Physicochemical Controls on Spatiotemporal Distribution and Benthic Mat Severity of Didymosphenia geminata in Pine Creek, an Unregulated Watershed in Northern Pennsylvania

Didymosphenia geminata (Didymo) is a benthic freshwater diatom that has been globally expanding its range and extracellular stalk production in freshwater ecosystems. Didymo has been observed in reaches downstream of hypolimnetic reservoir releases in the northeastern US since 2007. This study focused on a newly observed (2013) Didymo occurrence in Pine Creek, a highly forested and unregulated watershed in north-central Pennsylvania. Study objectives included comparing contemporary distribution with historical data to provide insight on historical occurrence, quantifying physicochemical controls on Didymo distribution and benthic mat severity, and examining historical changes in water chemistry that might affect habitat suitability. At present, Didymo cellular distribution is limited to upper reaches of Pine Creek where median soluble reactive phosphorus (SRP) is 2.7 µg/L; median SRP was 4.8 µg/L at sites where Didymo was absent. At the epicenter of distribution in Pine Creek where SRP was consistently