Post hoc test

Hi all

I am currently undergoing my undergraduate studies in Marine Biology. I am trying to look for significant differences in cell density of different species plankton that I have exposed to a variety of oil concentrations.

I have conducted a Repeat measures Anova to test for significant change over time.

I now what to see if there is a difference between the different types of plankton which I presume is a Post-hoc Tukey test. However the starting cell densities are different between the two samples. Does that mean I can still use the Tukey test or is there some kind of normalization I can use to standardize the start point?

Any help would be much appreciated.