Beneath the Surface: Be careful what you wish for

Weed management might also push a lake over its threshold point.

Over the years, I’ve heard numerous people express their desire to just get rid of all of the “weeds” in Lake Windermere. People have asked: “Why can’t we use machines and just pull them out?”

In 2013, the University of Michigan published an article with a good answer to that question.  Following is a summary of the article.

Why can’t we just pull the weeds out?

The Short Answer: Managing plants in a lake through cutting or removing them can sometimes result in murky water and algae growth over the long-term, especially in shallow lakes. So if you don’t have rooted plants, you will have algae and muddy water.

How it Works: Ecology has a theory called “alternative stable states” that offers a useful way of understanding lake processes.

It means lakes and ponds can be in one of two conditions: weedy and clear, or devoid of weeds and muddy. This is determined by nutrients (primarily phosphorus) which have an important role in determining the balance between rooted plants (“weeds”) and the suspended phytoplankton (algae) that contributes to turbidity (murkiness).

At low nutrient levels, the rooted plants will dominate because water is clear and plenty of light reaches the bottom of the lake. At high nutrient levels, the algae win out and effectively shade out rooted plants — this means extremely low water clarity and sometimes harmful algal blooms. At intermediate nutrient levels, things get a bit tricky. In this case, lakes can be pushed to either having more rooted plants, or more algae.

In these “intermediate” nutrient lakes, additional nutrient inputs (perhaps from septic fields or urban and agricultural runoff) can push a lake past its threshold and result in “catastrophic transition” to a muddy and algae-dominated state. When this happens, it can be very difficult to restore water clarity and rooted plant communities.

What mechanically removing rooted plants would do: Weed management might also push a lake over its threshold point.

One study referenced by the University of Michigan used computer simulations to investigate the outcome of various management strategies and found that management for “intermediate” vegetation density can be impossible to attain in shallow lakes with intermediate nutrient levels.

While not achieving desired outcomes, aquatic plant removal would also lead to murky water. Rooted plants prevent mucky bottoms from being stirred up by wind-driven currents, boating activity, and other disturbances. They also suppress algae growth by taking up nutrients. Some plants even release chemicals that further impede algae growth.

When rooted plants are destroyed, mucky bottoms get stirred up and re-suspend nutrients. Competition with algae ceases and foul blooms occur. If cut plant biomass is not mechanically removed, the rotting vegetation further adds to nutrient availability, turbidity, and algae growth.

In conclusion, if you want a clear, blue lake, be wary of large-scale vegetation control programs on shallow lakes. In other words, be careful what you wish for.

Kirsten Harma is the program co-ordinator for the Lake Windermere Ambassadors. She can be reached by phone at 250- 341-6898 or by email at info@lakeambassadors.ca.

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