BLACK OAK LAKE WATER QUALITY TESTING 

by John Annin

History of water quality monitoring on Black Oak Lake

BOLROA has been monitoring water quality on Black Oak Lake since 2002. The monitoring was started by Stu Hunt and Jill Graf of the Conserve School and is done as part of the Wisconsin Department of Natural Resources Citizens Lake Monitoring Program (CLMN). In recent years the testing has been done by John Annin and Walt Bates. This statewide program now enrolls over 1,000 trained volunteers monitoring over 1,500 lakes. Additionally, two Landsat satellites (Landsat 5 and 7) have been programmed to take water clarity measurements from space and these cover an additional 8,000 lakes in Wisconsin alone.

Purpose of Sampling Water

The purpose of this testing is to monitor the health of the lake and to establish a long time data base against which to compare newer data to watch for trends.

Water Sampling Procedure

Water samples are taken at the “Deep Hole” in 85 feet of water. Sampling is done at the surface and at the lake bottom. The samples are forwarded to the Wisconsin State Lab of Hygiene for analysis. The water is tested for Phosphorus, Nitrogen and Chlorophyll plus other chemical compounds. In addition, we take water temperature readings in 10 foot increments from the surface to the bottom and a Secchi Disk reading for water clarity. The sampling is done in January and February through the ice and in May, June, July, August, September and October by boat. In 2005, BOLROA purchased a Dissolved Oxygen (DO) Meter that now allows us to take DO readings at the same depths at which we take temperature readings.

                                                                                       A Secchi Disk and cord

Historical Water Sampling Observations

In the spring of 2005 many people remarked that the lake seemed unusually clear. In May, 2005, our water clarity measurement (Secchi Disk reading) was 42 feet. The historical range for May was 13 to 15.5 feet. The Secchi readings throughout the summer of 2005 continued to be well above average, including a reading of 24 feet in August – it is normal for visibility to decrease as summer goes on. While a clear lake may be desirable for our use, it is not necessarily a sign of a healthy lake. After all, many of upper New York’s lakes are crystal clear but it is due to acidification by acid rain. White Water Associates President Dean Premo, Phd, has reviewed this data. In January, 2006, he explained that clearer water allows sunlight to illuminate more lake bottom encouraging more aquatic growth. More such growth absorbs more lake nutrients that would otherwise fuel algae blooms. Thus the water gets clearer and clearer. Hopefully this is what is happening though it will take several years of observations to confirm this. The opposite can happen as well and should it go far enough the lake will be unrecoverable.

Our 2006 through 2009 Secchi readings have been even clearer. The CLMN program posts a spreadsheet at the end of each year showing all recorded data for all lakes. In 2008 and 2009 Black Oak Lake had the distinction of being the clearest lake in Wisconsin! To see the entire 2008 spreadsheet go to: www.blackoaklake.com/lakeinformation/waterquality/clmnavgs2008.xls. and for the 2009 data go to: www.blackoaklake.com/lakeinformation/waterquality/2009averages.xls. Notice on those charts that the WI State Hygiene Lab tests water samples for Phosphorous and Chlorophyll, nutrients that fertilize algae growth. And notice that it is therefore quite consistent that Black Oak’s clearest water contains very little of these two elements.

In any case a significant clarity increase happened in 2005 and we don't know the cause of the change. The DNR theory is that the Cisco population has been decimated, possibly by the population of large Walleye, and that this in turn has caused an increase in zooplankton which Cisco feed upon. This would increase the water clarity as zooplankton consume algae. If in fact the Cisco population has been reduced, Black Oak's population of indigenous and unique Lake Trout could be in jeopardy, because Ciscos are their primary food source.  So far this is only a theory but as a precaution the DNR has halted the planting of walleye in Black Oak Lake. We will continue to follow what is happening and keep you informed. …..Update, September, 2009……..Steve Gilbert, DNR Fisheries Biologist of Vilas County, conducted a gill netting on Black Oak in late August, 2009, and found a significant rebound in the Cisco population. But the 2009 Secchi readings have been the highest ever. This would argue that the Cisco / zooplankton / algae theory is not correct. Click here for Gilbert’s results and note that 50 Cisco were taken in less than 24 hours.

DNR sites for Black Oak Lake data follow:

Annual tables of water temperature, transparency, dissolved oxygen, and chemical content at several depths: http://dnr.wi.gov/lakes/CLMN/reportsanddata/station.asp?stationid=643126

Water transparency (Secchi depth) is conveniently graphed here for year to year comparisons. Notice the large increases starting in 2005: https://dnrx.wisconsin.gov/swims/public/reporting.do?type=11&title=Secchi+Graph&stationNo=643126&format=html&action=post

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