Chlorophyll a is the primary pigment used to capture light energy during photosynthesis. All plants, including algae and cyanobacteria, contain chlorophyll, and it is a widely used metric for algal biomass in water bodies. “Algae,” a term describing a diverse group of mostly microscopic organisms, typically are the base of aquatic food webs. Algal abundance and species composition are key water quality factors. Chlorophyll absorbs sunlight at specific wavelengths and reflects light that is not absorbed. The differential absorption and reflection of specific wavelengths of light is the basis for lab and remote sensing methods to measure chlorophyll.
Chlorophyll usually is determined by filtering algae from water samples, extracting the pigment from the filter by an organic solvent, and measuring absorbance by spectrophotometry. Remote sensing methods measure light reflected from water using wavelengths characteristic of the absorbance spectrum of chlorophyll. Remote sensing methods for water clarity also can provide information on algal biomass because water clarity typically is inversely correlated with algal biomass (increasing algae decreases clarity) if other substances that affect clarity (CDOM and mineral turbidity) are not present at high levels.
Chlorophyll distribution in Minnesota lakes
We recently began measuring chlorophyll levels in Minnesota’s more than 10,000 lakes using imagery from the Sentinel-2 satellites. The figure below illustrates statewide distribution of chlorophyll (and hence algal biomass) in late summer of 2016, and the bar graph shows the statistical distributions of chlorophyll in Minnesota lakes averaged over the 2017 and 2018 summer seasons. For more detailed information and results for a particular lake of interest, click the LakeBrowser button above.
Spatial and temporal patterns of lake chlorophyll
Chlorophyll concentrations (and algal densities) vary widely, even for lakes within small geographic regions (such as shown below for the Lake Minnetonka area in the west metro Twin Cities). These data reflect differences in nutrient loadings from surrounding landscapes, as well as inherent differences among lakes in their ability to assimilate nutrients.
Chlorophyll concentrations are highly dynamic, with large variations in concentration during the summer growth season (click on graphic below to see how concentrations change from May to October).