HOW IS IT MEASURED ?

Chlorophyll is measured by first taking a sample of lake water from the upper layer of water in the lake. This layer is called the epilimnion.

The epilimnion is a distinct layer because it is warmer and therefore less dense than the layers below it. We expect to find most of the algae living in the lake to be in the epilimnion because they need light to photosynthesize.

Scientists can figure out the depth of the epilimnion by taking temperature readings at every meter of depth in the lake by using a temperature meter (often combined with a dissolved oxygen meter).

We can tell where the bottom of the epilimnion is by looking for a change in at least 1º Celsius at each meter of depth. The water sample to test for chlorophyll is taken from one meter below the epilimnion all the way up to the surface. Lake scientists call this an epilimnetic core sample. Epilimnetic core samples are also used when scientists are testing for total phosphorus, color, and phytoplankton species.

University of Maine graduate student lowers the D.O. probe into the water while performing water quality tests on Jacob Buck Lake in Bucksport, Maine.

Above, the student lowers the tube into the water to rinse it out before the sample is taken.

Teeth make a good clamp!

Once the tube is at one meter below the epilimnion, she clamps the tube and quickly brings it to the surface. The water stays in the tube by suction. (This is just like putting your finger on top of a straw in water and take it out of the water. As long as your finger is on the top of the straw, the water stays in the straw.) Once the end of the tube is at the surface, she places the end of the tube in a sampling bottle, and then unclamps the tube to empty the water.

The sample bottles are labled and placed in a cooler until they reach the lab.

In the lab, the samples are run through a filter that concentrates the algae onto the filter paper.

The filter paper is then placed in a solvent, to dissolve the chlorophyll from off of the filter. The solution is placed in either a spectrophotometer or a fluorometer, which are able to determine how much chlorophyll is in the solution. Data can be compared from season to season and from year to year to look for patterns in chlorophyll concentrations.