A possibility: We had two unusually heavy rains upstream which carried 3 to 4 times the usual concentrations of nitrogen and phosphorus into Upper Whitefish. These nutrients provided great amounts of food for the already present algae. It is well known that, with warm water and ample nutrients, algae can double and redouble their mass in just a matter of hours.
Evidence like this makes it convenient for those of us on the lake to attribute the cause to upstream sources.
But the lake may be already pre-loaded with nutrients by the “routine” water runoff from our own lake shore properties. We cannot immediately change what happens upstream, but we can change our own contribution to lake nutrients. The floating plant fragments may be accounted for by luxuriant plant growth (again, think nutrients). The plants are disturbed and uprooted by the propellers and water turbulence of near-shore boating activity. They float to the surface; in this case in amounts great enough to slow boat movement and clog propellers. So what we were seeing may have been a combination of both increased algal activity and dislodged native plants.
Link to a MN Pollution Control Agency fact sheet: https://www.pca.state.mn.us/sites/default/files/Blue%20green%20algae%20fact%20sheet.pdf