Water Quality

Measuring Water Quality on Highland Lake

Secchi disk measurements are one of the simplest ways to measure a lake's water quality.  The clearer and more transparent the lake water, the less silt, algae and other particulate we have.  By slowly lowering a plain black and white, circular disk into the water we can visually measure water transparency. The disk is mounted on a measuring tape, and lowered into the water until the disk is no longer visible.  

This depth is recorded as a measure of the water transparency. Secchi measurements on Highland Lake are taken frequently in the open water periods with records dating back almost 50 years.  The plot below contains the last few years of Secchi measurements showing the variation of the lake's water quality over multiple months.  You see the deepest water clarity in June and shallower visibility in late July through August.  

The main causes for poor water transparency in Highland Lake has been due to an unusually high l growth in algae towards the end of July.   While the algae found in Highland Lake is not considered toxic, most would prefer less of it and clearer water throughout the year.

Some may remember that during the summer seasons, 2014-2017 scientists noted the first occurrences of the algae bloom, officially called a “nuisance bloom”  on the lake. A nuisance bloom is when the Secchi measurement falls below 2 meters of visibility.  This triggered HLA to perform more detailed scientific analysis which uncovered several hypotheses of what was happening.

There are two main factors that regulate the amount of algae produced in the lake.  What the algae eats, and what eats algae (also known as the food web.)  algae consume the phosphorus that enters the lake through natural and man made sources.  The more we limit the amount of phosphorus that enters the lake, the healthier it will be.  

The other hypothesis to the algae levels on Highland Lake is the reintroduction of a large number of spawning alewives in the last decade.  The alewives come from the ocean through Mill Brook, up the fish ladder and into the lake to spawn.  While their young are in their larval stage they consume the lakes nanoplankton (the smallest of the zooplankton naturally occurring in the lake) as their food source.  These same nanoplankton are being consumed by the alewife young are the primary predator of algae

More food + less predators = increase in algae and larger bloom.

Looking again at the graph above you will also see a quick fall off after each years' peak and clearer water returns at the end of the summer.  This is believed to be because as the alewife young mature, they start feeding on larger food sources causing a resurgence in nanoplankton that feed again on the algae. The lake does seem to be adjusting to this food web cycle.  As seen in the last two years, Secchi disk readings did not fall below 2 meters, (level of a nuisance bloom).  We will continue to investigate this hypothesis and support the general health of the lake.

One thing that is very clear - watershed residents are encouraged to reduce phosphorus input to the lake.  This is the one thing that everyone can do to support improved lake water quality. 

The Water Quality Committee

Water Quality Team Members

           Keith Williams

           Kevin McElearney

           Tim Hawkins

           Dennis Brown

           David Nadeau

      Rosie Hartzler

  Tom Bannen

  Bill Flahive