DOI: 10.5061/dryad.m0cfxpp5p
title: Data from: High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems

description: Coastal methane (CH4) emissions dominate the global ocean
 CH4 budget and can offset the “blue carbon” storage
 capacity of vegetated coastal ecosystems. However, current estimates lack
 systematic, high-resolution, and long-term data from these intrinsically
 heterogeneous environments, making coastal budgets sensitive to
 statistical assumptions and uncertainties. Using continuous
 CH4 concentrations, δ13C-CH4 values, and
 CH4 sea-air fluxes across four seasons in three globally
 pervasive coastal habitats, we show that the CH4 distribution is
 spatially patchy over meter-scales and highly variable in time. Areas with
 mixed vegetation, macroalgae, and their surrounding sediments exhibited a
 spatiotemporal variability of surface water CH4 concentrations ranging two
 orders of magnitude (i.e., 6 – 460 nM CH4) with habitat-specific seasonal
 and diurnal patterns. We observed (1) δ13C-CH4 signatures that
 revealed habitat-specific CH4 production and consumption
 pathways, (2) daily peak concentration events that could change
 &gt;100% within hours across all habitats, and (3) a high thermal
 sensitivity of the CH4 distribution signified by apparent activation
 energies of ∼1 eV that drove seasonal
 changes. Bootstrapping simulations show that scaling the
 CH4 distribution from few samples involves large
 errors, and that ∼50 concentration samples per day are
 needed to resolve the scale and drivers of the natural variability and
 improve the certainty of flux calculations by up to 70%. Finally, we
 identify northern temperate coastal habitats with mixed vegetation and
 macroalgae as understudied but seasonally relevant atmospheric
 CH4 sources (i.e., releasing ≥100 μmol
 CH4 m−2 day−1 in summer). Due to the large
 spatial and temporal heterogeneity of coastal environments,
 high-resolution measurements will improve the reliability of
 CH4 estimates and confine the habitat-specific contribution to
 regional and global CH4 budgets.
description: We quantified the partial
 pressures of surface water and atmospheric CH<sub>4</sub> and
 CO<sub>2</sub> along with the related C-isotopes (i.e.,
 δ<sup>13</sup>C-CH<sub>4</sub>, and
 δ<sup>13</sup>C-CO<sub>2</sub>, respectively) in
 three coastal habitats during five measurement periods in 2020 and 2021
 (i.e., 18 – 29 May; 06 – 17 July; 17 – 29 August; 30 November – 08
 December 2020; 01 – 06 March 2021). For the measurements, we used an
 adapted version of the Water Equilibration Gas Analyzer System
 (WEGAS) (details in
 Humborg et al., 2019) coupled to a CRDS. The system consists of four
 major components: (i) a submersible seawater intake pump at around 0.3m
 water depth mounted to a movable raft that can be deployed non-invasively
 over the various habitats; (ii) a water handling system comprised of a
 showerhead equilibrator (1 L headspace volume) and a thermosalinograph
 (Seabird TSG 45) fed via a hose by the seawater intake pump; (iii) a gas
 handling system with circulation pumps for the showerhead and ambient air;
 and (iv) the CRDS gas analyzer for CH<sub>4</sub> and
 CO<sub>2</sub> concentration and related C-isotope
 measurements (model G2201-i, Picarro Inc.). The use of a large seawater
 intake pump results in the combined measurement of CH4 from ebullition
 (bubbles) and the dissolved form in the water. The individual contribution
 of the two forms can, however, not be resolved using the current system.
 For CH<sub>4</sub> and CO<sub>2</sub> analysis,
 gas in the showerhead of the equilibrator was measured for 35 min,
 followed by gas measurements of ambient air for 10 min (i.e., one complete
 cycle was 45 min). These measurement cycles (i.e., 35 min water and 10 min
 air measurements) ran continuously during the five measurement periods
 mentioned above. The raft with the water intake pump was moved between the
 defined habitats every 24h from the shore with ropes. Measurements in
 March were distinct from the other sampling periods due to the ice cover
 that had been present for 4 to 6 weeks prior to the time of sampling.
 Here, holes were drilled into the ice and the pump lowered to measure
 “under-ice” concentrations. We validated the CRDS analyzer’s performance
 according to the manufacturer’s instructions with “ALPHAGAZTM Stable
 Isotope Ratio Gases” for Picarro instruments. Specifically, before each
 deployment period, we injected three standards with varying
 CO<sub>2</sub> and CH<sub>4</sub> bulk
 concentrations, and varying δ<sup>13</sup>C-CO2 and
 δ<sup>13</sup>C-CH<sub>4</sub> isotope values
 (i.e., low = 1.00 ppm CH<sub>4</sub>, -24.20‰
 δ<sup>13</sup>C-CH<sub>4</sub>, 250.00 ppm
 CO<sub>2</sub>, -5.00‰
 δ<sup>13</sup>C-CO<sub>2</sub>; natural = 1.77 ppm
 CH<sub>4</sub>, -48.30‰
 δ<sup>13</sup>C-CH<sub>4</sub>, 399.00 ppm CO2,
 -8.50‰ δ<sup>13</sup>C-CO2; and high = 10.00 ppm
 CH<sub>4</sub>, -68.60‰
 δ<sup>13</sup>C-CH<sub>4</sub>, 1000.00 ppm CO2,
 -20.10‰ δ<sup>13</sup>C-CO<sub>2</sub>).
 Measurements with each standard ran for 10 minutes, and 3-point
 calibration lines were constructed whose regression coefficients were used
 to scale the unknown sample data if needed.
 Concentration and isotope
 measurement at 1Hz frequency were averaged and logged every 10 seconds.
 The recorded data were filtered by removing data from the transition
 period between ambient air and water measurements due to the response time
 of CRDS to sharp changes in concentrations of
 CH<sub>4</sub> and CO<sub>2</sub>. Data was also
 removed during improper functioning (e.g., low water flow).
 CH<sub>4</sub> concentrations in water (in ppm obtained by the
 CRDS) were converted to molar concentrations (i.e.,
 CH<sub>4</sub> in nM) and CO<sub>2</sub> was
 converted to pressure units (i.e., pCO<sub>2</sub> in
 μatm) (Humborg et al.,
 2019). Alongside CRDS
 measurements, several other environmental and meteorological variables
 were recorded. Water temperature and salinity were also recorded with
 every CRDS measurement with a thermosalinograph (Seabird TSG 45) that was
 positioned before the showerhead equilibrator. Wind data observations
 (wind speed) were obtained from a Metek uSonic-3 heated 3D sonic
 anemometer mounted on a 1.5 m high meteorological mast. The mast was
 located at the waterline in a coastal bay, approximately 400 m to the
 North West of the sampled habitats. Mean winds were adjusted to a 10 m
 reference height assuming a logarithmic profile with neutral
 stability (Haugen,
 1973).
description: Please refer to the ReadMe file for more details on this
 dataset.