A Study of Diatom Communities of the Upper Main Stem of the Susquehanna River during Summer 2017

The upper main stem of the Susquehanna River is formed by the confluence of the West and North Branch, both of which are chemically and physically distinctive. The upper main stem retains the signatures of the two branches due to weak lateral mixing, and we refer to them as the West Branch plume (WBP) and the North Branch plume (NBP). Thus, characterization of the diatom communities requires samples taken from sites that occur in the plumes of both branches. Since 2009, we have monitored the upper main stem at an established transect that straddles Byers Island near Shamokin Dam, PA and below the Adam T. Bower inflatable dam at Sunbury, PA. Attached diatom communities were sampled from stones which were prepared for examination by scanning electron microscopy. The Pollution Tolerance Index (PTI) and Shannon Diversity Index (SDI) values showed very little variation between all sites (2.38-3.03 and 2.35-2.93). We found the greatest species richness in the diatom communities of the NBP (Site 3- 30; Site 4- 29). Proportional Bray-Curtis Similarity analyses of samples showed low to moderate overlap between the diatom communities. Across all sites we identified 57 different species. Habitats of the NBP were dominated by Discostella pseudostelligera (a small centric), Rhoicosphenia abbreviata (a biraphid), and Achnanthidium minutissium (a small monoraphid). Similar habitats of the WBP were dominated by Ach. minutissium, Encyonema appalachianum (a biraphid). Despite similarities between 2014 and 2017 regarding discharge, the diatom communities were depauperate this year (57 taxa; 104.470 cfs) compared to other high discharge summers (June, July, and August) especially 2014 (93 taxa; 80,590 cfs). Difference in taxa richness could possibly be explained by lingering effects of a prior low discharge year (2016- 36,562 cfs)