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.
In most shallow lakes, the relative abundance of phytoplankton is higher in the summer months, and so water clarity is lower. This is a natural cycle that is related to water temperatures: warmer water generally results in better algae growth.
There has been variability in what types of algae have been found in monthly samples and how abundant they are. In May, the spike in chlorophyll was associated with relatively large algae--they did their thing, then went away, replaced by other algae that were common last summer as well.