Didn't the number of houses surrounding the pond increase by more than double between 2003 and 2006? If that's the case, then the corresponding nutrients washing into the pond would significally increase as well. Forget about rainwater and surface run-off, if you put in a new lawn, you really need to pour the water to it (IIRC 1"/week?), and the water would find it's way into the pond thru groundwater, bringing along some of the starter fertilizer as well.
