Discharge and Diatom Biofilm Communities in the Susquehanna River During the Summers of 2016 - 2018

Elaine Langone Center, Terrace Room

The Susquehanna River is the fourth oldest river in the world and second in the United States. Two branches, the North and West branch converge to form the upper main stem of the Susquehanna River, and the two branches are distinct from each other both chemically and physically. The properties of the West branch and North branch remain distinct in the upper main stem, and we refer to them as the North branch plume (NBP) and the West branch plume (WBP). Since 2009, we have monitored the upper main stem at an established 4 site transect that straddles Byers Island near Shamokin Dam, PA and below the Adam T. Bower inflatable dam at Sunbury, PA. Diatom communities were sampled from cobbles 0.5m below the permanently wetted shore. Diatom frustules were removed from the stones and cleaned for identification by a JEOL 6010 LV scanning electron microscope. Taxa richness ranged from 18 to 36 species per site in 2018. In the previous years (2016 and 2017), taxa richness averaged 35 and 26, respectively. Dominant taxa included Achnanthidum deflexum, Ach. minutissimun, Cocconeis placentula, Discostella stelligera, D. pseudostelligera, Stephanodiscus parvus, and Gomphoneis clevei. Shannon Diversity (SDI) is based on the taxa richness and how evenly they are distributed. Average SDI values ranged from 2.79 (2016) to 2.60 (2017) to 2.15 (2018). Riverine discharge is variable, but can reflect stochastic seasonal trends. The Pollution Tolerance Index (PTI) is a metric related to organic pollution (0 = no organic pollution and 4 – high organic pollution), and specific diatom species have been assigned PTI values. The summer samples from 2016 to 2018, the average PTI metric rose from 2.57 (2016) to 2.70 (2017) to 2.97 (2018). The trends of the past three summers appear to be associated with discharge. The general weather patterns for the summers of 2016-2018 ranged from almost drought conditions in 2016 to the very wet summer of 2018. Discharge averaged from May through July relative to the 80-year average for those months was 48% (2016), 98% (2017), and 174% (2018). Note that during highest taxa richness (35) and SDI (2.79) were during the drought year of 2016. The PTI metric, however, showed a reverse trend. We will explain these results in light of discharge.