Effects of Low versus High Freshwater Discharge on the Concentration and Character of Dissolved Carbon, and on the Microbial Community Composition in the Skidaway River Estuary (Southeast USA).

Thais Bittar1, Stella Angela Berger2, Tina L Walters3, Elizabeth L Mann4, Robert G Spencer5, Aron Stubbins6, Marc Emil Frischer7 and Jay A. Brandes7, (1)Skidaway Institute of Oceanography, University of Georgia, Marine Sciences Department, Savannah, GA, United States, (2)Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Dep. 3, Experimental Limnology, Stechlin/OT Neuglobsow, Germany, (3)University of Georgia Skidaway Institute of Oceanography, Savannah, GA, United States, (4)Skidaway Institute of Oceanography, Savannah, GA, United States, current address: 34 Old Marlboro Road, Maynard MA 01754, (5)Florida State University, Department of Earth, Ocean and Atmospheric Science, Tallahassee, FL, United States, (6)Skidaway Institute of Oceanography, University of Georgia, Savannah, GA, United States, (7)University of Georgia, Skidaway Institute of Oceanography, Savannah, GA, United States
The 2012-2013 timeframe encompassed contrasting levels of precipitation in the Southeast USA, which resulted in low freshwater discharge into coastal waters in 2012, and high freshwater discharge in 2013. In the tidally-dominated Skidaway River Estuary, salinity was 32 ppt during summer 2012, and decreased to 25 ppt in summer 2013, at high tide. This shift in freshwater inputs and salinity affected the concentration and character of dissolved inorganic and organic carbon (DIC and DOC), and the composition of the picophytoplankton and bacterioplankton communities in the estuary. DIC concentrations were close to equilibrium throughout 2012, and decreased with freshwater discharge in 2013. DIC isotopes (δ13C-DIC) shifted towards a more depleted state in summer 2012, presumably due to respiration of isotopically depleted DOC, and throughout 2013, possibly due to a combination of respiration and freshwater inputs. During low discharge in 2012, DOC appeared enriched in autochthonous material, based on high N-content and protein-like fluorescence. High freshwater discharge led to higher DOC concentrations in 2013, and shifted DOC towards a more chromophoric, aromatic, humic and N-depleted character, indicating contributions from terrestrially-derived DOC. The picophytoplankton bloom in summer 2012 was co-dominated by two groups of cyanobacteria, including a high-salinity Synechococcus. In contrast, the 2013 summer bloom was dominated by a single group, as Synechococcus was absent, possibly due to lower salinities. Two groups of bacteria were distinguished by flow cytometry based on intensity of DNA fluorescence (low-FL and high-FL). In 2012, bacterioplankton was dominated by the low-FL group, while in 2013 the two groups comprised similar fractions of the community. These shifts in bacterioplankton community composition could be associated with changes in salinity and/or in DOC character.