Is the Ocean acidification inconsistent with the Bjerrum plot?
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I think I see the confusion. You are right that directly adding carbonate, perhaps in the form of $\ce{Na2CO3}$, would increase the pH. However, ocean acidification, as shown in your figure, is from atmospheric $\ce{CO2}$ dissolving in the ocean. The relevant equilibrium: $$\ce{CO2_{(aq)} + H2O &lt;=&gt; H2CO3_{(aq)} &lt;=&gt; HCO3^{-}_{(aq)} +H+_{(aq)}&lt;=&gt;CO_{3(aq)}^{2-} +H+_{(aq)}}$$

The Bjerrum plot is simply a graphical representation of this equilibrium. As you add $\ce{CO2}$, you push the equilibrium towards the right, forming $\ce{H+}$, lowering the pH. Conversely, when you add $\ce{CO3^{2-}}$, the equilibrium move towards the left, consuming some $\ce{H+}$ and raising the pH. Although adding $\ce{CO2}$ leads to an increase in the absolute concentration of carbonate, the ratio of concentrations goes the other way.

You can use this two ways:


If you have a change in pH, you can move along the x-axis to predict how the concentrations of the carbonate species change.
If you change the ratios of the species, e.g. by adding $\ce{CO2}$, you can predict how the pH will change.