Find in Library

<p>Surface ocean carbon dioxide (<span class="inline-formula">CO₂</span>) measurements are used to compute the oceanic air–sea <span class="inline-formula">CO₂</span> flux. The <span class="inline-formula">CO₂</span> flux component from rivers and estuaries is uncertain due to the high spatial and seasonal heterogeneity of <span class="inline-formula">CO₂</span> in coastal waters. Existing high-quality <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> fugacity (<span class="inline-formula"><i>f</i>CO₂</span>) at different stages of the tidal cycle. Changes in salinity and <span class="inline-formula"><i>f</i>CO₂</span> 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 <span class="inline-formula"><i>f</i>CO₂</span>. The correlation between salinity and <span class="inline-formula"><i>f</i>CO₂</span> 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 <span class="inline-formula"><i>f</i>CO₂</span> 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 <span class="inline-formula">CO₂</span> flux in the estuarine–coastal zone. Air–sea <span class="inline-formula">CO₂</span> flux variability in the estuarine–coastal boundary region is influenced by the state of the tide because of strong <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing uncertainty in carbon budgets.</p>