In planning this summers solo trip, I was looking at the water clarity of the lakes I plan to visit on the Minnesota Lake Browser (Gull, Gun, Beartrap, ect) and for many of the lakes the clarity had really gone down since 1975. While they state they are doing this via remote sensing, methods and sensor have greatly changed since the mid 70's so I wonder if it is a function of technology changing and the lakes are generally the same now as back then, or if the lakes are indeed are less clear now. ("less clear" being relative of course). I understand some lakes have been impacted over the years by blow downs and fires which will impact them for a few years.

https://www.dnr.state.mn.us/lakefind/index.html https://water.rs.umn.edu/sites/water.rs.umn.edu/files/rs_historical_methods_plb.pdf

Much of the Mn DNR clarity is done at the lake site with what is called a secchi disc lowered into the water until it disappears.
I am pretty sure the satelite(sp) image used by PCA will be different. Time of year makes a big difference also.
Being familiar especially with the Mn DNR method on some lakes I know.the PCA reading will vary between the two when I looked at it in the past.

Remote sensing probably didn't start to the 1980's or so?

It can be hard to compare clarity data, because there can be plenty of natural variation throughout the open water season---even on pristine lakes.