Abstract
The Mediterranean Sea comprises just 0.8 % of the global oceanic surface, yet considering its size, it is regarded as a disproportionately large sink for anthropogenic carbon due to its physical and biogeochemical characteristics. An underwater glider mission was carried out in March–April 2016 close to the BOUSSOLE and DyFAMed time series moorings in the northwestern Mediterranean Sea. The glider deployment served as a test of a prototype ion-sensitive field-effect transistor pH sensor. Dissolved oxygen (O2) concentrations and optical backscatter were also observed by the glider and increased between 19 March and 1 April, along with pH. These changes indicated the start of a phytoplankton spring bloom, following a period of intense mixing. Concurrent measurements of CO2 fugacity and O2 concentrations at the BOUSSOLE mooring buoy showed fluctuations, in qualitative agreement with the pattern of glider measurements. Mean net community production rates (N) were estimated from glider and buoy measurements of dissolved O2 and inorganic carbon (DIC) concentrations. Glider and buoy DIC concentrations were derived from a salinity-based total alkalinity parameterisation, glider pH, and buoy CO2 fugacity. The spatial coverage of glider data allowed calculating advective O2 and DIC fluxes. Mean N estimates for the euphotic zone between 10 March and 3 April were (−17 ± 35) for glider O2, (44 ± 94) for glider DIC, (17 ± 35) for buoy O2 and (49 ± 86) mmol m–2 d–1 for buoy DIC, all indicating net metabolic balance over these 25 days. However, these 25 days were actually split into a period of net DIC increase and O2 decrease between 10 and 19 March and a period of net DIC decrease and O2 increase between 19 March and 3 April. The latter period is interpreted as the onset of the spring bloom. The regression coefficients between O2 and DIC-based N estimates were 0.25 ± 0.08 for the glider data and 0.54 ± 0.06 for the buoy, significantly lower than the canonical metabolic quotient of 1.45 ± 0.15. This study shows the added value of co-locating a profiling glider with moored time series buoys, but also demonstrates limitations in achievable precision.
Highlights
20 Around a quarter of anthropogenic carbon dioxide (CO2) emitted between 2011 and 2020 was absorbed by the oceans (Friedlingstein et al, 2021)
305 This study demonstrates the capability of estimating N using measurements obtained by an autonomous glider
Both c(O2) and c(DIC) were used to derive mass budgets, taking into account physical fluxes relating to horizontal advection, air-sea exchange, and mixing
Summary
20 Around a quarter of anthropogenic carbon dioxide (CO2) emitted between 2011 and 2020 was absorbed by the oceans (Friedlingstein et al, 2021). The surface of the Mediterranean Sea (2.5 × 106 km2) represents just 0.8 % of oceans globally, but relative to its size, it is regarded as an important sink for anthropogenic carbon dioxide emissions due to higher levels of anthropogenic carbon than in 30 the Atlantic or Pacific Oceans (Lee et al, 2011; Schneider et al, 2010). This is due to a low Revelle factor related to relatively warm, salty, and high alkalinity waters, encouraging a net flux of carbon dioxide from the atmosphere to the ocean. Carbon dioxide absorbed by the ocean is thought to reach the interior via deep water
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