Chlorophyll is the pigment that algal and plant cells use to absorb solar energy in the form of sugars. This process known as photosynthesis, involves chlorophyll, sunlight, water, and CO2. In lakes most of the chlorophyll is associated with phytoplankton and periphyton. Since chlorophyll is mostly found algae, this method gives a way to estimate total algal biomass in the lake and its distribution.
We measure chlorophyll at Castle Lake by taking samples from different depths. After zooplankton are filtered out, the samples are filtered again so that all the algal cells (which contain chlorophyll) are collected on a paper filter. The amount of chlorophyll in each sample is measured from their respective filters, giving an accurate estimate of how much chlorophyll there is at each depth. Depending upon the year and climate, chlorophyll concentrations (algal biomass) varies dramatically at different depths. Generally, the concentration changes as the seasons change.