Proceedings of the 2003 Georgia Water Resources Conference, held April 23-24, 2003, at the University of Georgia.Relationships between water chemistry, microbial activity, and hydrology were examined in Chickasawhatchee Creek, a small coastal plain stream in southwestern Georgia. Microbial activity in the creek was determined by measurement of water-column respiration rates and growth-limiting substrates through oxygen consumption experiments. Water chemistry parameters (nitrate-N, soluble reactive phosphate, dissolved organic and inorganic carbon, major cation species) were measured in order to evaluate introduction and removal of various compounds and to identify ground water  contributions.  Results indicated substantial differences in water chemistry between the upstream and downstream reaches that were related to streamflow variation. During low flow periods, the upper reach of the creek was regulated by surface runoff while groundwater inflow played a more important role in the lower reach.  Conversely, during periods of high flow, surface water runoff dictated streamflow for the entire system. There is strong evidence that dissolved organic carbon (DOC) always served as the growth-limiting substrate, and that DOC concentrations varied based on dilution by low-DOC groundwater. However, there was no correlation between measurements of water-column respiration and dissolved organic carbon

Opsahl, Stephen P.

Wheeler, Kit

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HYDROLOGIC CONTROLS ON WATER CHEMISTRY AND MICROBIALACTIVITY IN A SMALL COASTAL PLAIN STREAMKit Wheeler and Stephen P. OpsahlAUTHORS:  J.W. Jones Ecological Research Center, Route 2 Box 2324, Newton, GA 39870.REFERENCE:  Proceedings of the 2003 Georgia Water Resources Conference, held April 23-24, 2003, at the University of Georgia. Kathryn J.Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia.    Abstract.  Relationships between water chemistry,microbial activity, and hydrology were examined inChickasawhatchee Creek, a small coastal plain streamin southwestern Georgia.  Microbial activity in thecreek was determined by measurement of water-columnrespiration rates and growth-limiting substrates throughoxygen consumption experiments.  Water chemistryparameters (nitrate-N, soluble reactive phosphate,dissolved organic and inorganic carbon, major cationspecies) were measured in order to evaluateintroduction and removal of various compounds and toidentify ground water contributions.  Results indicatedsubstantial differences in water chemistry between theupstream and downstream reaches that were related tostreamflow variation.  During low flow periods, theupper reach of the creek was regulated by surfacerunoff while groundwater inflow played a moreimportant role in the lower reach.  Conversely, duringperiods of high flow, surface water runoff dictatedstreamflow for the entire system.  There is strongevidence that dissolved organic carbon (DOC) alwaysserved as the growth-limiting substrate, and that DOCconcentrations varied based on dilution by low-DOCgroundwater. However, there was no correlationbetween measurements of water-column respiration anddissolved organic carbon.INTRODUCTION    Information that distinguishes the effects of ground-water inputs and overland runoff on stream waterchemistry and biological activity is especially lackingin the lower Flint River Basin, where extensiveagricultural development over the past thirty years hasbeen associated with an impairment of water chemistry,particularly when accompanied by loss of ripariancorridors (Lowrance et al., 1983, 1984).  Recentdrought periods have heightened concerns about furtherdegradation of surface- and ground-water chemistry,and clarified the need for research efforts designed toinvestigate the impacts of land use on regional waterresources.    One of the lower Flint River Basin’s most uniquefeatures is the Chickasawhatchee Swamp, which is thesecond largest wetland complex in the state of Georgia(Georgia News, 2000).  Since access to the swamp waslimited until recently, little is known about the role itplays in the regional hydrology, or to what extent itinfluences surface- and ground-water quality insouthwestern Georgia.  Previous studies (Golladay andBattle, 2001, 2002) have identified theChickasawhatchee Swamp as a water quality buffer anda provider of important ecosystem services in theregion.    Three sites along the Chickasawhatchee Creek,which flows south through the swamp, were monitoredmonthly for surface-water chemistry and microbialactivity from the end of 2000 through the end of 2001.The goal of this effort was to examine changes insurface-water chemistry and microbial responseresulting from a wide variety of flow conditions.STUDY SITE    The Chickasawhatchee Creek watershed has acatchment area of 86,800 ha (Golladay and Battle,2002).  Row-crop agriculture and managed forestlandsconstitute the primary land uses, although theChickasawhatchee Creek watershed contains slightlyless (10-13%) agricultural area and slightly more (7%)wetland area than adjacent watersheds (Golladay andBattle, 2002). The Chickasawhatchee Swamp accountsfor most of the wetland area within the watershed.    Low topographic relief in combination with poroussurface geology results in low stream drainage densityand a dominance of subsurface water flow in regionalhydrology (Hicks et al., 1987).  Winter is the primaryseason when most groundwater recharge occurs in theregion.  During typical winters both the regional water-table and streamflow increase in response to extendedrainfall and inundation of riparian areas.  During thesummer months, most precipitation is lost throughevapotranspiration, groundwater recharge is minimal,and base flows in regional streams are largelysupported by discharges from the aquifer (Hicks et al.,1987).METHODS    Monthly monitoring at three stations alongChickasawhatchee Creek was initiated in December2000 (Figure 1).  Stations were located so that theimpact of passage through the ChickasawhatcheeSwamp could be determined.  There was a stationupstream of the swamp complex (CR234), a middlestation at the northern swamp boundary (CR62), and alower station downstream of the swamp (CR37).    At each station, a grab sample was collected justbelow the water surface in well-mixed areas withmeasurable flow.  This single sample was collected inan acid-washed polypropylene bottle, stored on ice, andtransported back to the lab for further processing.Samples were filtered (ashed Whatman 4.25 cm GF/F)and filtrate subsamples were collected to allow forsubsequent analysis of various chemical constituents.Dissolved organic carbon (DOC) and dissolvedinorganic carbon (DIC) were measured using aShimadzu TOC-5050 analyzer.  Nitrate-N (NO3-N) andsoluble reactive phosphate (SRP) samples wereanalyzed using a Lachat QuikChem 8000.Concentrations of major cation species, includingcalcium (Ca), were obtained using a Perkin-Elmer5100PC atomic absorption spectrophotometer.    During each monthly sampling event, water-columnmicrobial respiration rates and growth-limitingsubstrates were measured at each of the creek sites.Although primary production was not measureddirectly, the contribution of photosynthetic respirationwas probably minimal in this system.  SamplingFigure 1. Study site location in southwest Georgia.locations were low-order, shaded, turbid, surrounded byintact riparian vegetation, and had high dissolvedorganic carbon concentrations (range of 1.87 to 19.4with a mean of 8.5 mg/L).  All of these conditions havebeen shown to favor heterotrophic, rather thanautotrophic, processing (Minshall et al., 1983, Websteret al., 1995).  An acid-washed 10-L nalgene carboy wasfilled at each station, and water from these carboys wasused to fill up a series of 60-mL glass BOD bottles ateach site.  Four bottles from each site were fixed in thefield using MnSO4 and NaI/NaOH to scavenge theavailable O2 and served as T0 samples.  Four nutrientsolutions (NH4Cl, NaNO3, Na2HPO4, and Glucose)were used as the limiting substrates.  Bottles wereamended with one of the four solutions in the field.After field processing was complete, the headspace ineach bottle was filled with water, covered to minimizethe introduction of atmospheric oxygen to the samples,and incubated at room temperature (~20°C).  Thebottles were also kept in the dark to eliminate thepossibility of photosynthetic oxygen production.  Aftera period between 4 and 10 hours had elapsed, a secondset of four bottles was fixed to serve as the T1 samples.Reagents were added to bottles used for nutrientadditions approximately 24 hours after the T0 sampleshad been fixed.  Winkler titrations were performed onall of the samples using a Mettler-Toledo DL50Graphix.RESULTS AND DISCUSSION    There were substantial differences in surface-waterchemistry between the upstream and downstreamreaches of Chickasawhatchee Creek, and thesedifferences were related to variable hydrologicconditions.  Streamflow was highest during late Marchand early April following spring rains (Figure 2).Summer rains were much less frequent than normal,and as a result, there was low (<20 cfs) streamflow formuch of the time between mid-July and late November.    There was strong evidence of groundwater inputs inthe downstream reach of the creek, particularly duringlow-flow conditions.  Calcium and DIC are both knownto be high in groundwater from the Upper Floridanaquifer (Hicks et al., 1987) relative to surface runoff,and downstream levels of both parameters exceededlevels reported upstream (Table 1).  Measurements ofNO3-N provided additional evidence of groundwaterinflow because groundwater is known to have higherNO3-N concentrations (1,000-3,000 µg/L) than creeksand wetlands ( 1000 µg/L) (Hicks et al., 1987).During periods of reduced streamflow, NO3-NCHICKASAWHATCHEEWMA  BOUNDARYCHICKASAWHATCHEECREEKNCR 62CR 37CR 234AREA OFINTEREST06001200180012/01/00 03/11/01 06/19/01 09/27/01 01/05/02Daily Mean Streamflow (cfs)0123Daily Precipitation (in.)QPrecipFigure 2.  Daily mean streamflow values forChickasawhatchee Creek, along with dailyprecipitation measurements.  Streamflow data arefrom U.S. Geological Survey (USGS) Gage 02354500on Chickasawhatchee Creek.  Rainfall data are fromthe Georgia Automated Environmental MonitoringNetwork weather station in Dawson, GA.concentrations were higher at the downstream stationthan at either of the upstream stations (Figure 3A).    Nutrient dynamics were different in the two reachesof Chickasawhatchee Creek and varied withstreamflow.  Looking over the entire creek stretchcovered in this study, concentrations of SRP werereduced as the creek flows south (Figure 3B).  In mostinstances, the majority of the removal occurred in theupstream reach of the creek between CR234 and CR62.Elevated levels of SRP in the creek were probably dueto the presence of an upstream wastewater treatmentplant.  The upstream reach also appeared to have NO3-N removal capacity as concentrations were alwayslower at CR62 than at CR234 (Figure 3A).  However,NO3-N concentrations at CR37 were higher than thosefrom upstream stations during more than half of thesampling events.  The results suggested that baseflowthrough the swamp was more reliant on groundwaterinflows than other runoff, particularly during low-flowconditions.  It was only during the highest flow periodsthat NO3-N concentrations decreased over the entirecreek reach.  Removal of both SRP and NO3-N waslowest during high-flow periods.Table 1.  Calcium and dissolved inorganic carbon(DIC) measurements in Chickasawhatchee CreekReported concentrations represent the mean of all samplingevents.  Standard deviations are given in parentheses.CR234 CR62 CR37[DIC] (mg/L) 16.30 (3.68) 18.01 (3.95) 30.59 (4.92)[Ca] (mg/L) 17.22 (2.16) 20.08 (3.00) 44.43 (6.07)0200400600800100012/10/00 3/10/01 6/8/01 9/6/01 12/5/01NO3-N (ug N/L) CR234CR62CR37A030609012/10/00 3/10/01 6/8/01 9/6/01 12/5/01SRP (ug P/L)CR234CR62CR37BFigure 3. Measured concentrations of nitrate-N(NO3-N) (3A) and soluble reactive phosphorus(SRP) (3B) in Chickasawhatchee Creek.    Nutrient removal in streams is typicallyaccomplished by a combination of processes includingbiological uptake, sedimentation, and adsorption.  Onepossible explanation for the decrease in nutrientconcentrations in the upstream reach of the creek wouldbe dilution by groundwater from the Upper Floridanaquifer.  However, if there were substantial ground-water inputs in this reach of the creek, NO3-N andcalcium concentrations would be significantly higher atCR62 than at CR234 (Figure 3A; Table 1).  Therefore,groundwater dilution appeared to play a minimal role inreducing nutrient concentrations between CR234 andCR62.  In contrast, observed increases in NO3-N andcalcium levels between CR62 and CR37 indicate thatthere are significant groundwater inputs to thedownstream reach, and that during dry periods theseinputs make up a greater relative percentage ofstreamflow.  Therefore, as the creek flowed through theswamp between CR62 and CR37, nutrient reductionwas at least in part due to dilution.  However, it is likelythat other mechanisms including biological uptake andabiotic removal were also actively involved in nutrientcycling.    Swamp hydrology likely played a role in governingwater-column microbial activity in ChickasawhatcheeCreek.  During wet, high-flow periods, overland runoffdominated streamflow for the entire system.  DOCconcentrations increased as water moved through thecomplex and the swamp effectively exported organiccarbon downstream (Figure 4A).  As it became drierover the summer months and streamflow decreased,groundwater inputs became increasingly influential inthe southern reach of the creek, while the northern04812162012/01/00 03/11/01 06/19/01 09/27/01 01/05/02DOC (mg/L)CR234CR62CR37A012305/09/01 07/18/01 09/26/01 12/05/01Resp. Rate (uM O2/hr) CR234CR62CR37BFigure 4. Measured concentrations of dissolvedorganic carbon (DOC) (4A) and water-columnrespiration rates (4B) in Chickasawhatchee Creek.reach was still regulated by surface runoff.  Regardlessof streamflow conditions, organic carbon was alwaysfound to be the growth-limiting substrate in the creek.However, our measurements of water-columnrespiration (Figure 4B) did not correlate well with DOCconcentrations (r2=0.0363, p<0.05).  This indicates thatmicrobial respiration is not dependent on theconcentration of DOC, but rather its bioavailability.  Itshould be noted that in this study, we did not assessbenthic respiration, which has been shown to be asignificant portion of overall microbial respiration instreams (Edwards et al., 1990, Webster et al., 1995).  Itis possible that benthic communities respond morestrongly than those in the water column to large inputsof organic matter.CONCLUSIONS    The role that the Chickasawhatchee Swamp plays inregional hydrology is only beginning to be understood.Areas just upstream of the swamp complex have thecapability to reduce nutrient levels, which could help tominimize downstream enrichment problems.  Passagethrough the swamp does not have much of an impact onSRP concentrations, but can serve to increase NO3-Nconcentrations, especially during dry, low-flowconditions, through groundwater inputs.  Swamp-derived organic carbon export during elevatedstreamflow conditions is more biologically availableand likely contributes to downstream aquaticproductivity.  Further studies addressing the role ofbenthic metabolism in overall ecosystem productionwould provide more complete insight to carbon andnitrogen cycling in the swamp. Overall, anunderstanding of the surface water/groundwaterinteractions and associated water quality improvementsis necessary to ensure appropriate wetland protectionmeasures are taken, and so that comprehensive regionalwater resource plans can be developed.ACKNOWLEDGMENTS    A. Liner, R. Lane, J. Jenkins, N. Peneff, T.Humphries, J. Warren, and B. Penhallegon assistedwith the laboratory and fieldwork.  The R.W. WoodruffFoundation and J.W. Jones Ecological Research Centerprovided funding for the research.LITERATURE CITEDEdwards, R.T., J.L. Meyer, and S.E.G. Findlay, 1990.  Therelative contribution of benthic and suspended bacteria tosystem biomass, production, and metabolism in a low-gradient blackwater river. Journal of the North AmericanBenthological Society 9:216-228.Georgia News, 2000.  Historic purchase protects vital waterresources.  The Nature Conservancy of Georgia. Winter2000.  p.4-5.Golladay, S.W., and J. Battle, 2001.  Does theChickasawhatchee Swamp influence water quality?Proceedings of the Georgia Water Resources Conference,2001.  pp. 337-340.Golladay, S.W., and J. Battle, 2002.  Effects of flooding anddrought on water quality in gulf coastal plain streams inGeorgia.  Journal of Environmental Quality 31:1266-1272.Hicks, D.W., H.E. Gill, and S.A. Longsworth, 1987.Hydrogeology, chemical quality, and availability ofgroundwater in the Upper Floridan Aquifer, Albany Area,Georgia.  USGS Water-Resources Investigations Report87-4145, Atlanta.Lowrance, R., R.L. Todd, and L.E. Asmussen, 1983.Waterborne nutrient budgets for the riparian zone of anagricultural watershed.  Agriculture, Ecosystems &Environment 10:371-384.Lowrance, R., R. Todd, J. Fail Jr., O. Hendrickson Jr., R.Leonard, and L. Asmussen, 1984.  Riparian forests asnutrient filters in agricultural watersheds. Bioscience 34:374-377.Minshall, G.W., R.C. Petersen, K.W. Cummins, T.L. Bott,J.R. Sedell, C.E. Cushing, and R.L. Vannote, 1983.Interbiome comparison of stream ecosystem dynamics.Ecological Monographs  53:1-25.Webster, J.R., J.B. Wallace, and E.F. Benfield, 1995.Organic processes in streams of the eastern United States.In: C.E. Cushing, K.W. Cummins, and G.W. Minshall(eds.) River and Stream Ecosystems. Elsevier, New York.

Hydrologic Controls on Water Chemistry and Microbial Activity in a Small Coastal Plain Stream

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Hydrologic Controls on Water Chemistry and Microbial Activity in a Small Coastal Plain Stream

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