The 2009 results from monitoring programs in the middle section of the Huron River watershed show that phosphorus levels have continued to decrease – almost to target levels set by state regulation. The reductions are dramatic. Compared to data going back to 2003, phosphorus concentrations in the last two years have declined by 17% on average in the river, and by 29% in tributary streams. The median phosphorus concentration across all tributary sites was 0.042 mg/l for 2009 – below the regulatory target of 0.050 mg/l.
The phosphorus decreases do not appear to be uniform across the middle Huron River watershed. In a study sponsored by the City of Ann Arbor, a team of researchers at the University of Michigan led by Dr. John Lehman found that phosphorus concentrations dropped at river sites within the city and downstream of the wastewater treatment plant, but there was no significant change upstream of the city. Further, they found that concentrations of other nutrients did not decline over the same period of time. These findings suggest that something happened in Ann Arbor that did not occur upstream. Monitoring conducted by HRWC under the Middle Huron Nutrient Monitoring Program found significant declines in tributaries both within and outside Ann Arbor, but the declines were much greater in urban tributaries.
This decline in phosphorus is consistent with the implementation of Ann Arbor’s phosphorus fertilizer ordinance in 2007. That ordinance restricts the application of phosphorus containing fertilizers within the city limits. A competing explanation is that, as construction activities have declined, so have phosphorus concentrations in the river and streams. Further research and refinement of results is needed to clarify a potential causal explanation.