Carbon fixation of a temperate plankton community in response to calcium- and silicate-based Ocean Alkalinity Enhancement using air-sea gas exchange measurements

<p>Ocean Alkalinity Enhancement (OAE) is a carbon dioxide removal strategy that aims to chemically sequester atmospheric CO<span class="inline-formula">₂</span> in the ocean while potentially alleviating localized effects of ocean acidification. Depending on the implementation approach, OAE can considerably alter seawater carbonate chemistry, resulting in temporarily reduced CO<span class="inline-formula">₂</span> partial pressure (<span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span>) and elevated pH before re-equilibration with the atmosphere or mixing with unperturbed waters. To investigate the effects of OAE on biogeochemical processes and organisms under close-to-natural conditions, a large-scale mesocosm experiment was conducted in a temperate fjord ecosystem near Bergen, Norway, during late spring. A non-CO<span class="inline-formula">₂</span>-equilibrated OAE approach was chosen, simulating OAE with calcium- and silicate-based minerals. A gradient of five OAE levels was achieved by increasing total alkalinity (TA) by 0–600 <span class="inline-formula">µmol kg⁻¹</span>. The added TA remained relatively stable over the 47 d experiment and measured CO<span class="inline-formula">₂</span> gas exchange rates reached up to <span class="inline-formula">−15</span> mmol C m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>. We estimated that full equilibration (95 %) by air-sea gas exchange for a <span class="inline-formula">Δ</span>TA of 600 <span class="inline-formula">µmol kg⁻¹</span> would take <span class="inline-formula">∼1050</span> d. Furthermore, various mineral-type and/or <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c6f00d13d95b9183e3e2526db4298e27"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-137-2026-ie00001.svg" width="8pt" height="14pt" src="bg-23-137-2026-ie00001.png"/></svg:svg></span></span> pH effects were found. Coccolithophore calcification followed an optimum curve response along the <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span> gradient, consistent with findings from single-species laboratory cultures. In contrast, in-situ net community production (NCP) was higher in the silicate-based treatments, but was not modified by changes in <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span>. Zooplankton respiration, estimated from in-situ NCP and in-vitro NCP incubations, was lower for the silicate-based treatments and negatively correlated with <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span>. These complex findings suggest both direct and indirect effects of mineral type and OAE level and provide a valuable foundation for designing future OAE field trials. For a safe application of OAE, non-equilibrated alkalinity additions must balance efficiency and environmental impact.</p>