m of depth) since 1970 (IPCC, 2013), with longer and more frequent marine heat waves over the last century (Oliver et al, 2018). The maximum surface seawater temperature (SSTmax) in the Balearic Islands, in the western Mediterranean Sea, is projected to increase by 3.4 ± 1.3 ◦C by 2100 under a scenario of moderate greenhouse gas (GHG) emissions (A1B scenario from the IPCC Special Report on Emissions Scenarios, equivalent to the RCP6.0 scenario of the IPCC Fifth Assessment Report) (Jordà et al, 2012), with important consequences for marine organisms and ecosystems
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