I have two waterbodies that were sampled for a single water quality variable over time. Both waterbodies were sampled in the fall of each year from 2009-2012. In each year of sampling, 5 replicate samples were taken (so I would have 20 samples in total for each waterbody). I want to compare the two waterbodies over time to see if the water quality variable in each waterbody follows a similar trend. I feel like this is similar to an ANCOVA problem where a person could test for equal slopes. But this problem is a bit different because instead of having two continuous time series, I have 5 replicates of samples for each waterbody in each year. Anyway I'm wondering if ANCOVA is not reasonable, what is the best method to proceed with? A colleague suggested planned linear contrasts might help but I don't understand how that would be applicable to comparing 2 different waterbodies.