PART II: What happens when those we love start behaving badly? – the Didymo backstory

by Mark Edlund |

The scenario that the kids and scientists on Minnesota’s North Shore are studying exemplifies the problems that didymo has been causing throughout the world. Didymo seems to appear out nowhere but loves its nice new habitats and grows in profusion to coat everything in a mucilaginous goo. We’ve seen it in New Zealand, Chile, Argentina, and throughout North America as didymo colonizes some of our nicest stream habitats, especially those with cool clear low nutrient waters, sunshine, constant flow, and larger substrates. Quite a change from what we used to think of didymo.

In the early 1990s, I made a special trip to Lake Superior to specifically make my first collection of didymo. I had seen the pictures, seen it arranged on Victorian era slides, and ran into it in sediment cores we were analyzing from Lake Baikal. Didymo was a diatom that every diatomist loved. Big, bold, beautiful, cool shape, and not very common in everyday samples. I had to have it in my collection! And then didymo went bad.

In the 1990s didymo started appearing in places it had never been seen before. And when it appeared, it took control of the new habitat, covering everything in a thick periphytic mat and causing ecological shifts in stream ecology. Didymo was an ecosystem engineer. The centimeters-thick mats were mostly didymo stalk material with live didymo cells lining the periphery and extending into the water column (Fig. 1). The stalks provided habitats for other smaller diatoms, but, deep in the mat, conditions were anoxic, allowing bacteria to grow that could help garner what few nutrients were present in the water to support the mat.

Fig. 1. Didymo is an ecosystem engineer, forming layered mats of cells, stalk material, smaller diatoms, and bacterial communities.

In Minnesota, the global didymo story repeated itself. First seen in one river in 2018, didymo was found in eight more streams by 2025. Where it showed up became predictable. Streams with low nutrients, cool water temperatures, open forest canopy, cobble, boulder, or bedrock substrate, and with lengthy periods of baseflow conditions were suitable for didymo colonization.

We don’t know where it came from, but the streams — long-time destinations for hikers, tourists, and flyfishing anglers — are popular stops. Was it anglers’ felt-soled wading boots? They’ve long been criticized for moving organisms between streams, and in many places felt-soled waders have been banned to prevent the spread of non-native organisms. The source issue is confounded in Minnesota. Didymo has long been known to happily live in nearby Lake Superior. Hmmm, Lake Superior? That’s the same place I went to make my first didymo collection in the 1990s. Molecular barcoding suggests that the populations found in Lake Superior and the North Shore rivers do not significantly differ genetically from each other or from populations found throughout North America. Evidence suggests it’s all the same didymo; we haven’t identified a specific invasive strain.

The other hypothesis that we are exploring is whether environmental conditions on the North Shore have changed to promote didymo colonization or maybe even allow proliferation of what used to be a rare taxon. Are climate-warmed water temperatures slightly up? Are the rivers less flashy? Have we seen shifts in spring runoff or longer growing seasons? Historical discharge records are pointing to shifts in peak runoff. Rather than peak discharge in March-May during spring snowmelt, recent years are seeing peak discharge in November-January from late fall rains and repeated melting of the initial snowpack. Is this the environmental shift that let didymo colonize so many North Shore streams?

We’re still working to come up with a clear answer based on evidence to help guide management and resource protection in the North Shore streams, but the kids have already figured out this didymo problem. Let the rock snotters (Fig. 2) take care of it, they’ll eat anything!

Fig. 2. A rock snotter will eat anything, especially rock snot!

Funding for this project was provided by the Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR), “Didymo II The North Shore Threat Continues”.