Net community production in the northwestern Mediterranean Sea from glider and buoy measurements

<p>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 (O<span class="inline-formula">₂</span>) 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 CO<span class="inline-formula">₂</span> fugacity and O<span class="inline-formula">₂</span> concentrations at the BOUSSOLE mooring buoy showed fluctuations, in qualitative agreement with the pattern of glider measurements. Mean net community production rates (<span class="inline-formula"><i>N</i></span>) were estimated from glider and buoy measurements of dissolved O<span class="inline-formula">₂</span> and inorganic carbon (DIC) concentrations, based on their mass budgets. Glider and buoy DIC concentrations were derived from a salinity-based total alkalinity parameterisation, glider pH and buoy CO<span class="inline-formula">₂</span> fugacity. The spatial coverage of glider data allowed the calculation of advective O<span class="inline-formula">₂</span> and DIC fluxes. Mean <span class="inline-formula"><i>N</i></span> estimates for the euphotic zone between 10 March and 3 April were (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">17</mn><mo>±</mo><mn mathvariant="normal">36</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="72722262efff25e0f14c9d145645b397"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-18-1245-2022-ie00001.svg" width="46pt" height="10pt" src="os-18-1245-2022-ie00001.png"/></svg:svg></span></span>) for glider O<span class="inline-formula">₂</span>, (<span class="inline-formula">44±94</span>) for glider DIC, (<span class="inline-formula">17±37</span>) for buoy O<span class="inline-formula">₂</span> and (<span class="inline-formula">49±86</span>) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mmol</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="e2e1b050bb1cf0596ed782afacc19a6e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-18-1245-2022-ie00002.svg" width="67pt" height="13pt" src="os-18-1245-2022-ie00002.png"/></svg:svg></span></span> for buoy DIC, all indicating net metabolic balance over these 25 d. However, these 25 d were actually split into a period of net DIC increase and O<span class="inline-formula">₂</span> decrease between 10 and 19 March and a period of net DIC decrease and O<span class="inline-formula">₂</span> increase between 19 March and 3 April. The latter period is interpreted as the onset of the spring bloom. The regression coefficients between O<span class="inline-formula">₂</span> and DIC-based <span class="inline-formula"><i>N</i></span> estimates were 0.25 <span class="inline-formula">±</span> 0.08 for the glider data and 0.54 <span class="inline-formula">±</span> 0.06 for the buoy, significantly lower than the canonical metabolic quotient of <span class="inline-formula">1.45±0.15</span>. This study shows the added value of co-locating a profiling glider with moored time series buoys, but also demonstrates the difficulty in estimating <span class="inline-formula"><i>N</i></span>, and the limitations in achievable precision.</p>