Tidal mixing of estuarine and coastal waters in the western English Channel is a control on spatial and temporal variability in seawater CO 2

Surface ocean carbon dioxide (CO 2 ) measurements are used to compute the oceanic air–sea CO 2 flux. The CO 2 flux component from rivers and estuaries is uncertain due to the high spatial and seasonal heterogeneity of CO 2 in coastal waters. Existing high-quality CO 2 instrumentation predominantly utilises showerhead and percolating style equilibrators optimised for open-ocean observations. The intervals between measurements made with such instrumentation make it difficult to resolve the fine-scale spatial variability of surface water CO 2 at timescales relevant to the high frequency variability in estuarine and coastal environments. Here we present a novel dataset with unprecedented frequency and spatial resolution transects made at the Western Channel Observatory in the south-west of the UK from June to September 2016, using a fast-response seawater CO 2 system. Novel observations were made along the estuarine–coastal continuum at different stages of the tide and reveal distinct spatial patterns in the surface water CO 2 fugacity ( f CO 2 ) at different stages of the tidal cycle. Changes in salinity and f CO 2 were closely correlated at all stages of the tidal cycle and suggest that the mixing of oceanic and riverine endmembers partially determines the variations in f CO 2 . The correlation between salinity and f CO 2 was different in Cawsand Bay, which could be due to enhanced gas exchange or to enhanced biological activity in the region. The observations demonstrate the complex dynamics determining spatial and temporal patterns of salinity and f CO 2 in the region. Spatial variations in observed surface salinity were used to validate the output of a regional high-resolution hydrodynamic model. The model enables a novel estimate of the air–sea CO 2 flux in the estuarine–coastal zone. Air–sea CO 2 flux variability in the estuarine–coastal boundary region is influenced by the state of the tide because of strong CO 2 outgassing from the river plume. The observations and model output demonstrate that undersampling the complex tidal and mixing processes characteristic of estuarine and coastal environment biases quantification of air–sea CO 2 fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing uncertainty in carbon budgets.