A pond near me is covered by green scum in the summer. What causes this and should I be worried about it?

This is caused by an excess of nutrients in the water and is called eutrophication. For an overview see:

http://en.wikipedia.org/wiki/Eutrophication

The green scum is an algal bloom that results from the high nutrient levels and can cause numerous problems ranging from visual ugliness and bad smells to harmful toxins in the water. Hop Brook and its associated ponds are one of several examples in the Town of Sudbury. Some species of blue-green algae (cyanobacteria) release harmful toxins into the water. Other species are benign or even beneficial. An overview of cyanobacteria is at:

http://en.wikipedia.org/wiki/Cyanobacteria

The World Health Organization has produced very detailed information on toxic cyanobacteria in water at:

http://www.who.int/water_sanitation_health/resourcesquality/toxicyanbact/en/

Additional (very technical) information can be found in the following USEPA report:

http://www.epa.gov/OGWDW/ucmr/pdfs/meeting_ucmr1_may2001.pdf

Remediation of eutrophication in the ponds and waterways in Sudbury is a complex issue facing the Town of Sudbury as well as other towns and our State and Federal governments.