Ponds in Peril - Again.  The way to reduce vegetation overgrowth is to protect the watershed from fertilizers and pesticides that run off into the Cove. EG
Article form Newton PATCH:

Improvements Coming to Purgatory Cove in Newton/Waltham

Purgatory Cove in Waltham and Newton to have vegetation removed.

• By Ryan Grannan-Doll February 15, 2012

If all goes according to plan, Purgatory Cove on the Waltham/Newton line will be rid of overgrown vegetation by this summer.

The state Department of Conservation and Recreation plans to start removing the vegetation at the cove this coming May, according to Waltham City Councilor Stephen Rourke. DCR officials will to monitor the cove, which connects to Auburndale Cove, over the next several years and possibly perform chemical treatments. Hopefully, the work will open the cove for boating and fishing again, Rourke said.

An informational meeting will be held in Waltham tonight at 6:30 p.m. to present the project to neighborhood residents. The meeting will be held at the Hovey House Apartments at 315 Crescent St., Waltham. Parking is available on Woerd Avenue or on the DCR boat launch.

The vegetation has overrun the cove over the past several years making it unavailable for boating and fishing, according to Rourke.

“Over the past couple decades ... it has gotten more and more vegetated to the point now where there is no open water,” Rourke told Patch. “If you leave nature to run its course, over time it would become a swamp and eventually a meadow.”

The water became stagnant over time, partly because a stream that used to flow from the former Newton landfill on Rumford Avenue was cut off from the cove, Rourke said.

If chemical treatments are proposed, Rourke said officials would need to get the proper permits from both Newton and Waltham before performing any treatments.

The work has been “a long time coming,” Rourke said. For the past several years, officials and residents have asked DCR for assistance, even submitting petitions, according to Rourke. No funding, however, was available, until now.

Last fall, Step Rep. John Lawn, D-Watertown, toured the area and agreed to push for funding for the project, which resulted in the creation of the recent plans, Rourke said.