A study of diatom communities in the upper main stem of the Susquehanna River during various late-summer discharge regimes, 2014-2016

The upper main stem of the Susquehanna River is formed by the confluence of the West (WBP) and North Branches (NBP); each chemically and physically distinctive. Characterization of the diatom communities requires samples taken from sites representative of both branches, which was done at an established transect straddling Byers Island near Shamokin Dam, PA. From 2014-2016 the upper main stem experienced low flow regimes in September but different regimes in August: high (2014), moderate (2015), and low (2016). Diatom biofilm samples were taken from stones from both sites and examined by scanning electron microscopy. Various indices showed little variation between the sites. We found the greatest species richness in the diatom communities during 2016, while years 2014 and 2015 were somewhat comparable. Despite similarities in community metrics, the Proportional Bray-Curtis similarity values between the three years was 62% for WBP, 32% for NBP, and 49% between WBP and NBP. Taxa richness was highest with low discharge, but there was no clear relationship with the other indices