by John Annin

In the above photo, some of the lake bottom you see is under 30 feet of water. View is to the west with our sand bar in mid-photo.



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 7 and 8) have been programmed to take water clarity measurements from space and these cover an additional 8,000 lakes in Wisconsin alone.



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 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. In 2013 we purchased a Conductivity Meter and now test our samples' electrical conductance to see if water is entering or leaving the lake through underground flow. (see explanation below). We also have placed several piezometers around the lake which show the differential pressure between ground water and lake water. This is another indication of the direction and strength of underground flow.

A Secchi Disk and cord



In the spring of 2005 many people remarked that the lake seemed unusually clear. Typical May Secchis were in the 13' to 16' range. The DNR was called in and on May 25, 2005 they measured a Secchi depth of 42 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. In any case a significant clarity increase happened in 2005 and we don't know the cause of the change. The DNR theory was that the Cisco population had been decimated, possibly by the population of large planted Walleye, and that this in turn had caused an increase in zooplankton on which Cisco feed. This would increase the water clarity as zooplankton consume algae. If in fact the Cisco population had 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 was only a theory but as a precaution the DNR halted the planting of walleye in Black Oak Lake.

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 were the highest to date. This would argue that the Cisco / zooplankton / algae theory is not correct. Below are Gilbert’s results and note that 50 Cisco were taken in less than 24 hours. 


Update, Fall, 2010 and again Fall, 2011………. DNR fisheries folks did two days of work catching, tagging, and milking our Lake Trout. See Lake Information > Black Oak Lake Trout. Their new population estimate for the entire lake is now over 700 Lake Trout. This is more than double the 2004 estimate.

The Lake Trout program continued in the Fall of 2015. Again, the Lake Trout population was numerous and healthy. Fishery expert, Steve Gilbert, did note a relatively small number of small trout, though, and plans to return in the Fall of 2016. See his 2015 results here.

Our 2006 through 2009 Secchi readings were even clearer than 2005 though the 2010 values were off about 5 to 10% from 2009. 2011, 2012 and 2013 Secchis were each slightly higher yet, exceeding 50’ in mid-June of all three years. Then in 2014, the dramatic increase in our water level inundated a lot of mainly terrestrial plants that had grown along the shorelines during the seven year drought dropping our clarity by about 20% from 2013. The CLMN program posts a spreadsheet at the end of each year showing all recorded data for all lakes. Since 2007 Black Oak Lake has had the distinction of being the clearest lake in Wisconsin in 8 out of the last 10 years! The complete spreadsheets showing all measured parameters for all the monitored lakes in the state are below with Black Oak's clarity ranking shown after each year (full 2017 data not yet available): 

2005 (2)    2006 (2)     2007 (1)     2008 (1)     2009 (1)     2010 (1)     2011 (1)     2012 (1)     2013 (1)     2014 (2)    2015 (1)     2016 (12)    2017 (3)

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 fact, our June, 2010,  August, 2012, June, 2013, July, 2013, and June, 2014 analyses set records. The phosphorous level was so low it was below the state lab's minimum level of detection of 5 micrograms per liter.



What looks like one of those Verizon Wireless coverage TV ads is actually a satellite image of central Vilas County lakes. See if you can find Black Oak Lake. On this Landsat "Spectral Radiance" image you'll notice that all the lakes are of slightly different colors. This is all part of the DNR's project that started in 1999 using satellite sensors to estimate lakes' water clarity from earth orbit. To learn more about this amazingly sophisticated project go to Part of the text that catches your eye explains why Black Oak's estimated clarity using this satellite data may be unreliable. About midway through it you read:


 ".....shallow lakes' estimates are unreliable, because the spectral radiance measured by Landsat may be coming from the lake bottom rather than the lake water itself. Thus, lakes that are known to be "optically shallow" (having a maximum depth less than twice the normal Secchi disc measurement) are so noted"  (My italics).  Well, two times Black Oak's normal Secchi depth (2013's was 37 feet) is 74 feet which covers over 95% of the lake! For a visual example of this problem check the photo at the top of this page. It seems ironic that having such clear water inhibits the satellite's ability to measure that clarity.


By the way, if you haven't found Black Oak yet........ North is up and the Sylvania and Cisco Chain lakes are in the upper right corner with Black Oak directly below them. Then Big Portage is just to our south and Forest to the west. True shapes are slightly distorted because their on-orbit photo pixels are 30 x 30 meters.


If Black Oak Lake were located in Minnesota it would be the clearest lake in that state as well. The only two lakes that come close to Black Oak's clarity are Wazee Lake in Wisconsin and Sabin Lake in Minnesota. But both of these are former man dug open pit taconite mines that were abandoned and then filled naturally with water. Their substrates are mostly iron ore with the result that the water is very low in Phosphorous and Chlorophyll.

The search for the reason that our clarity took such a large jump in 2005 continues. After all, if we don't understand why the lake suddenly got so clear it could just as easily go back with no notice. So, during the September 2012 AIS survey with Dr Susan Knight we took a water sample that she took back to her lab to have tested for conductivity. Following is an enlightening report on what she found.


Dear BOL friends,

The BOL sample Walt collected last week had a specific conductance (conductivity) of 51 umhos, in contrast to the 1960 value of 85 umhos.  Conductivity is usually quite constant within a lake over long periods of time (although it is sensitive to the temperature of the water) so this is quite odd. It is unfortunate we only have the one value from 1960, but if accurate, this represents a substantial drop in conductivity. Conductivity is a measure of how well the lake water conducts electricity; it means there are now fewer ions in the water.  This is almost certainly due to less input of ground water.  Fewer ions, including nitrogen and phosphorous ions could be responsible for less primary productivity or “greenness” of the lake and consequently your increased Secchi depth.  This would all make sense if your increase in Secchi happened simultaneously with the drought, which could have shut off some discharge of ground water into the lake but I know you say this is not true.  I am still investigating this, and it is possible you were losing groundwater inputs before you saw a change in lake level.  My working hypothesis at this point is that ground water stopped coming in from your only wetland-dominated area in the north bay of the lake in the earliest years of the drought.  That wetland-lake link might not even have contributed that much water, so the lake level might not have been affected, but perhaps had a disproportionate effect on the specific conductance.  The groundwater from the wetland would be both highly stained (colored) and rich in dissolved organic carbon, contributing to a higher conductance.  I believe my last hypothesis was also related to cutting off the wetland/lake link and that the loss of the ground water from the wetland, which would be very darkly stained, could be directly responsible for the change in clarity.  That was knocked down, probably because the lake level changes were not co-occurrent with the changes in Secchi.  However, the ground water from the wetland also has a high concentration of dissolved organic carbon, and if cut off, could have contributed to the lower conductivity.  I see flaws in my logic, so this still needs thought.  Anyway, it is another intriguing piece of the puzzle.  I will keep working on this and hopefully be able to pull all the strings together.



On another subject related to water sampling......Whitewater Associates took water samples in August, 2013, from which they determined that Black Oak's calcium concentration is 6.3mg/L which is low for the suitability of zebra mussels to grow their shells. It seems that zebra and quagga mussles have very robust shells and also are very poor processors of calcium. Therefore, they need far more calcium in their water than ordinary shelled animals. You can go to the Smart Prevention website to view the lakes on which they have done the calcium threshold study at and click on the online mapping tool. It has an interactive mapping tool that shows susceptibility of lakes to zebra mussel invasion based on calcium concentrations. The model used seven studies that measured effects of calcium concentrations on zebra mussel development and growth. The value derived for the lower threshold of lake suitability to zebra mussels was 10 mg/L. The UWEX model presents three classes of suitability: (1) “not suitable” to zebra mussel invasion (<10 mg/L Ca), (2) “borderline suitable” (10 to 21 mg/L Ca), (3) “suitable” (> 21 mg/L Ca). Our low level of calcium could also be the reason that our rusty crayfish population, which we have had for decades, has never become a problem. Crayfish also need calcium to build their shells though not as much as mussels.



You have probably heard the term "turnover".  If it seems mysterious to you here is a link to an article describing exactly what happens in both spring and fall. Click on "Density Stratification". You'll see lab movies of colored water being injected into large tanks to illustrate the how and why explanations in the text. Additional text describes a plethora of lake science issues. Pick a subject from the column down the left side.



Annual tables, including current year data to this date, of water temperature, transparency, dissolved oxygen, and chemical content at several depths are shown here:

Water transparency (Secchi depth) is conveniently graphed here for year to year comparisons. Notice the large increases starting in 2005: type=11&title=Secchi +Graph&stationNo=643126&format=html&action=post

Secchi depths throughout a recent eleven year period are graphed and the regularity of their intra-summer changes is explained:



Considerable information can be gleaned from a core of bottom soils through the science of Paleolimnology. A Boulder Junction area lake known as Max Lake was cored in 2009. Articles describing results are linked below and cover the entire Northern Wisconsin area.   

          Max Lake Sediment Core Report
          Max Lake Sediment Charts and Graphs

Then in July, 2012, Black Oak was cored. This was a "top/bottom" analysis meaning that the top of the roughly 24 inch-long core was compared to the bottom. The top contains today's stuff while the bottom material was from about 150 years ago, before any significant impact by humankind.

          Black Oak Lake Sediment Core Report


Read this article about Wisconsin's lakes: 15,081 Jewels . It was an article appearing in a Travel Magazine that accompanied the Milwaukee Sentinel in 2009. It's sort of an all-you-ever-wanted-to-know-about-Wisconsin's-Lakes piece. Lengthy to be sure but interesting to anyone who has more than an average level of curiosity about the subject. Black Oak appears in a clarity chart near the end.


         Start by Clicking HERE!