Didymosphenia geminata (Didymo) is a freshwater diatom that forms thick benthic mats (i.e., blooms) in cold,
oligotrophic rivers, disrupting benthic processes and altering aquatic ecosystems. The St. Marys River, a critical
hydrological connection between Lake Superior and Lake Huron, has experienced increasing Didymo
proliferation, raising concerns about its ecological impacts within the Laurentian Great Lakes. This study
examines the seasonal variation (May–December 2024) of Didymo biomass (ash-free dry mass, AFDM, g/cm²)
and cell density (cells/cm²), as well as the relationship between Didymo metrics (biomass and cell density) and
nutrient parameters, along with potential shifts in macroinvertebrate assemblages in the Main Rapids of the St.
Marys River. Methods included weekly collection of water samples for nutrient analysis (e.g., TN, TP, SRP,
NOx, and NH4), while Didymo samples were collected biweekly by scraping rocks to assess biomass and cell
densities. Macroinvertebrates were collected biweekly using a Hess sampler to assess community composition
and potential shifts in response to Didymo blooms. Linear regressions revealed no significant relationships
between nutrient concentrations and biomass (R² <0.16, p >0.05). Biomass and cell densities also were not
correlated (R² = 0.089, p = 0.24). These findings suggest Didymo blooms in the St. Marys River are driven by
multiple factors, and that nutrients may not be the strongest driver in the St. Marys River. Seasonal flow
dynamics, light availability, and substrate composition likely play key roles. Ongoing macroinvertebrate
identification will provide further insight into Didymo’s ecological effects in an area that supports a key
recreational fishery. A comprehensive understanding of these drivers is crucial for developing effective
management strategies aimed at mitigating Didymo impacts and preserving the ecological integrity of the
Laurentian Great Lakes region.