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.