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<p>We analyse 400 perturbed-parameter simulations for two configurations of an optimality-based plankton–ecosystem model (OPEM), implemented in the University of Victoria Earth System Climate Model (UVic-ESCM), using a Latin hypercube sampling method for setting up the parameter ensemble. A likelihood-based metric is introduced for model assessment and selection of the model solutions closest to observed distributions of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="57a4663cbf0d11bf294d99bb32c9ae29"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00001.svg" width="25pt" height="16pt" src="gmd-13-4691-2020-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">PO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">3</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="32f5d7f7abed950e70340840c377a456"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00002.svg" width="29pt" height="17pt" src="gmd-13-4691-2020-ie00002.png"/></svg:svg></span></span>, <span class="inline-formula">O₂</span>, and surface chlorophyll <i>a</i> concentrations. The simulations closest to the data with respect to our metric exhibit very low rates of global <span class="inline-formula">N₂</span> fixation and denitrification, indicating that in order to achieve rates consistent with independent estimates, additional constraints have to be applied in the calibration process. For identifying the reference parameter sets, we therefore also consider the model's ability to represent current estimates of water-column denitrification. We employ our ensemble of model solutions in a sensitivity analysis to gain insights into the importance and role of individual model parameters as well as correlations between various biogeochemical processes and tracers, such as POC export and the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="91b2e19ca239409a7665981c17575147"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00003.svg" width="25pt" height="16pt" src="gmd-13-4691-2020-ie00003.png"/></svg:svg></span></span> inventory. Global <span class="inline-formula">O₂</span> varies by a factor of 2 and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a02883d0956e7dc256b9fe9fffa70b09"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00004.svg" width="25pt" height="16pt" src="gmd-13-4691-2020-ie00004.png"/></svg:svg></span></span> by more than a factor of 6 among all simulations. Remineralisation rate is the most important parameter for <span class="inline-formula">O₂</span>, which is also affected by the subsistence N quota of ordinary phytoplankton (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>Q</mi><mrow><mn mathvariant="normal">0</mn><mo>,</mo><mi mathvariant="normal">phy</mi></mrow><mi mathvariant="normal">N</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="88b8921026fbebf4bc45d721d9427e74"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00005.svg" width="30pt" height="18pt" src="gmd-13-4691-2020-ie00005.png"/></svg:svg></span></span>) and zooplankton maximum specific ingestion rate. <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>Q</mi><mrow><mn mathvariant="normal">0</mn><mo>,</mo><mi mathvariant="normal">phy</mi></mrow><mi mathvariant="normal">N</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="bde0d786b395dc62e9ff22c1bd562c0f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00006.svg" width="30pt" height="18pt" src="gmd-13-4691-2020-ie00006.png"/></svg:svg></span></span> is revealed as a major determinant of the oceanic <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="eb51cd45ba2a21283d090226a04e61ba"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00007.svg" width="25pt" height="16pt" src="gmd-13-4691-2020-ie00007.png"/></svg:svg></span></span> pool. This indicates that unravelling the driving forces of variations in phytoplankton physiology and elemental stoichiometry, which are tightly linked via <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>Q</mi><mrow><mn mathvariant="normal">0</mn><mo>,</mo><mi mathvariant="normal">phy</mi></mrow><mi mathvariant="normal">N</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="8088180cdd74c3f81a250f7ca7686902"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-13-4691-2020-ie00008.svg" width="30pt" height="18pt" src="gmd-13-4691-2020-ie00008.png"/></svg:svg></span></span>, is a prerequisite for understanding the marine nitrogen inventory.</p>