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Information Bulletin #29

posted Jun 7, 2014, 7:52 PM by Keith Gilbert
Date:  June 3, 2014
From:  LJIS Board


We've received a number of inquiries regarding the floating algae that's currently on the lake surface.  The following Q&A may address concerns about the algae.

•   What is filamentous algae?

Filamentous algae, also called “pond moss or scum,” are single cell plants joined end-to-end with a thread-like appearance.  It grows in shallow water in late winter and early spring when water temperature exceeds 39° and sunlight reaches the lake bottom.  The algae adheres to submerged rocks, trees, and other vegetation in dense, hair-like masses.

•   Why is it on the surface of Lake Johanna?

As it grows, filamentous algae produces oxygen that’s trapped in the tangled strands and submerged mats.  The trapped oxygen causes clumps of algae to rise to the surface where it appears as floating, slimy or cotton-like mats.  These mats may stay in shallow water near the shoreline or drift with the wind.

•   What happens to the floating mats?

High winds and rain can free the trapped oxygen, breaking up the mats and re-submerging the algae, which may rise again or remain on the lake bottom.  The algae eventually turns brown and dies, or other natural controls limit growth as the water temperature warms.  The algae usually disappears from the lake surface in a few weeks.

•   Will it be a nuisance every year?

The size of nuisance “blooms” of filamentous algae that interfere with boating, fishing, and swimming, and are unsightly and smell bad, varies each year.  Four factors affect the algae’s presence: 1) large rainfall events that add nutrients to the lake, 2) warming temperatures above 39°, 3) lengthier periods of sunlight, and 4) winds that break-up or re-submerge the algae mats.

•   Is it toxic to humans?

No, but it’s always a good practice to wash your hands after contact.

•   Does it harm the lake?

No.  Filamentous algae provides cover for aquatic insects, snails, and other small animals that are valuable fish food.  Lake organisms are well-adapted to periodic “blooms,” although excessive growth that’s present for long periods of time can reduce oxygen levels in the water.

•   How can we remove it from the lake in the short-term?

The best method is to remove the floating clumps from your lakeshore with a rake.  You can compost the algae or use it in your garden as mulch.  Hand removal of non-rooted vegetation does not require a DNR permit.

•   Do our vegetation treatments affect the algae?

Yes.  The twice-annual applications permitted by the DNR will eliminate the filamentous algae where the chemicals are applied.  However, we can only treat the approved DNR footage along our lakeshore, and we “…get only two bites of the apple.”  If our chemical treatments are not carefully timed, we could miss other submerged vegetation that appears later in the spring or summer.

•   What about the long-term?

Preventative measures that limit the nutrient flow of phosphorous and nitrogen into the lake will lessen the severity of nuisance filamentous algae during high precipitation years.  These include conservation practices that limit shoreline erosion, reduce stormwater runoff from streets, and prevent construction erosion.  The most effective measure is replacement of a few feet of lawn on the shore with native vegetation that catches and filters runoff to the lake.


Tuesday, June 10th, 5:30pm, dinner at 6:00pm, Island Beach Club.
Chicken, potato salad, cole slaw, beans, and drinks provided, bring a side dish or desert to share.
Access at the Intersection of Ridgewood Road and Arden Place.  Park on the adjacent streets - there's very little parking space on the Island
(Rain check - same time and location, June 17)