Preprints
https://doi.org/10.5194/bg-2021-166
https://doi.org/10.5194/bg-2021-166

  01 Jul 2021

01 Jul 2021

Review status: this preprint is currently under review for the journal BG.

Tidal mixing of estuarine and coastal waters in the Western English Channel controls spatial and temporal variability in seawater CO2

Richard Peter Sims1, Michael Bedington2, Ute Schuster3, Andrew Watson3, Vassilis Kitidis2, Ricardo Torres2, Helen Findlay2, James Fishwick2, Ian Brown2, and Thomas Bell2 Richard Peter Sims et al.
  • 1Department of Geography, University of Calgary, Calgary, T2N 1N4, Canada
  • 2Plymouth Marine Laboratory, Plymouth, PL1 3DH, United Kingdom
  • 3Department of Geography, University of Exeter, Exeter, EX4 4QE, United Kingdom

Abstract. Surface ocean CO2 measurements are used to compute the oceanic air–sea CO2 flux. The CO2 flux component from rivers and estuaries is uncertain. Estuarine and coastal water carbon dioxide (CO2) observations are relatively few compared to observations in the open ocean. The contribution of these regions to the global air–sea CO2 flux remains uncertain due to systematic under-sampling. Existing high-quality CO2 instrumentation predominantly utilise 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 CO2 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 CO2 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 CO2 fugacity (fCO2) at different stages of the tidal cycle. Changes in salinity and fCO2 were closely correlated at all stages of the tidal cycle and suggest that the mixing of oceanic and riverine end members determines the variations in fCO2. The observations demonstrate the complex dynamics determining spatial and temporal patterns of salinity and fCO2 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 CO2 flux in the estuarine–coastal zone. Air–sea CO2 flux variability in the estuarine–coastal boundary region is dominated by the state of the tide because of strong CO2 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 bias quantification of air-sea CO2 fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing uncertainty in carbon budgets.

Richard Peter Sims et al.

Status: open (until 14 Aug 2021)

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Richard Peter Sims et al.

Richard Peter Sims et al.

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Short summary
The amount of carbon dioxide (CO2) being absorbed by the ocean is relevant to the earths climate. CO2 values in the coastal ocean and estuaries are not well known because of the instrumentation used. We used a new approach to measure CO2 across the coastal and estuarine zone. We found that CO2 and salinity were linked to the state of the tide. We used our CO2 measurements and model salinity to predict CO2 draw down. Previous studies overestimate how much CO2 the coastal ocean draw down.
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