Hello and thank you for your report! It's hard to tell for sure without a closeup photo, but I believe that this is a water fern (an Azolla species). Azolla is a floating plant native to our part of the world, and it really can grow quickly in slow-moving water bodies with lots of nutrients. The plant divides itself mostly and in the summer (as we're seeing) turns this reddish color.
It’s a native plant playing an important function (food for ducks, mitigating algal blooms, providing cover for critters).
Lots of nutrients from septic tanks, fertilizer, and natural sources can lead huge populations. Reducing nutrient loads like nitrogen and phosphorus can help slow the spread.
Some people will apply a chemical called a flocculant to bind to the nutrients and settle them to the bottom; they also might physically remove a portion of the Azolla.
But just to be clear, it's not a concern to natural resource professionals across the state and control should be considered only on a case by case basis.

Jacob Rose
June 20, 2024, 7:26 p.m.