The claim that the amount of phosphorus getting into the lake has decreased in recent years is without foundation, and nobody knows whether the target of 75 metric tonnes per year makes any sense at all.
The reported improvement relies on a comparison of findings in consecutive study periods. The more recent period findings suggest that the amount of phosphorus entering the lake from the atmosphere has fallen roughly in half from one study period to the next, an apparent reduction in atmospheric deposition of close to 20 tonnes. The bad news is that the atmosphere did not suddenly get cleaner.
Nor is it likely that the urban runoff, which increased from 10 tonnes to 20 tonnes during the first study period, suddenly fell in half.
What happened is that the measurement techniques changed, what was measured changed, and climate changed. Here’s what the Conservation Authority has to say:
“These differences limit our ability to compare the estimates of total phosphorus load for the two time periods.” And specifically with regard to urban runoff: “Due to technological advances and methodological changes in the way that urban area was estimated, we cannot directly compare the contribution of direct urban runoff to the lake load between the two reports.”
Something else changed too. Let me quote the CA again: “The Lake has changed dramatically under the influence of zebra mussels and other invasive plant species. The programs have not. The programs have not changed to address the impacts associated with these trends.”  
The CA concluded that if we are to have self-sustaining cold water fish stocks and avoid excessive growths of algae and weeds further phosphorus loads must be reduced. Phosphorus concentrations in all gauged rivers continue to exceed level recommended to avoid nuisance plant growth.
But you don’t have to be a scientist to observe changes that are inconsistent with reports of improvement. The sediment has changed. Where there used to be stones on the lake bottom there are now weeds that grab ankles and fowl propellers. The lake floor is now a nutrient rich plant growing medium. Zebra mussels and weeds absorb phosphorus and transfer it to the sediment. Aside from how unpleasant this it, it is also dangerous. There may come a time when the amount of phosphorus cycled back from the sediment to the water column itself becomes a lethal source of phosphorus.
The problem is that nobody knows.
Robert Eisenberg, Founding Chair of the Rescue Lake Simcoe Coalition