A Study of the Diatom Communities in the Upper Main Stem of the Susquehanna River During an Unusually Wet Summer

The upper main stem of the Susquehanna River is formed by the confluence of the West and North Branches, each of which is 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 North Branch plume (NBP) and West Branch plume (WBP). Thus, characterization of the diatom communities required samples taken from sites that occur in the plumes of both branches. Since 2009 when our river monitoring program began on the upper main stem, the summer of 2015 was an unusually wet year. Furthermore, monthly average discharge levels of the river during July exceeded those of any year during the 77 years of USGS records for the upper main stem. Indeed, the high water required that we reduce the scope of active sampling to one set of collections at each site together with artificial substrates. Our samples were taken from an established transect that straddles Byers Island near Shamokin Dam, PA and below the Adam T. Bower inflatable dam at Sunbury, PA. Diatom samples from diatometers, stones and other substrates were prepared for examination by electron microscopy. Within the plumes of the two branches, we focused on two particular habitats inhabited by diatom communities: stone, and diatometer. Overall, we identified approximately 93 different species in this study. Taxa richness in WBP (sites 1 and 5) was relatively low 16-24 and 19 species in the stone (16-24) and diatometer (19) communities, respectively. Similar communities from NBP (sites 2, 3 and 4) were richer and more diverse (e.g. 29-48 taxa on stones and 22-34 taxa on diatometers. Habitats of the NBP were dominated by a small centric and biraphid species (e.g. Discostella pseudostelligera and Rhoicosphenia abbreviata ) and similar habitats of the WBP were dominated by monoraphid species (e.g. Achnanthidium deflexum, Achananthidium minutissium, and Cocconeis placentula). Our preliminary results suggest that the high levels of discharge through the summer until we made our collections on July 27 contributed to differences in diatom communities between substrates