I found this for you Bill:
"If phosphorus gets into the water in which muskrats occupy, it can cause algae blooms, once the algae blooms die and decay it leads to murky, warm water. This is toxic to muskrats and can cause a sudden downfall in population. If the nitrogen in the soil around a pond leaks into the water from runoff after a heavy rainfall, it can cause the oxygen to be "dissolved" by the nitrogen. This can "kill" the affected bodies of water."
http://prezi.com/n1yiir9hnrw-/the-cl...n-of-muskrats/