Monitoring Water Quality

Fish Swimming 

            Water quality has been monitored on Blockhouse Lake regularly since the summer of 2000 by Blockhouse Lake Association members Jim Williams and Cliff Verrill.  The Wisconsin DNR maintains a self-help monitoring program that teaches lake dwellers how to use a Secchi disc and other instruments to gather data about their lake. A Secchi disk is an eight-inch diameter metal disc that is painted black and white in alternating quadrants.   To measure water clarity, the disc is lowered into the water until it can no longer be seen, and the depth recorded.  These readings are taken every 10 days or so throughout the summer.  These measurements of water clarity or “transparency” are good indicators of how much algae is present in the lake.  Lakes with large amounts of algae present are termed “eutrophic,” and are green in color.  Clear lakes are termed “oligotrophic,” and have very little algae.  Secchi readings taken over time are useful in determining whether a lake is becoming eutrophic, and how rapidly this is occurring.  A sudden change in clarity is sometimes an indication that a nearby change in land use is causing sediments to be deposited in the lake, which also affects Secchi readings.  The readings are also affected by the amount of tannic acid in the lake, which are produced by decaying plants. 

            Here are the Secchi disc readings for Blockhouse Lake from 2000 to the present:

 

Year                 2000                2001                2002                2003

Av. Depth        4.0 ft                3.8 ft                3.9 ft                4.1 ft  

 

            These readings are fairly constant, and show that few changes have occurred in water clarity during the past four years.  The readings are on the low side, and indicate that Blockhouse Lake is moving into the eutrophic phase of lake development.  A strong mitigating factor in this judgment is the brown water color of the lake, revealing the presence of lots of tannin.  Tannins help suppress algae growth because they reduce light penetration. 

 

            In 2002 Jim Williams received a DNR grant to support monitoring of phosphorus, chlorophyll and water temperature in Blockhouse Lake.  The results of two years of testing are generally consistent with the Secchi readings in terms of the trophic index.  The chlorophyll level is less than would be expected, however, indicating that algae are not really a problem in the lake at this time.