Didymosphenia geminata in Maryland Trout Streams: Is Phosphorus Limitation a Necessary Condition for Seasonal Bloom Formation?

Reports of nuisance levels of the stalk-forming benthic diatom Didymosphenia geminata in freshwaters have captured the attention of scientists, resource managers, policy makers, and anglers.  The reasons for D. geminata blooms remain unclear.  Phosphorus limitation may be a contributor to stalk growth and mat development that characterize nuisance blooms.  D. geminata was first documented in Maryland in 2008, in Gunpowder Falls, a Piedmont stream that supports an important tail-water brown trout fishery.  D. geminata blooms have also been documented in three other Maryland trout waters:  Savage River (2009), North Branch Potomac River (2011), and Big Fishing Creek (2012).  Monthly water chemistry data were analyzed to explore a prevailing hypothesis that D. geminata blooms do not form in waters where mean soluble reactive phosphorus (SRP) concentrations over a 24-month period exceed ~2 ppb.  Frequency plots showed that in only 3.1% of 96 monthly samples were SRP concentrations less than or equal to 2 ppb in Gunpowder Falls, and in only 5.2% of the 96 samples in Savage River and North Branch Potomac River.  D. geminata blooms form in Maryland waters where SRP concentrations typically exceed 2 ppb, hence phosphorus limitation does not appear to be a necessary condition for bloom formation.