Methodology: We compiled available data published in peer-review articles and grey literature reports until year 2009 as well as our own unpublished data. We searched for data with the engine ISI web of Knowledge, using the keywords Posidonia oceanica AND (regression OR decline OR progression OR recovery OR status OR cartography OR limits OR cover OR density OR biomass OR dynamics), and conducting a back search of cited papers.
More information about the methodology to be found at: Calleja, M.L., N. Marbà, C.M. Duarte (2007). The relationship between seagrass (Posidonia oceanica) decline and sulfide porewater concentration in carbonate sediments. Estuarine coastal and shelf science 73: 583-588; Waycott M, CM. Duarte, T.J. B. Carruthers, R.J. Orth, W.C. Dennison, S. Olyarnik, A. Calladine, J.W. Fourqurean, K.L. Heck, Jr, A.R.ll Hughes, G A. Kendrick, W. J Kenworthy, F T. Short, S L. Williams (2009). Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proceedings of the National Academy of Science USA 106: 12377–12381; Moreno, D., P. A. Aguilera, H. Castro (2001). Assessment of the conservation status of seagrass (Posidonia oceanica) meadows: implications for monitoring strategy and the decision-making process. Biological Conservation 102: 325-332., Content and values displayed: The data set includes general information of the study conducted: the year or period of years of the study, site, country, water depth; and of the seagrass meadow investigated: extent, depth of deep and shallow limit, cover, shoot density, meadow status -decline, expansion, steady-state-, rate of change, Index of Conservation, Moreno et al. 2001) and types of coastal pressures present. The data set includes qualitative and quantitative data, since it contains studies where changes in seagrass meadow extension, cover and /or density are identified from expert judgement as well as studies that quantified the magnitude and rates of the changes reported.
The absolute and relative rates of change of seagrass extent, meadow depth limits, cover and shoot density are calculated as described in Marbà et al (submitted).
The meadows are categorized as declining (final area < 90% of initial area, net % cover abosolute change < -10, or µ density < -5% yr-1), increasing (final area >110% of initial area, net cover change > 10 % yr-1, or µ density > 5% yr-1) or without detectable change (final area within <90 % of initial area, -10 % yr-1 < net cover change < 10 % yr-1, or -5 % yr-1 < µ density < 5 % yr-1) following Waycott et al (2009) for area and Calleja et al (2007) for density criteria., Access and reuse: This dataset is subject to a Creative Commons Reconocimiento-NoComercial-CompartirIgual 4.0 Internacional License., This dataset compiles qualitative and quantitative information on the stability (i.e. decline, steady state, expansion) of the seagrass (Posidonia oceanica) in the Mediterranean Sea between years 1842 and 2009. Data on meadow extent, shallow and depth limits, cover and shoot density as well as rates of change for the study period are provided. A ReadMe file, the full list of references and explanation of dataset values are attached., SESAME Southern European Seas: Assessing and Modelling Ecosystem changes(EU, Project no: 036949, Integrated Project of the Thematic Priority: 6.3 Global Change and Ecosystems). Lenght: November 1, 2006-December 1, 2011., OPERAS (EU, Project no: 308393, Collaborative Project of Theme ENV.2012.6.2-1)., EstresX, Sinergia y antagonismo entre múltiples estreses en ecosistemas marinos Mediterráneos (Ministerio de Economía y Competividad, ref. CTM2012-32603). Lenght: January 1, 2013-December 31, 2015., Peer reviewed

In March 2012 we collected 3-replicated sediment cores (9 cm diameter and 12-15 cm long) per restored site along the planting chronosequence (i.e. years 1994, 1997, 2003, 2004, 2006). Similarly, we collected 3 sediment cores in two bare sites, previously colonized by seagrasses, and in a large seagrass patch that survived the disturbances in the second half of the 20th Century that we considered a mature and thus reference meadow. We measured sediment bulk density and organic matter content and carbon stocks along all sediment cores sliced at 1 cm interval. Organic carbon content (% Corg) was estimated from loss of ignition (% LOI) at 550ºC for 5 h using the empirically fitted equation for Oyster Harbour sediments,Log % Corg = -0.62 + (1.33 * Log % LOI)
SEintercept = 0.01, SEslope =0.10, N = 55, R2 = 0.77, P<0.0001
We analyzed the ƌ3C of the organic carbon in the top 3 cm sediment layer (ƌ13Csediment) along the chronosequence to estimate the fraction of seagrass (X) and sestonic (1-X) deposition as
ƌ13Csediment = [X . Log % Corg = -0.62 + (1.33 * Log % LOI)
SEintercept = 0.01, SEslope =0.10, N = 55, R2 = 0.77, P<0.0001
We analyzed the ƌ3C of the organic carbon in the top 3 cm sediment layer (ƌ13Csediment) along the chronosequence to estimate the fraction of seagrass (X) and sestonic (1-X) deposition as
ƌ13Csediment = [X . ƌ13Cseagrass] + [(1-X) . ƌ13Cseston]
being ƌ13Cseagrass -9.65 ‰ and ƌ13Cseston -22 ‰ (Dauby 1989)., Access and reuse: This dataset is subject to a Creative Commons Reconocimiento-NoComercial-CompartirIgual 4.0 Internacional License., This dataset contains the values of ƌ13C, concentration and density of organic carbon measured along sediment cores collected where seagrasses (Posidonia australis) where lost, in re-vegetated plots and the continuously vegetated meadow at Oyster Harbor., Financial acknowledgements: CSIRO-Carbon Cluster (Australia);
Australian ARC Linkage projects (LP100200429, LP1301000155); Opera (EU FP7, Project No. 308393);
MEDEICG (CTM2009-07013); EstresX (CTM2012-32603).
Mobility grant of CSIC (PA1003258) to Núria Marbà;
Gledden Visiting Fellowship of the Institute of Advanced Studies (UWA) to Núria Marbà;
Gledden Visiting Fellowship of the Institute of Advanced Studies (UWA) to Pere Masqué;
IM by a PhD fellowship by the Government of the Balearic Islands; PM by ICREA Academia
Generalitat de Catalunya (2014 SGR – 1356);
PhD grant by the Govern of the Balearic Islands;
PhD grant by Obra Social “la Caixa”., Peer reviewed

The dataset contains data on profiles of bulk density, concentrations of organic matter, carbonate, organic carbon and inorganic carbon, density of organic and inorganic carbon, δ13C and abundance of 210Pb along sediment cores. The sediment cores were collected inside three Zostera marina meadows growing in Western Greenland: Ameralik (64°15’N, 51°35’W), Kapisillit (64°28’N, 50°13’W) and Kobbefjord (64°09’N, 51°33’W) (Marbà et al 2018). The sediment cores were sliced every 1 or 2 cm depending on the core. Dry bulk density was measured by dividing the weight after oven-drying them at 60 oC for 48 h by the wet volume of the sediment sample. Concentration of total 210Pb was determined by alpha spectrometry following Sanchez-Cabeza et al. (1998) . The concentration of excess 210Pb was calculated as total 210Pb minus supported 210Pb, estimated as the average of total 210Pb concentration at the base of each sediment core profile. Supported 210Pb values were comparable to the 226Ra concentrations obtained at selected depths in each core. The depth of the sediment horizon accreted since year 1900 was identified by applying constant flux: constant sedimentation (CF:CS) model (Krishnaswamy et al 1971) and the year of sediment accretion at the top of each slice by applying the constant rate of supply (CRS) model (Appleby and Oldfield 1978) . Organic matter concentration (OM, % DW) was measured using the loss of ignition technique. Sediment organic carbon concentrations (Corg, % DW) were estimated from measured organic matter concentrations (OM, % DW) using the relationship described by Fourqurean et al. (2012). Concentration of inorganic carbon (Cinorg, %DW) was measured by conducting a second combustion of the sediment samples at 1000 oC for 2 h and multiplying the amount of CO2 released from the carbonate by 0.27 (i.e. the ratio of the atomic weight of carbon (12 g) to the molecular weight of CO2 (44 g)). Densities of Corg (g Corg cm-3) and Cinorg (g Cinorg cm-3) were calculated by multiplying, respectively, Corg and Cinorg concentrations by the sediment dry bulk density of each sediment sample. We analyzed the 13C of the sediment organic carbon in acidified samples by an isotope ratio mass spectrometer (Thermo fisher scientific) and report it in the δ notation as the ratio of the 13C to the 12C isotope in the sample (Rsample) relative to that of a standard (Standard) i.e., δ sample = 1000 [(Rsample/ Rstandard) − 1]. The primary standard is Vienna Pee Dee Bellemnite (VPDB) and secondary standards are Acetanilide (Schimmelmann) and sucrose. The seagrass contribution in the sediment organic carbon pool after year 1900 was estimated by applying a two source-mixing model,
δ13Csed after 1900 = δ13Cseagr * f + [δ13Csed before 1900 * (1-f)],
that considered Z. marina (δ13Cseagr Ameralik = -7.31 ± 0.02 ‰, δ13Cseagr Kapisillit = -6.58 ± 0.33 ‰, δ13Cseagr Kobbefjord = -7.83 ± 0.15 ‰) and a business as usual carbon source scenario, represented by the average δ13Csed observed in sediments accreted before year 1900 (δ13Csed after 1900 = -30.44 ± 0.38 ‰), as end members. We corrected for the historical change in the δ13C source signatures due to 13C depletion in the atmospheric CO2 and oceanic DIC δ13C signature towards present derived from the burning of fossil fuels (i.e. Suess effect, Keeling 1979). This was done by applying the model described by Schelske and Hodell (1995) and modified by Verburg (2007):
δ13Catm = 4577.8 – 7.343 *Y + 3.9213 * 10-3 * Y2 – 6.9812 * 10-7 * Y3
to estimate the δ13C of atmospheric CO2 (δ13Catm) over time (years, Y) since year 1840. These values were subsequently normalized to δ13Catm in year 1840, and the resulting time-dependent depletion in δ13C since1840 was subtracted from the measured δ13Csed for each dated sediment section., The dataset provides data on organic matter (OM), carbonate (CaCO3), organic carbon (Corg), inorganic carbon (Cinorg), 13C in organic carbon and 210Pb in sediment cores collected at three Zostera marina meadows from Western Greenland., EU FP7 (project Opera’s, contract number 308393) and King Abdullah University of Science and Technology (KAUST). DKJ received support from the COCOA project under the BONUS program funded by the EU 7th framework program and the Danish Research Council and from the NOVAGRASS (0603-00003DSF) project funded by the Danish Council for Strategic Research. P.M. was supported by the Generalitat de Catalunya through its grant 2017 SGR-1588., Peer reviewed