Electrochemical concentration cells -- how anion levels change It seems clear that in the anode, the anion concentration will increase as it enters from the salt bridge to balance the cations leaving from the oxidized electrode. But what is happening to the anion concentration in the cathode?  Cations are leaving the salt bridge to replace those being reduced on the electrode.  So is the anion concentration just constant?
Considering you have chosen a $\ce{Zn}$ concentration cell:
The cell representation is:
$$\ce{Zn(s)/ZnSO4(M1)||ZnSO4(M2)/Zn(s)}$$

Anodic reaction puts more $\ce{Zn^2+}$ ions in the solution:
$$\ce{Zn(s) -&gt; Zn^2+(aq) + 2e-}$$
To balance these extra $\ce{Zn^2+}$ ions salt bridge pulls some anions into anodic compartment.

Cathodic reaction removes $\ce{Zn^2+}$ ions from cathodic compartment:
$$\ce{Zn^2+(aq) + 2e- -&gt; Zn(s)}$$
To make up this equivalent amounts of cations are pulled off from the salt bridge.

So activity of $\ce{SO4^2-}$ is not changed in either of the solutions. It is only those of $\ce{Zn^2+}$ ions in both the compartments, which undergoes a change leading to redox reaction.