This is probably (but not necessarily) due to to fall turnover, the reversal of water layers. A similar event occurs in spring. See these links: http://www.google.com/search?q=hydrogen%20sulfide%20fall%20turnover&ie=utf-8&oe=utf-8

Here in RI we have a number of locations notorious for the stench of turnover, and each year the phone lines practically melt from the panic stricken calls of nearby residents. This year's turnover has already started due to unseasonably cool weather. My daily dog walk borders an old beaver pond which is already stinking. The water is pristine, there is nothing wrong except a normal, annual cycle which will disappear in a few days.

Agape,

Bob