Seasonal dynamics of air-sea CO2 fluxes in the inner and outer Loire estuary (NW Europe)

We investigated partial pressure of CO2 (pCO2) dynamics in the inner and outer Loire estuary (coastal margin of north-western Europe) over an annual cycle (2009–2010) using a comprehensive dataset of pCO2, dissolved oxygen concentration (DO) and ancillary data collected during four cruises conducted in different seasons. In the inner estuary, pCO2 exhibited very large spatial and inter-seasonal amplitudes (from 294 to 2744 μatm) mainly driven by strong heterotrophic respiration from spring to fall, thermodynamic effects in summer and mixing during winter floods. In the outer estuary, during spring and to a lesser extent during summer, the large extension of the plume resulted in stratification of the water column and autotrophic processes induced pCO2 below atmospheric equilibrium in the plume’s surface waters (mean values of −65 μatm compared to the atmosphere). During fall, mixing with inner estuary waters influenced by heterotrophic processes induced pCO2 above atmospheric equilibrium (mean values of +86 μatm) in the very restricted and shallow plume. During winter floods, thermodynamics as well as episodic winter phytoplankton blooms were responsible for pCO2 values below atmospheric equilibrium in the plume (mean values of −42 μatm), whereas the adjacent surface waters were at equilibrium with the atmosphere. Air-sea CO2 fluxes in the inner estuary ranged from 41 ± 4 mmol m−2 d−1 during spring to 140 ± 18 mmol m−2 d−1 during fall, compared to an outer estuary minimum of −9.0 ± 1.0 mmol m−2 d−1 during spring and maximum of 7.7 ± 1.1 mmol m−2 d−1 during fall. Integrated over their respective areas, annual CO2 emissions from the inner estuary of 3.2 109 mol C yr−1 were offset by the annual CO2 sink of −17.9 109 mol C yr−1 from the plume alone. Based on four seasonal cruises, it seems that because of its large size during spring and to a lesser extent during winter, the Loire plume significantly impacted estimates of air-sea CO2 fluxes at a regional scale. Comparison with other major estuarine plumes in diverse continental shelf zones seems to indicate a predominance of CO2 sinks in these systems suggesting that other estuarine plumes might counteract inner estuary CO2 emissions at seasonal to annual level.

► We assessed the seasonal pCO2 dynamics in the inner and outer Loire estuary.
► Pelagic biological processes were the main driver of pCO2 dynamics in the Loire plume.
► The expansion of the plume during spring resulted in an important CO2 sink.
► The CO2 sink in the plume can offset the CO2 emissions from the inner estuary.
► Most estuarine plumes from major rivers studied to date are important CO2 sinks.