Phosphate in the oceans, does it tend to bond with sodium?
The pH of ocean water is currently about 8.1, which lies between the second and third dissociation constants (7.20 and 12.37, respectively).  So most of the phosphate inthe ocean in its twice-dissociared form, as $\text {HPO}_4^{2-}$.  For comparison, our own blood is around pH 7.4, so phosphate in our blood would be a mixture of $\text {HPO}_4^{2-}$ and $\text {H}_2\text{PO}_4^-$.
In the EPA reference above, it is noted that the slightly lower pH today versus in the relatively past implies a larger increase in hydrogen ion concentration.  But it's worse than that.  Because of the way changing pH is coupled with changing dissociation of acids including phosphoric acid, a seemingly small change in pH could mean a lot more potentially acidic material (in the case of phosphate, $\text {H}_2\text{PO}_4^-$ versus $\text {HPO}_4^{2-}$) in the oceans.