There is an ongoing effort to monitor Winni water quality including phosphorus levels. This past summer we volunteered as part of the Lakes Lay Monitoring Program (UNH LLMP).
http://www.winnipesaukee.org/unh-llmp.htm
It is part of an overall effort to develop a water quality management plan for the lake
http://www.lakesrpc.org/lwwmp/
The most recent water quality that I found on line is from 2007:
http://www.winnipesaukee.org/images/...Report2007.pdf
When the 2010 data is available I'll post a link.