Insights from year-long measurements of air–water CH 4 and CO 2 exchange in a coastal environment

Air–water CH 4 and CO 2 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, CH 4 flux was almost always positive (annual mean of ∼0.05 mmol m −2 d −1 ) except in December and January, when CH 4 flux was near zero. At times of high rainfall and river flow rate, CH 4 emission from the estuarine-influenced Plymouth Sound sector was several times higher than emission from the open-water sector. The implied CH 4 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 CH 4 measurements near the mouth of the river Tamar. CO 2 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 CO 2 flux and implied partial pressure of CO 2 in water ( p CO 2 ) 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 CO 2 efflux may be underestimated if sampling strategies are limited to daytime only. Combining the flux data with seawater p CO 2 measurements made in situ within the flux footprint allows us to estimate the CO 2 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.

Abstract licence: CC BY

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