Eutrophication is a growing problem in urban lakes, but it can be challenging to attribute changes in water quality to individual practices when lakes are impacted by a variety of land-use shifts, in-catchment management practices, and in-lake treatments. The Minneapolis-St Paul metropolitan area (MSP) contains over 1,000 lakes and has some of the longest water quality records for urban lakes in the world, primarily collected by 33 watershed management organizations as well as city, county, and state agencies, and a metro-wide monitoring program. We have leveraged this data to explore the impact of aluminum sulfate (alum) treatments on nutrient cycling. Alum treatments reduce algal growth in eutrophic lakes by reducing internal phosphorus (P) loading. However, many lakes are co-limited by nitrogen (N) and P, and the ultimate impact of alum on nutrient limitation and the N:P ratio is unclear because N data are more rarely reported. We aggregated data from 60 monitoring organizations to compare lake responses to 45 alum treatments with long-term trends in untreated lakes. We found that 82% of lakes decreased in P after alum treatment, and of those lakes 87% also decreased in N. The percent decrease in N and P were positively correlated, offsetting some of the increase in the N:P ratio that would be assumed if N remained unchanged. This complicates the attribution of successful remediation to solely enhanced P-limitation as opposed to an overall reduction in nutrient concentrations. The increase in N:P after alum treatment is similar in magnitude or smaller than a broad trend in increased N:P ratios across untreated lakes in MSP, possibly attributed to in-catchment remediation efforts. These results show that urban lakes are improving due to a variety of management actions that reduce P, but more studies are needed to understand the mechanisms that govern the resulting N:P ratios.