Environmental constraints on Didymosphenia geminata occurrence and bloom formation in Canadian Rocky Mountain lotic systems

The first documented bloom of Didymosphenia geminata in Alberta occurred in 2003 and subsequent field investigations revealed that D. geminata was present in the periphyton of a number of lotic systems, yet did not always form blooms. We sampled 76 sites in Canadian Rocky Mountain National Parks, chosen to provide ranges in exposure to D. geminata propagules and environmental conditions thought to affect D. geminata growth and bloom formation. Generalized Linear Models were then used to determine environmental constraints on habitat suitability and requirements for blooming and non-blooming populations across the landscape. D. geminata was detected at 88% of sites and of those, 34% had blooms, defined as visible mats of D. geminata stalks. D. geminata cell abundance was significantly negatively related to stream velocity, stream order and TP concentration, yet there was no significant relationship between D. geminata blooms and any of environmental variables measured. There was a significant negative relationship between ash free dry mass (AFDM) and temperature when cells were present but blooms absent, and a positive relationship between temperature and stalk abundance when blooms were present. Total phosphorus, flow velocity and temperature were significant environmental correlates of cell abundance, but we detected no thresholds related to D. geminata blooms. The widespread occurrence of D. geminata over 1000s of km2 and close proximity of sites with and without blooms suggests that local, rather than regional factors drive bloom development.

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