Chlorophyll a is one of the plant pigments that is used as a measure of phytoplankton abundance--the tiny algae that are suspended in lake water.
Values in 2019 have been lower than in 2018, both before and after the alum treatment. The 2018 alum treatment caused a 75% drop in water column algae, and other more typical algae (for example, the data point labeled "Dinobryon" shows a bloom of a golden algae that likes lower pH water and is often seen in the lake--it doesn't stick around long, though) were able to come and go over the rest of the summer and fall.
Nutrient levels have remained relatively low this year, which serves to limit algae growth. pH levels have remained relatively constant this year, and are below levels that sustain cyanobacteria (the bad algae).