Abstract
Abstract. Air–water CH4 and CO2 fluxes were directly measured using the eddy covariance technique at the Penlee Point Atmospheric Observatory on the southwest coast of the United Kingdom from September 2015 to August 2016. The high-frequency, year-long measurements provide unprecedented detail on the variability of these greenhouse gas fluxes from seasonal to diurnal and to semi-diurnal (tidal) timescales. Depending on the wind sector, fluxes measured at this site are indicative of air–water exchange in coastal seas as well as in an outer estuary. For the open-water sector when winds were off the Atlantic Ocean, CH4 flux was almost always positive (annual mean of ∼0.05 mmol m−2 d−1) except in December and January, when CH4 flux was near zero. At times of high rainfall and river flow rate, CH4 emission from the estuarine-influenced Plymouth Sound sector was several times higher than emission from the open-water sector. The implied CH4 saturation (derived from the measured fluxes and a wind-speed-dependent gas transfer velocity parameterization) of over 1000 % in the Plymouth Sound is within range of in situ dissolved CH4 measurements near the mouth of the river Tamar. CO2 flux from the open-water sector was generally from sea to air in autumn and winter and from air to sea in late spring and summer, with an annual mean flux of near zero. A diurnal signal in CO2 flux and implied partial pressure of CO2 in water (pCO2) are clearly observed for the Plymouth Sound sector and also evident for the open-water sector during biologically productive periods. These observations suggest that coastal CO2 efflux may be underestimated if sampling strategies are limited to daytime only. Combining the flux data with seawater pCO2 measurements made in situ within the flux footprint allows us to estimate the CO2 transfer velocity. The gas transfer velocity and wind speed relationship at this coastal location agrees reasonably well with previous open-water parameterizations in the mean but demonstrates considerable variability. We discuss the influences of biological productivity, bottom-driven turbulence and rainfall on coastal air–water gas exchange.
Highlights
Methane (CH4) and carbon dioxide (CO2) are two of the most important greenhouse gases (GHGs)
Air–sea CH4 and CO2 fluxes measured by eddy covariance from a coastal location in the southwest UK over 1 year demonstrate significant variability on seasonal timescales
CH4 flux in the coastal seas varied on a semi-diurnal scale, while CO2 flux at times varied diurnally
Summary
Methane (CH4) and carbon dioxide (CO2) are two of the most important greenhouse gases (GHGs). Models estimate that 2.4 ± 0.5 GtC yr−1 of CO2 (a quarter of anthropogenic emissions) have been absorbed by the global ocean over the last decade (Le Quéré et al, 2018) Shelf seas, despite their relatively small area, support high primary productivity, cause a large drawdown of CO2 in the mean (Frankignoulle and Borges, 2001; Chen et al, 2013) and might be responsible for as much as 10 %–40 % of global oceanic carbon sequestration (Muller-Karger et al, 2005; Cai et al, 2006; Chen et al, 2009; Laruelle et al, 2010). The high-frequency fluxes allow us to characterize their variability across a range of timescales (semi-diurnal to diurnal to seasonal) Combining these data with in situ observations of dissolved gas concentrations as well as supporting physical and biogeochemical measurements enables us to quantify the gas transfer velocity at this coastal location and examine its controls
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