I am working with a dissolved oxygen dataset. continuous readings are taken at 15 minute intervals and we have been recording these data at 12 stations along the savannah river for two years now. The longest set of readings that are continuous without interuption is 53 days. I would like to look at the power spectral density at each of these sites (most likely one day will be the overridding trend due to photosynthesis and respiration). I would also like to look at the cross spectrums between to contiguous site to look for advective transport from the upstream sections.I have used spectrum to get a periodogram. This I don't believe is the power spectral density. If it is I am unsure how to determine the # in the span argument to smooth the periodogram. I imagine that I am overlooking something- I appreciate all of your help. stephen -- Let's not spend our time and resources thinking about things that are so little or so large that all they really do for us is puff us up and make us feel like gods. We are mammals, and have not exhausted the annoying little problems of being mammals. -K. Mullis [[alternative HTML version deleted]]