This year will surely be a poor visibility year in the lake, with all those heavy rains.
But the overall decline in water quality is ultimately due to changes in the watershed.
A really good page, worth a read, regarding Lake Eutrophication:
http://www.des.state.nh.us/factsheets/bb/bb-3.htm
And an incredible resource regarding all the various issues of watershed management:
http://www.des.state.nh.us/bb.htm