Interesting discussion. The fertilizer ordinance did not address overall fertilizer use — only P content. Fertilizer sales and use would be expected to be about the same, thus, one would not expect a change in N levels following the implementation of the ordinance.

I cannot speak for Dr. Lehman’s sampling, but our tributary sampling program runs during the May-September growing season, because those are the months that the TMDL regulation is in effect. Effects from overnutrification are not seen outside of the growing season. That is the target, so that is when we sample, along with the fact that winter sampling is logistically much more difficult.

It is very difficult to EVER draw direct conclusions from environmental results. However, it is reasonable to put all the available information together and then look for the most likely explanation if there is a consistent result. Additional sampling would always improve our understanding, but as of now one cannot rule OUT a positive effect from the ordinance and the preponderance of evidence suggests a positive correlation.

Thanks for the comments. Let me know if you have direct links to the research you cite above. We recognize that there are many reasons why phosphorus levels could drop. Fertilizer, fewer building projects (less soil runoff), and you have added street sweeping. I will check on how that has changed or not. I believe we have added one additional city sweep in the spring for stormwater. I am not sure if that would provide the magnitiude of change Dr. Lehman is seeing or would provide the change for the whole may, june, july season. Thanks for taking the time to think about this with us.

“Further, non-target variables sampled at the same time as phosphorus (e.g., nitrate, dissolved organic matter, silica, specific conductance, and pH) did not change over the sampling period.”

Most lawn fertilizers have all free nutrients, usually 9 to 10 times more nitrogen than phosphate. Nitrogen is a lot more mobile than phosphate, so if you have not seen any reductions in nitrogen I would ask if the city has increased the frequency of their street sweeping program.

Did the city buy a new more effective street sweeper?

Given that more than 75% of phosphate losses from turf occur when the ground is frozen [peer reviewed research from Wisconsin (Kussow) and Minnesota (Horgan)] you need to sample year round before you start to make conclusions.