Didymosphenia geminata aka Didymo aka Rock Snot has been, and remains, incompletely understood. It is a diatom which develops on a stalk. In certain conditions, called blooms, is has the ability to completely cover the substrate of a stream with serious consequences to the biological environment. The appearance of blooms in parts of the world like New Zealand, Australia, Chile, parts of the Himalaya over the recent past where it was not noted previously has led to its identification as an invasive species, and for several years, specifically between 2010 and 2016 here in Vermont, it was treated as an invasive, leading among other things to a ban on felt-soled fisherman's waders. If invasive, felt-soled waders were an ideal mechanism for its dispersal. During that same time frame, the White River Fish Hatchery in Bethel, Vt was shut down, the fish destroyed, and the hatchery rebuilt after Didymo was found present in the water supply. Since 2016 in Vermont, Didymo has been considered 'endemic' but only a 'nuisance' that develops blooms under low-nutrient conditions. Didymo is not treated like that everywhere, and remains on the invasive list of the National Invasive Species Information Center.
For the reasons above, I started looking for Didymo almost as soon as I started collecting aquatic invert. samples, without knowing much about diatoms at all. It is easy to identify because of its characteristic bottle shape, and its size, about 100 microns, which is large for a diatom, and the fact that it may occur as single individuals, or attached to a stalk. I have found it off-and-on for the past 2 years, last year (2023) more frequently than this year which has been low on stream algae in general. Below are some pictures taken Aug 28, 2024 from a sample from the North Branch of the Middlebury River in Ripton Vt. The filamentous bundles of Didymo in the first picture (with the mayfly) were not readily apparent to the naked eye, and there was nothing like a nuisance bloom at the macro-level, but the presence of the diatom is curious and leaves future developments in question.
I have logged my observations of Didymo on iNaturalist, which can be seen
here
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