Zebra Mussel explosion may have already doubled water clarity in Lake Minnetonka

Image: Minnesota Pollution Control Agency

The odd looking white circular disk is smaller than a Frisbee.  The attached line measures the depth when the bright white circle disappears into the cloudy lake water. It's called a Secchi disk.

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Image: Minnesota Pollution Control Agency

Each week during boating season for the past 20+ years, a member of the Backes family has boated to a spot off of Lake Minnetonka's Big Island and lowered the circular white metal disc down into the water until it disappears from view. They are part of an army of citizen volunteers who monitor water quality in Minnesota for the Minnesota Pollution Control Agency.

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Millie rides along with on the Backes family boat to take Secchi disc readings. Image: Steve Backes

In the late 1970s, a normal reading was 5 or 6 feet. In the 90s, it was closer to 10 feet.

Then sudden changes in water clarity began to emerge in 2012 in the big lake. For a decade, median water clarity readings hovered around 10 feet. Now suddenly in 2012, Secchi disk readings of 14-15 feet were common. In 2013 the trends became even more stark.

Steve Backes now tracks the white disk a full 23 feet down into the previously too murky depths of Lake Minnetonka.

Introducing the Zebra Mussel:

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On July 27, 2010, zebra mussels were discovered in Lake Minnetonka by a local resident.

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Adult Zebra mussels on a rock in Minnehaha Creek. Image MCWD

The small, fingernail-sized mussels appeared in Lake St. Clair near Detroit in 1988. This native of the Caspian Sea region in Asia thrives in a wide range of environmental conditions. Now, they have spread to parts of all the Great Lakes, the Mississippi River, and are spreading into inland lakes like Minnetonka.

Zebra mussels are "filter feeders."  They suck up and filter plankton from surrounding water and if their numbers are great enough, this process can increase water clarity.

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Swarm of zebra mussels cover cement weight on the lake bottom. Image: MCWD


Critical Mass: Tonka's zebra mussel population explodes in 2011

For the past 3 years, scientists at the Minnehaha Creek Watershed District placed "samplers" into various parts of Lake Minnetonka in an effort to gauge the spread of zebra mussels. In 2011, numbers grew slowly. But in 2012, samples show the zebra mussel population took off.

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Lake Minnetonka Zebra Mussel Veliger Data Water samples from 13 sites on Lake Minnetonka were analyzed for the presence of zebra mussel veligers, which is the microscopic larval life stage of zebra mussels. Zebra mussel density is typically measured by the # veligers/Liter. This represents, in theory, the number of veligers that you could find at a particular site if you took a liter of water from the lake. Ultimately, the density of veligers gives a relative picture of zebra mussel reproduction in Lake Minnetonka. Source: MCWD

The eastern end of Lake Minnetonka has the highest concentration of zebra mussels.

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Zebra mussel counts are highest in the eastern end of Lake Minnetonka. Image: MCWD

Here's an excerpt from the MCWD report issued at the end of the 2012 season prepared by Eric Fieldseth an AIS Specialist with MCWD.

Discussion – Unfortunately, there were a lot of missing seasonal samplers in 2011, but overall this map shows a smaller distribution of zebra mussels lake-wide. Not counting the missing samplers, only Spring Park Bay, Coffee Cove and Crystal Bay appeared to have zebra mussels attached to the seasonal samplers on the west side of the lake. You can see the density of Crystal Bay in 2011 was in the range of 1 – 100 zebra mussels/m^2, while in 2012 they were in the range of 50,001 – 61,149 which is a huge increase from 2011. Many sites in the east side had high densities of zebra mussels, with Robinsons Bay and Wayzata Bay having the highest densities. In 2012, you will find a high density of zebra mussels throughout the eastern half of the lake, with densities starting to increase in the western half of Lake Minnetonka. Overall, the zebra mussel population in Lake Minnetonka is increasing and spreading to other areas of the lake which did not have noticeable populations in the past couple years.

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Image: Steve Backes

Zebra explosion may be dramatically increasing water clarity

Long term trends in water clarity in Lake Minnetonka have been improving for decades. Better practices, such as the reduction in phosphorus based lawn fertilizer which can feed algae growth through stormwater runoff.  The Freshwater Society in Navarre has lead the way for decades in improving water quality in Lake Minnetonka. It has been a successful, but slow process.

The dramatic increase...a doubling in water clarity reading from around 10 feet a decade ago...to 20 feet this year in parts of Lake Minnetonka can likely be attributed to the simultaneous explosion of the zebra mussel population. More science is needed, and is in progress, but the trends are likely too strong to be a coincidence.

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Water clarity in Lake Minnetonka. reading in 2012 begin to show a spike in clarity that match the explosion of the zebra mussel population. Image: Minnesota Pollution Control Agency

The readings for 2013 show an even bigger "clarity spike." I asked Big Island based water quality volunteer Steve Backes (A fellow Minnetonka High School classmate) about trends he is seeing in water clarity on Lake Minnetonka. Steve sent me this reply regarding his Secchi disc readings for 2013. (I added bold type to highlight soaring clarity readings over 20 feet.)

Hey Paul: 

Regarding water data, we're happy to be your "Big Island Connection".

Volunteers use a Secchi disk and sample the depth in the same location each time, ideally once a week or so during the summer months. In our case we are going to a spot in the lower Lake in an area between Big Island, Gale Island and the Greenwood shoreline in about 60' of water. We took over this year for my mother who sampled data for 20+ years in the same spot.

So far this year the lake has been very clear: 

5/12/2013 10:00 AM 23.5'

5/30/2013 10:00 AM 19.5'

6/12/2013 3:00 PM 19'

6/16/2013 4:00 PM 18'

7/1/2013 1:00 PM 19'

7/3/2013 1:00 PM 23'

7/11/2013 12:00 PM 18'

7/20/2013 11:00 AM 17.5'

7/21/2013 11:00 AM 18.5'

Steve's water clarity data is eye opening. More work needs to be done, but it appears likely that the explosion of zebra mussels in Lake Minnetonka and other Minnesota lakes is changing the basic properties of  lake water. The effects may be both positive and negative in the future.

One thing is certain now. The introduction and expansion of invasive species like the zebra mussel means we are quite literally entering uncharted waters in the Land of 10,000 Lakes.