Effect of discharge and habitat type on the occurrence and severity of Didymosphenia geminata mats in the Restigouche River, eastern Canada

Since 2006, the Restigouche River watershed, eastern Canada, has been affected by nuisance growths of the mat‐forming diatom, Didymosphenia geminata . In 2010, in view of the potential impacts of this alga on the local Atlantic salmon fishery, we created a volunteer monitoring network to assess D .  geminata mat severity within the watershed. Over the course of 6 monitoring summers, more than 1,200 observations of D .  geminata mat severity were reported in 20 subwatersheds of the Restigouche River basin. Observations were mapped to illustrate the yearly severity of D .  geminata mats throughout the watershed. Metrics were then extracted from this dataset to assess the spatial and temporal variability of mat severity. At the reach scale, D .  geminata occurrence was predominantly found in riffles compared to any other river habitat type. At the watershed scale, a two‐sample Kolmogorov–Smirnov test highlighted a significant effect of maximum spring discharge on mean annual D .  geminata mat severity, indicating that when maximum spring discharge is high, severity of D .  geminata mats in the following months is significantly lower. Additionally, maximum spring discharge explained 71% of the variability in annual mat severity. This study contributes to the understanding of mat severity dynamics and illustrates the value of volunteer monitoring networks for studying complex ecosystem dynamics.