Download this Review
Download version has more complete data.

Review of Data on Ultrasound Algae Control Demonstration

Ultrasound algae control units were installed in Lake Pocotopaug with increasing numbers over the summer(see Appendix 1). Because of the water flow in the lake complete coverage of the lake is required for definitive proof that the units will clean the whole lake.  Each ultrasound unit treats an area about 1000 feet by 200 feet. We have measured water flows between 1 and 15 feet/minute.  Even at the lowest flow measured the treated water is replaced by adjacent algae laden water in less than 4 hours.  The algae control units take up to 4 days to treat an area and remove the algae so fresh algae is constantly entering the area. Complete coverage of the lake eliminates this effect.

We were hoping that two measurement strategies would give indications that the units were controlling algae. 1) We would make Secchi disc readings near the units and compare them to the readings at the deep spots in the Lake.  Higher transparency at the units would indicate algae control. The water flow into the area of a unit replaces the water in the area of a unit in less than 8 hours so only a small effect should be expected. 2) We placed units in Markham Bay and compared the Markham Bay deep spot with the Oakland Basin deep spot.  Again the water flow into and out of Markham Bay makes the expected change from these readings small.

1. Secchi Disc Readings at Deep Spots and Near Ultrasound Units

Below are the readings in two tables, those in the main body of the lake compared to Oakwood Basin and those in Markham Bay compared to the Markham Bay deep spot. Note that the units in Markham Bay are near the Markham Bay deep spot.

Where there is no reading there was no ultrasound unit at the location

Note that the water flow in the Lake, which was measured, is surprisingly high and would tend to eliminate differences in algae between different points on the Lake

An average of all these readings showed an increased transparency at the ultrasound units of .08 meters(.26 feet). A Student’s T test¹on these differences between the transparency at the units and that at the two Deep Spots indicated 85% significance for increased transparency at the Ultrasound units compared to the deep spots. This increased transparency is small but the point of this demonstration was to show that the units will reduce the algae concentration.  In order to clear the lake of algae, sufficient units must be installed to handle the algae in the whole lake.

2. Comparison of Oakwood Basin Deep spot and Markham Bay deep spot.

We had one unit in Markham Bay starting in early May and two units starting on June 1. Three more units were installed in Markham Bay at the end of July.  Again because of the water flow in the lake carrying algae into the treated area we are looking for small differences in Secchi depth readings.

Comparison of Oakwood Basin and Markham Bay deep spots

A comparison of these readings shows that Markham Bay has higher transparency of .12 meters or .4 feet. The Student’s T test showed a confidence of 85% in this case also.  After we moved additional units into Markham Bay we continued to see a small increase of transparency but it was not statistically significant, just three readings.

In both the Markham Bay test and the test at the individual units the water flow in the lake would tend to make the readings at all locations equal.  We do not claim this is definitive proof that the units are adequate only that they are having an effect and there is reason to expect that a full installation will control algae in the lake.

Appendix 1 Ultrasound Unit Timing

Timing of Installation of Ultrasound Units