Because I know I have friends who like to swim there:
High levels of microcystis algae were detected at Spy Pond during water
sampling performed by volunteers from the Mystic River Watershed
Association. This particular type of algae can be toxic if ingested in
high doses and can cause skin irritation and a rash after wading or
swimming. The Arlington Board of Health strongly advises residents not to
swim or stand in the water and especially urges residents to keep pets
away from the water.
Read the full notice as well as learn more about microcystis algae online
at
http://www.arlingtonma.gov/Public_documents/ArlingtonMA_Home/014BF4AF-000F8513.