Fig. 2: Physiological measurements associated with the incubations.

a Nitrate (NO₃, black), phosphate (PO₄ x 10, white), and silicate (H₄SiO₄, gray) concentrations (µmol L⁻¹). The initial (T0) concentrations are those before amending at the concentrations shown in Table 1. b Chlorophyll a concentrations (µmol L⁻¹) from every 24 h interval for the 400 (red), 800 (blue), and 1200 ppm CO₂ treatments (green). At Experiment 1, the 800 ppm and 1200 ppm treatments are significantly different (P = 0.026) after 72 h. At Experiment 3, the 400 ppm treatment is significantly different from both the 800 ppm (P = 0.018) and 1200 ppm treatments (P = 0.010) after 96 h. c Ratios of particulate organic carbon (POC) to particulate organic nitrogen (PON). The expected Redfield ratio (106:16) is denoted with horizontal dashed line. d Short-term iron-59 uptake rates normalized to chlorophyll a (µmol Fe mol C⁻¹ hr⁻¹) (pmol Fe µg Chl⁻¹ hr⁻¹) for inorganic iron (FeCl₃, black, left y-axis) and organically-complexed iron as ferrioxamine B (Fe-DFB, white, right y-axis). At Experiment 3, the FeCl₃ uptake rates are significantly different between the 400 and 1200 ppm treatments (P = 0.016). Both pCO₂ treatments and time points in hours are denoted on the x-axis in (a, c, d). Error bars represent the standard deviation of the mean (n = 3 biologically independent samples or field replicate samples for T0 except the Experiment 4800 ppm samples where n = 2). Significant differences (P < 0.05, Two-way ANOVA with Tukey’s HSD test) are denoted with an asterisk (*).