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.
Last winter, mats of filamentous algae could be seen on the bottom, and they persisted through mid-summer. This winter there has been none, but there has been an increase in chlorophyll values, although once the latest sampling results are in, we will likely see a drop again as the clarity levels are improving.