Winona - For various reasons, including your ponds presumed geography and a hunch, I suspect your annual pond-visitor is
LYNGBYA.
I certainly can't verify this hunch simply by viewing your photos
Photo 1 Photo 2 . However, one notable characteristic might support the suspected ID. Lyngbya usually has an odor similar to freshly dug soil. I don't know how else to describe it, but you'll probably know what I mean if you take a whiff and it is truly one of the lyngbya species of algae.
In my limited encounters with this very tough and tenacious pest, it often forms on the bottom of ponds and lakes - even in relatively deep, but clear water. At some point, it becomes bouyant; floating to the surface in broken mats and bringing with it plant fragments (including pine needles) that have previously sunk to the bottom, and also providing a floating substrate upon which seeds are able to germinate and grow.
Check it out from this perspective and see what you're able to determine.
