What is the best correlation test to use when you want to compare two variables in a spatiotemporal context? I have 9 sampling sites. I did the sampling in May, July, and September in the sediments and the water. So, I have 27 samples. I want to know if there is a linear correlation between total nitrogen concentration in the water and the sediments. What is the best correlation test if :
a) the data is normally distributed?
b) the data is not normally distribute?
Please explain it as simply as possible because I don't have a lot of knowledge in this field. If possible can you give some references too?
Thanks,

With only 27 samples, I think that you can not use any relevant spatio-temporal method. Controlling for time and space will require the estimation of several parameters and you don't have enough observations to do so.
You could use a descriptive approach by calculating a locally weighted correlation (the weight will be a function of the distance between observations) in time, space and both, but you will not be able to do inference.
