Dataset: Experimental carbon emissions from degraded Mediterranean seagrass (Posidonia oceanica) meadows under current and future summer temperatures.

Experimental carbon emissions from degraded Mediterranean seagrass (<em>Posidonia oceanica</em>) meadows. Guillem Roca, Javier Palacios, Sergio Ruíz-Halpern, Núria Marbà Contact details: Guillem Roca, [e-mail] Issue date: Identifier: Citation: Roca, Guillem; Palacios, Javier; Ruíz-Halpern, Marbà, Núria; Experimental carbon emissions from degraded Mediterranean seagrass (<em>Posidonia oceanica</em>) meadows. [Dataset] Abstract: The dataset provides data on sediment C0₂efflux rates (μmol CO₂m^-2s^-1), carbon emissions during the experiment (gm^-2), % Organic Carbon, Organic Matter content (g m^-2) of the <em>Posidonia oceanica</em> seagrass sediments collected in Pollença bay (North of Mallorca Island). Sediments were cultivated in 5 different seawater temperature treatments and two different agitation conditions. Keywords: C0₂efflux rates, C0₂ emissions, Sediment, Seagrass, <em>Posidonia Oceanica</em>, experiment, temperature treatment, Sediment suspension Blue carbon, Organic Carbon. Description: The dataset contains data on sediment C0₂efflux rates, carbon emissions during the experiment (gm^-2), % Organic Carbon, Organic Matter content of the <em>Posidonia oceanica</em> seagrass sediments collected in Pollença bay (North of Mallorca Island). Sediments were cultivated in 5 different seawater temperature treatments and two different agitation conditions. Sediments used in the experiment were extracted in October 2017 from the <em>P. Oceanic</em>a meadow of Pollença in Mallorca Island at six-meter depth Figure (1). Sediments were sampled in October 2017 using sediment cores (9 cm ID and 30cm long) and directly transported to the laboratory. Only the top 10 cm of the sediment cores were used since this fraction is the most susceptible to erosion. Living seagrass tissues (roots, rhizomes, and leaves) were removed and sediment was mixed and homogenized. 40ml of sediments were poured into glass containers of 750ml with 500ml of seawater. Finally, each recipient contained a sediment layer of approximately 1.1cm in each container. Containers were placed at five different temperature baths (26,27.5, 29, 30.5, 32 ºC) simulating summer temperatures in the bay (Garcias-Bonet et al., 2019) at different agitation regimes (agitation/repose) to simulate exposed and sheltered conditions.10 containers were sampled right after the experiment started to provide initial sediment conditions. Five containers per temperature and agitation treatment were removed 7, 21, 43, 67, and 98 days from the experiment start, to analyse sediment organic matter and CaCO₃ content. CO₂ incubations were run 5, 14, 56, and 91 days from the experiment start. Sampling times were distributed considering that organic matter remineralisation was likely to follow an exponential trend, including a rapid phase of loss of the more labile material followed by a slower loss of more recalcitrant substrates (Arndt et al., 2013). The experiment was run in the dark to avoid photosynthesis in an isothermal chamber at 21ºC. <strong>Organic Carbon analysis</strong> In each sampling time, organic matter content in sediments (OM %DW) was estimated as the percentage weight loss of dry sediment sample after combustion at 550ºC for 4 hours. Organic carbon (Corg) was calculated from OM content using the relation described in (Mazarrasa et al., 2017b) y = 0.29x – 0.64; (R2=0.98, p&lt; 0.0001, n=60) OM and POC stocks along the experiment (mg OM ml-1 and mg POC ml-1) were estimated by multiplying the OM and POC (%DW) by the sediment dry weight (mg) remaining in each experimental unit and standardized to the initial volume of sediment (40 ml) introduced in every glass container. Inorganic carbon was estimated as the percentage weight loss of already combusted sediment (550ºC) after combustion at 1000ºC. <strong>Sediment CO₂ production</strong> Container headspace CO₂ gas concentration was measured during 20 minutes continuum incubations (4 replicates) in each temperature and agitation treatment in all sampling times. CO₂ air concentration measures were carried out using an Infra Red Gas Analyser EGM4 from PPSystems. Concentration of dissolved CO₂ in seawater (in μmol CO₂ L⁻¹) was calculated from the concentration of CO₂ (in ppm) measured in headspace air samples after equilibration as described in (Garcias-Bonet and Duarte, 2017; Wilson et al., 2012). Briefly, we calculate the dissolved CO₂ remaining in seawater after equilibration with the air phase ([CO₂]SW−eq) by, [CO₂]SW−eq = 10−6 β [C CO₂]Air P where β is the Bunsen solubility coefficient of CO₂, calculated according to Wiesenburg and Guinasso (1979), as a function of seawater temperature and salinity; [CO₂]Air is the CO₂ concentration measured in containers headspace air (in ppm) and P is the atmospheric pressure (in atm) of dry air that was corrected by the effect of multiple sampling applying Boyle’s Law. Then, the initial CO₂ concentration in seawater before the equilibrium ([CO_2]SW−before eq) was calculated (in ml CO₂ /ml H₂O) by, [CO₂]_{SW−before eq} = ([CH₄]_SW−eq V_Sw + 10−6 ([CO₂]Air −[CO₂]_{Air background}) V_Air)/V_SW Where V_Sw is the volume of seawater in the core or in the seawater closed circuit, [CO₂]_{Air background} is the atmospheric CO₂ background level and V_Air is the volume of the headspace or the closed air circuit. Finally, the initial CO₂ concentration was transformed to µmol CH₄ L⁻¹ by applying the ideal gas law. CO₂ efflux values were calculated from CO₂ variation per time unit. Then, we converted the rates to aerial (taking in account container surface) base, and thickness (in μmol m^-2s^-1).