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Title: Interannual drought length governs dissolved organic carbon dynamics in blackwater rivers of the western upper Suwannee River basin

Author
item MEHRING, ANDREW - University Of Georgia
item Lowrance, Robert
item HELTON, ASHLEY - University Of Georgia
item PRINGLE, CATHY - University Of Georgia
item THOMPSON, AARON - University Of Georgia
item Bosch, David - Dave
item VELLIDIS, GEORGE - University Of Georgia

Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2013
Publication Date: 12/1/2013
Citation: Mehring, A.S., Lowrance, R.R., Helton, A., Pringle, C., Thompson, A., Bosch, D.D., Vellidis, G. 2013. Interannual drought length governs dissolved organic carbon dynamics in blackwater rivers of the western upper Suwannee River basin. Journal of Geophysical Research. 118:1-10. doi:10.1002/2013JG002415,2013.

Interpretive Summary: Climate change is expected to alter future temperature and rainfall in the southeastern U.S. In this study, we used long-term watershed data from Little River, in south Georgia to determine potential effects of climate change that might include higher temperatures and lower rainfall (i.e. more frequent drought) on the transport of dissolved organic carbon (DOC). DOC is important because blackwater rivers such as Little River provide carbon transport to coastal waters which in turn can serve as an energy source for organisms in coastal waters. The most obvious effects of increased drought frequency would be decreased transport of DOC due to decreased stream flow. A less obvious impact is the potential for increased rates of DOC decomposition by bacteria when the DOC concentrations are higher during droughts. Thus decreased transport could be due to both lower flow and to enhanced breakdown of DOC to carbon dioxide. These results indicate that the effects of climate change on DOC transport could be substantial if drought and no-flow periods continue to lengthen. The study also showed the value of long-term watershed data in providing a basis to assess the potential effects of climate change induced alterations to future stream flow regimes.

Technical Abstract: Rivers play important roles in global carbon cycling through carbon mineralization and transport to coastal areas. Climate-change-induced alterations to river discharge may have large implications for riverine carbon cycling. We assessed impacts of drought on carbon transport and mineralization by coupling long-term datasets with laboratory experimentation for the Little River (LR) in southern Georgia, USA. The LR has experienced lengthening droughts since monitoring began in 1972. The composition of DOC appeared to be stable despite dramatic hydrologic changes over two years: optical analyses suggested that LR DOC is dominated by terrigenous DOC (tDOC) and primarily composed of three terrestrial humic-like fluorescence groups that do not vary across changes in temperature, discharge or oxygen concentration. However, seven years of data show that annual DOC transport decreased by eight metric tons for each one-day increase in drought length, and that DOC concentration ([DOC]) in a hydroperiod is predictable by DOC exported in the previous hydroperiod. After years with short hydroperiods, resulting in less DOC exported, [DOC] was significantly higher. Also, [DOC] within hydroperiods increased at high temperature and low discharge. Chamber incubations indicated that bacteria consume more DOC and release more CO2 when [DOC] is high. Our findings suggest river carbon cycling will be significantly altered as droughts intensify, temperatures rise, and discharge decreases, resulting in reduced DOC transport to coastal waters.