Research ArticleECOLOGY

CO2 leakage alters biogeochemical and ecological functions of submarine sands

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Science Advances  07 Feb 2018:
Vol. 4, no. 2, eaao2040
DOI: 10.1126/sciadv.aao2040
  • Fig. 1 Chemico-physical conditions.

    (A) Daily variation of bottom water pHT over 15 days of in situ measurements with RBR sensors (dashed black line represents the average) at REF and CO2-R sites and the tide shown as variation in water depth (right y axis) of the CO2-R site in 2012. (B) Spatial variation of pHT and dissolved CO2 [CO2(aq)] at REF, CO2-G, and CO2-R at high resolution below and above sediment surface (asf; spatial scale, 200 μm). The profiles were obtained with in situ microsensors in 2012. (C) Bottom water and porewater profiles for total alkalinity (TA), dissolved inorganic carbon (DIC), and saturation of calcite (Ωcalc); dashed line is the Ωcalc state threshold for carbonate dissolution. TA and DIC are average data (±SD; n = 6) of samples collected during campaigns 2012 to 2013. Ωcalc was calculated from pHT and TA data.

  • Fig. 2 Biogeochemical conditions at investigated sites.

    All data are integrated (summed up) over the 0- to 5-cm sediment layer, except for extracellular enzymatic activity (ß-glucosidase and esterase) data, which are integrated over the 0- to 2-cm layer (error bars are ±SD; year and number of sampling are given in each plot). TOC, total organic carbon; SRRs, sulfate reduction rates. Stars indicate significant differences between sites [analysis of variance (ANOVA); *P < 0.05, ***P < 0.001; for details, see table S2C].

  • Fig. 3 Community composition of studied sampling site (top 5 cm of sediments).

    (A) Microbial cell numbers and bacterial community structure, as described by 454 MPTS, showing relative number of sequences for dominant bacterial classes (that is, OTUs > 0.1%) clustered according to similarity [based on the Bray-Curtis distance matrix, surface, and subsurface layer; analysis of similarities (ANOSIM); R = 0.948; P < 0.001]. (B) Nematode density and biomass and relative abundance of nematode feeding groups. (C) Polychaete density, macrofauna biomass, and relative abundance of polychaete feeding groups. Error bars are ±SD; year and number of sampling are given in each plot; stars indicate significant differences between one or both CO2-vented sites and the REF (ANOVA; *P < 0.05, **P < 0.01, ***P < 0.001; for details, see table S2E). Ind., individuals; dwt, dry weight; uncl., unclassified.

  • Fig. 4 Effect of medium-term transplantations on benthic community composition.

    Nonmetric multidimensional scaling ordination plot (based on the Bray-Curtis dissimilarity matrix) of bacterial (ARISA-OTUs–based) and nematode (genus level) community structure for undisturbed sediment (sed.), within-habitat, and across-habitat transplants (transpl.). (A) Bacterial community of REF sediment (top, 10-cm layer) transplanted into CO2-R sediment (REF/CO2-R) was significantly different from the source community after 1 year (ANOSIM; R = 0.982; P < 0.001). The bacterial community of CO2-R transplanted into REF sediment (CO2-R/REF) was also significantly different to that of the original site (ANOSIM; R = 0.961; P < 0.001). Both communities that were transplanted within their own habitat remained similar to the undisturbed ones. (B) Nematode community of the 0- to 4-cm sediment horizon 1 year after transplantation was significantly different in across-habitat transplants (ANOSIM; R = 0.723; P < 0.001).

  • Fig. 5 Synthesis scheme summarizing the effects of CO2 leakage on the benthic food web.

    Values depict the percentage of increased or decreased standing stock or activity in CO2-vented sediments relative to REF [(CO2-R/REF) − 1]. Continuous and dashed arrows show biotic and abiotic processes, respectively. C pool, detrital carbon pool; AR, aerobic respiration; AnR, anaerobic respiration; GPP, gross primary production; CaCO3 dis., CaCO3 dissolution. *P < 0 .05, **P < 0.01, ***P < 0.001 (Welch’s t test; n = 3 to 6). Average data of 2012 and 2013 sampling campaigns are shown, except for macrofauna/polychaete data, which are the average of 2012, and for AR and GPP data, which are the average of 2013.

  • Table 1 Main environmental characteristics of sampling sites at Basiluzzo Islet (Panarea Island, Italy).

    na, not available.

    SiteREFCO2-GCO2-R
    CoordinatesN 38°39.827′N 38°39.820′N 38°39.749′
    E 15°07.118′E 15°07.137′E 15°07.123′
    Water depthm14–172115–17
    Aream210035200
    Seagrass meadowsPosidonia oceanicaPosidonia oceanicaPosidonia oceanica
    Bottom water properties (10 cm asf)Temperature°C18.8–19.518.8–19.018.7–19.3
    Salinity383838
    O2§,***μmol liter−1243 (±9)na277 (±7)
    ORP§,***mV245 (±75)na133 (±64)
    pHT7.97.87.3
    DICmmol liter−12.1 (±0.1)2.3 (±0.1)25 (±0.2)
    TAmEq kg−12.3 (±0.1)2.3 (±0.1)2.4 (±0.2)
    Ωcalc||4.0 (±0.2)3.2 (±0.9)1.3 (±0.6)
    Si(OH)4††μmol liter−12.1 (±1.0)2.7 (±1.8)3.2 (±0.3)
    PO43–††μmol liter−100.3‡‡0
    NH4††μmol liter−14.9 (±1.3)1.8 (±1.5)2.6 (±1.0)
    NO2/NO3-††μmol liter−10.4 (±0.1)0.8 (±0.4)0.5 (±0.3)
    Fe††,*μmol liter−10.1 (±0.02)0.2‡‡0.7 (±0.1)
    Mn††μmol liter−100.5‡‡0.4 (±0.1)
    Sediment properties (0–10 cm layer)ColorGrayGrayRed (rusty)
    Median grain sizeCoarse sandCoarse sandCoarse sand
    Porosity§§%38–4440–4241–43
    Carbonate content¶¶,***mg g−19.34 (±1.13)0.04 (±0.02)0.08 (±0.02)
    Porewater pHT||||7.5–7.45.5–5.45.5
    Porewater fluxesGas bubblingNoYesYes
    CO2 content%90–9797–99
    Gas flow†††Liter m−2 hour−180120
    Porewater flow‡‡‡Liter m−2 day−111–6912–4511–85
    DIC flux§§§,***mol m−2 day−10.0–0.22.4–13.82.7–10.3
    Si(OH)4§§§,***mmol m−2 day−10.0–0.910.0–41.717.6–28.2

    †Patch of bare sediment within seagrass bushes.

    ‡Average temperatures in 2011 to 2013 measured in situ with SEAGUARD at 30 cm asf.

    §Average (±SD; n = 4000) of 2012 data collected in situ with RBR sensors over 15 days at 2 cm asf.

    ¶Average of 2011 to 2013 measurements (n = 9).

    ||Calculated using R package seacarb v 3.0.11; input variables, pHT and TA (for details, see table S1).

    ††Average (±SD; n = 3) of 2013 data; one sample available for PO43−; one sample available for dissolved Fe and Mn at CO2-G.

    ‡‡No replicates available.

    §§Average porosity assessed from sediment samples collected in 2011 to 2013 (n = 3).

    ¶¶Average (±SD; n = 8) of CaCO3 content in 0- to 2-cm and 4- to 6-cm layers for 2012 and 2013.

    ||||Average at top (0 to 2 cm) and bottom (8 to 10 cm) layers of sediment profile in 2011 to 2013 (for details, see table S1).

    †††At seafloor during low tide.

    ‡‡‡Range (2012 to 2013) of porewater efflux.

    §§§Range of fluxes measured in 2013 (n = 6).

    *P < 0.05, ***P < 0.001; Welch’s t test between REF and CO2-R.

    • Table 2 Benthic oxygen fluxes.

      Oxygen exchange (transparent chamber; net O2 flux), oxygen respiration (masked chamber; O2 respiration), and oxygen production [GPP = net O2 flux + (O2 respiration)] rates obtained from benthic chambers deployed in 2013; oxygen production–to–respiration (GPP/R) ratio, respiration per unit of total biomass (R/BTotal), and respiration per unit of heterotroph biomass (bacteria and animals; R/BHeterotrophs). Benthic masked chambers rates (n = 2) and average (mean with ±SD in parenthesis), maximum (Max), and minimum (Min) rates of transparent chambers (n = 3 to 4) and O2 production (n = 6 to 8) are given. nt, not tested for significance level; ns, not significant (P > 0.05).

      Net O2 flux (daylight)nsO2 respiration (masked)ntO2 production**GPP/RR/BTotalR/BHeterotrophs
      mmol m−2 day−1mmol m−2 day−1mmol m−2 day−1day−1day−1
      REFMean−7 (6)na10 (8)0.60.06/0.140.07/0.18
      Max−11−2321
      Min−2−100
      CO2-GMean−58 (63)−188130 (63)0.71.132.06
      Max−151na173
      Min−15na37
      CO2-RMean−18 (6)na55 (37)§0.80.16/0.490.41/1.14
      Max−24−10696
      Min−11−3815

      †Average (±SD; n = 6) of O2 production calculated from each Net O2 flux using O2 respiration from both masked chambers.

      ‡At CO2-R, only one masked chamber was available.

      §Average (±SD; n = 8) of O2 production calculated from each O2 flux using O2 respiration from both masked chambers.

      **P < 0.01 (Welch’s t test between REF and CO2-R).

      • Table 3 Summary of CO2 impact on benthic organisms and processes at the Basiluzzo Islet sites (soft sediments) and comparison with available benthic data from other shallow natural CO2 vents (soft and rocky seafloor) at Ischia, Vulcano, and Papua New Guinea.

        Significant deviations from the REF are described by upward (enhancing) or downward (declining) arrows, respectively, or by + for changes in community structure. 0, neutral; PNG, Papua New Guinea; OC, organic carbon; EEA, extracellular enzymatic activity; Undist., undisturbed sediments at CO2 vents; Transpl., medium-term (1 year) transplanted sediments from reference to CO2-impacted site (REF/CO2-R). For references and detailed description of CO2 affects on marine environments at natural CO2 vents, see table S9.

        BasiluzzoIschiaVulcanoPNG
        Undist.Transpl.
        InvertebratesCommunity structure+++na+
        Abundance↑↓na
        Biomassnana
        SeagrassDensity0na↑/0
        Biomassnanana
        Photosynthetic activitynana0
        MacroalgaeCommunity structurenana+na+
        Community structure+nana+na
        MicrophytobenthosAbundancenanana
        Biomassnana
        BacteriaCommunity structure++na++
        Abundance00nanana
        Primary production and OC remineralizationOxygen productionnananana
        Oxygen respirationnananana
        SRRnanana
        OC degradationβ-glucosidase (EEA)nanana
        Esterase (EEA)nanana
        Nutrients fluxSilicate, ironnanana

      Supplementary Materials

      • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaao2040/DC1

        Supplementary Materials and Methods

        fig. S1. Maps and seafloor images of the sampling area.

        fig. S2. Bottom water and porewater chemistry.

        fig. S3. Biogeochemistry of medium-term transplants and undisturbed sediments.

        fig. S4. Similarity between benthic communities at investigated sites.

        table S1. Bottom water and porewater chemistry at the study sites off Basiluzzo Islet.

        table S2. Outcome of one-way ANOVA.

        table S3. pHT change and in situ fluxes obtained from benthic chambers deployed in 2013.

        table S4. Change in relative sequence number of highly abundant, putative functional groups of bacteria based on 454 MPTS OTUs annotation (OTU > 0.1%).

        table S5. Outcome of distance-based multivariate regression analysis.

        table S6. Outcome of permutational ANOVA for environmental variables (Env. Setting), bacterial community structure (ARISA), and fauna community structure (that is, nematode species, meiofauna higher taxon, polychaete species, and macrofauna higher taxon).

        table S7. Relative abundances of macrofauna taxa, polychaete species, meiofauna taxa, and nematode species at the three investigated sites.

        table S8. Differences in porewater and sedimentary environmental settings in medium-term transplant experiments (n = 3).

        table S9. Overview of CO2/pH impacts on benthic organisms at natural CO2 vents, as observed in previous studies.

        References (72160)

      • Supplementary Materials

        This PDF file includes:

        • Supplementary Materials and Methods
        • fig. S1. Maps and seafloor images of the sampling area.
        • fig. S2. Bottom water and porewater chemistry.
        • fig. S3. Biogeochemistry of medium-term transplants and undisturbed sediments.
        • fig. S4. Similarity between benthic communities at investigated sites.
        • table S1. Bottom water and porewater chemistry at the study sites off Basiluzzo Islet.
        • table S2. Outcome of one-way ANOVA.
        • table S3. pHT change and in situ fluxes obtained from benthic chambers deployed in 2013.
        • table S4. Change in relative sequence number of highly abundant, putative functional groups of bacteria based on 454 MPTS OTUs annotation (OTU > 0.1%).
        • table S5. Outcome of distance-based multivariate regression analysis.
        • table S6. Outcome of permutational ANOVA for environmental variables (Env. Setting), bacterial community structure (ARISA), and fauna community structure (that is, nematode species, meiofauna higher taxon, polychaete species, and macrofauna higher taxon).
        • table S7. Relative abundances of macrofauna taxa, polychaete species, meiofauna taxa, and nematode species at the three investigated sites.
        • table S8. Differences in porewater and sedimentary environmental settings in medium-term transplant experiments (n = 3).
        • table S9. Overview of CO2/pH impacts on benthic organisms at natural CO2 vents, as observed in previous studies.
        • References (72–160)

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