Methane emissions from UK estuaries: re-evaluating the estuarine source of tropospheric methane from Europe

We report measurements of dissolved methane (CH4) and turbidity in axial surveys of six UK inner estuaries between February 2000 and October 2002: the Humber, Forth, Tamar, Tyne, Tees, and Tay. Dissolved CH4 was always supersaturated relative to air and strong spatial and temporal variability was a notable feature. Super-saturation was highest in the Tyne (max. 107,725%; mean 26,348±30,330%) and lowest in the Tay (max. 1294%; mean 584±425%). All six inner estuaries were atmospheric CH4 sources throughout their surveyed reaches. Broad mid-estuarine CH4 maxima were characteristic of all inner estuaries except the Tay and are consistent with substantial CH4 inputs from intertidal flats. In the Humber, Forth and Tay maximal CH4 in the low salinity turbidity maximum zone (TMZ) implied additional large CH4 inputs at these locations but overall CH4 — turbidity relationships were complex, potentially reflecting interactions between CH4 inputs in the TMZ and from intertidal sediments, with estuarine hydrodynamics. CH4 flux densities (emissions per unit area) varied over an order of magnitude, some of the highest values occurring in relatively small estuaries (e.g. Tees and Tyne). Evidently overall CH4 emissions from inner estuaries are disproportionate to area extent. Combining our measurements with all relevant published data, applying an aerially weighted approach that discriminated between inner and outer estuaries and using a revised estimate of the total European estuarine area, we derived total (inner plus outer) estuarine CH4 emissions for the UK and Europe as follows: UK, 5.8±5.8×109gyr−1; Europe (including UK), 2.7±6.8×1010gyr−1. We estimate the individual contributions to these totals from inner and outer estuaries at ~25% and 75% respectively. Our figure for European estuaries is around an order of magnitude lower than an earlier estimate that set this equivalent to 2–13% of total marine CH4 emissions. Adjusting for our revised estuarine area and for wind speeds that we consider to be more realistic, this comes more into line with our revised synthesis.