Urban lakes provide a myriad of benefits to users and residents but face high nutrient enrichment from anthropogenic sources leading to eutrophication. Eutrophication is of concern due to the resulting harmful algal blooms (HABs), which are potentially toxic to humans and wildlife and decrease the aesthetic value of the lake in addition to other deleterious effects. Aluminum sulfate (alum) is a common chemical management tool for both nutrient reduction and HAB treatment. Although there are decades of research on alum’s effectiveness and ecological impact, few studies have considered how repeated alum applications affect planktonic communities in lakes over time. To measure these effects, I examined the changes in zooplankton and phytoplankton communities over twelve years in a eutrophic and HAB impaired urban lake in western Oregon that used alum as its primary methods of HAB management. Phytoplankton and zooplankton samples were collected at least monthly from May through October over three management periods; (I) 2002-2004 before alum treatments began, (II) 2005-2010 during annual alum surface applications, and (III) 2011-2014 after alum surface applications ended. I used PerMANOVA tests to evaluate changes in phytoplankton and zooplankton communities across the three management periods and Pairwise Wilcoxon tests to identify changes in major phytoplankton and zooplankton groups across the periods. The phytoplankton communities were observed to be significantly different between periods I and II (p=0.004) and I and III (p=0.001) while no statistically significant difference was observed in the zooplankton communities. The proportion of diatoms (p=0.041) and cyanobacteria (p=0.009) significantly increased and decreased, respectively, between periods I and III. The proportion of rotifers (p<0.05) decreased significantly after each period, with more copepods (p=0.003) during period II and more cladocerans (p=0.009) during period III. This analysis shows that planktonic communities respond differently to ongoing alum applications than to the resulting change of environmental conditions within the lake following a single application. Further analysis into the relationship between factors associated with individual applications and changes in the planktonic populations will help inform managers planning alum applications to be more ecologically sound.