Coupling Source Water Chemistry to Microbiological Processes

Research Team: Christopher Wheeler (Research Technician III) and Dr. Stephen P. Opsahl (Assistant Scientist)

Beginning late in 2000, relationships between water chemistry and microbial activity were examined in Chickasawhatchee Creek, a small coastal plain stream in southwestern Georgia. Microbial activity in the creek was gauged by determination of respiration rates and growth-limiting substrates through oxygen consumption experiments. Important water chemistry parameters (nitrate, phosphate, ammonium, dissolved organic and inorganic carbon, major cation and anion species) were also measured in order to evaluate introduction and removal of various compounds and to identify groundwater contributions from the Upper Floridan aquifer.

As the creek flows south through the Chickasawhatchee Swamp, concentrations of both nitrate (NO₃) and phosphate (PO₄) are reduced. In the northern stretch of the creek, PO₄ reduction averages 20 mg/L, while NO₃ removal typically averages 150 mg/L. Nutrient removal in the northern reach of the creek is accomplished by a combination of processes including biological uptake, groundwater outflow, sedimentation, and adsorption. The southern reach of Chickasawhatchee Creek is quite different. Average PO₄ reduction is only about 2 mg/L, while NO₃ concentrations usually increase, in some cases by as much as 900 mg/L. The increase in measured NO₃ levels in this stretch indicates that there are significant groundwater inputs and that nutrient reduction in this stretch of the creek is in part due to dilution. However, it is likely that other removal mechanisms are still at work and probably dependent on stream characteristics like availability of dissolved organic carbon, oxygen levels in the water column and in the water-soil interface, and the degree of groundwater/surface water interaction.

Coupling Source Water Chemistry to Microbiological Processes During periods of high flow, surface water runoff dictates streamflow for the entire system. Dissolved organic carbon (DOC) concentrations increase as you move downstream and the swamp actually serves to export labile organic carbon. As it becomes drier over the summer months and streamflow decreases, groundwater inputs become increasingly influential in the southern reach of the creek, while the northern reach is still regulated by surface runoff. During these periods of low flow, the swamp acts as a sink for organic carbon. Since carbon always serves as the growth-limiting substrate in the creek, its dynamics have a profound effect on microbial activity. Measurements of community respiration over times of reduced streamflow show the lowest levels of microbial activity (0.12 mM O₂/hr) at sites where there has been physical dilution by low-carbon water.

The hydrologic regime, consisting of high flow periods regulated by surface runoff and low flow periods influenced by groundwater inputs from the Upper Floridan aquifer, plays a strong role in governing both water chemistry and microbial activity in Chickasawhatchee Creek as it flows through the Chickasawhatchee Swamp.

Additional Information:

Wheeler, K. and S. P. Opsahl. (in review). Hydrologic Controls on Water Chemistry and Microbial Activity in a Small Coastal Plain Stream. In: Proceedings of the 2003 Georgia Water Resources Conference. K. J. Hatcher, editor. Institute of Ecology, The University of Georgia, Athens, Georgia.

Funded by: The Joseph W. Jones Ecological Research Center