This individual was found in mucky organic soil at the edge of a small, spring-fed rivulet, which was running along the outer edge of a graminoid-dominated bog. The last photo is the best habitat shot I have, unfortunately. The small rivulet runs along the base of/somewhat undercuts the bluff, and thus is not super visible in the photo. The plant was collected near where the grassy edge of the bog and the edge of the rivulet meet, more or less directly in front of where the person in the photo is standing.
Ruler units are 0.5 mm.
https://www.inaturalist.org/observations/183415649
Here is an additional observation of the same individual, with more close-up photos of the plant taken in the field.
Second photo shows sample in aquarium temporarily until megaspores could be photographed
Clark Creek Natural Area, just upstream of first major waterfall
Plants numerous, but scattered. Growing in sand, silt, and gravel in 0.5 to 1 meter of water.
Leaves 6-10 cm in length.
Megaspores 450--550 μm in diamemter; uniformly echinate, spines thin and sharp, not reduced near the equator; girdle obscure.
Ligule conspicuous, deltoid, >1 mm long.
Note: Isoetes echinospora was previously recorded at Lake Padden by legendary botanist/plant collector Wilhelm Suksdorf in 1890.
Isoetes riparia ou echinospora?? Dans la rivière des Outaouais/ in the Ottawa River
photomicrograph taken after collection; common in shallow water along NW shore of lake (and probably elsewhere, but I wasn't attempting to assess distribution)