How to get started for monitoring a very small lake

I have a desire to assist with water quality monitoring of lakes and streams however I am a little stuck. Particularly small urban lakes.
I can use electronics and arduinos but I can’t seem to find the right information to help.

What I am looking for is some information on inexpensive continuous monitoring systems that actually work.
I can find a whole bunch of information describing a test system but not ones that are in actual use. Or describing data loggers in use and then vague statements about you can connect a bunch of sensors but not describing the actual sensors.

Ideally I would like to see:
– Parameters that indicate water quality.
What should I be worried about. Perhaps things I should be concerned about such as acid rain, fertilizer runoff, sewage.
Then what parameters are indicators of water quality problems with those worries.
So maybe fertilizer is nitrogen, but if that is difficult perhaps dissolved oxygen is an indicator of algae activity and available nutrients so can be an indicator.

– What sensors can be used to reasonable measure these.
So for example if I am looking for a fixed location continuous monitoring system in a small lake or large pond. What exact sensors might make sense and what trade-offs.
For example:
– Low cost version approximately $xxx
pH sensor model XXX
temperature sensor model xyz
Do sensor model yjk
this setup will require initial calibration, cleaning and re-calibration of sensor … every … sensor XX will likely last x to y years.

– Medium cost version approx $yyy
(sensor models)
(required maintenance…)

– Higher cost version approx $yyy
(sensor models)
(required maintenance…)

Or even just a good example of we decided to track the health of our lake and so built this system. With a pointer to all the components.

Maybe I am just not looking in the right places .

So any help appreciated, thanks!