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LONGITUDINAL AND SEASONAL VARIATIONS IN AMPLITUDE AND PHASE OF DIEL CARBONATE CYCLING IN CLEAR, SPRING-FED RIVERS
By
CAROLYN BALL
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2012
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© 2012 Carolyn Ball
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To my family and friends
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ACKNOWLEDGMENTS
I thank my advisor Dr. Jon Martin and the members of my committee, Dr. Matt
Cohen and Dr. Mark Brenner for offering their insight and advice. I acknowledge helpful
discussion with fellow graduate students at the Department of Geological Sciences,
University of Florida: Marie Kurz, Chad Foster, Bobby Hensley, Amy Brown, Kelly
Deuerling, John Ezell, Mitra Khadka, Pati Spellman, and Jason Gulley. Support for the
project has come from the National Science Foundation through grants: EAR0853956
and EAR0910794. Most importantly, I thank my fiancé and my parents for their support
and encouragement.
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TABLE OF CONTENTS
ACKNOWLEDGMENTS .................................................................................................. 4
LIST OF TABLES ............................................................................................................ 7
LIST OF FIGURES .......................................................................................................... 8
LIST OF ABBREVIATIONS ............................................................................................. 9
CHAPTER
1 INTRODUCTION .................................................................................................... 12
Biogeochemical Cycling .......................................................................................... 12 Transient Storage ................................................................................................... 13
Light Limitations ...................................................................................................... 13 Hypothesis .............................................................................................................. 14
2 STUDY SITE ........................................................................................................... 16
Geology and Hydrogeology .................................................................................... 16 Hydrology ................................................................................................................ 17
Vegetation ............................................................................................................... 18
3 METHODS .............................................................................................................. 21
Sites ........................................................................................................................ 21 Field Sampling and Analytical Methods .................................................................. 21
Model Estimates and Diel Cycling .......................................................................... 24 Flow Weighted Residence Time ............................................................................. 24
4 RESULTS ............................................................................................................... 28
Flow Weighted Residence Time ............................................................................. 28 Diel Cycles .............................................................................................................. 28
5 DISCUSSION ......................................................................................................... 37
Estimates of Residence Time ................................................................................. 37
Seasonal Effects of Diel Cycles .............................................................................. 39 Residence Time Controls on Diel Cycles ................................................................ 40 Longitudinal Variations in Ca2+ Concentrations and Carbonate Mineral
Diagenesis ........................................................................................................... 42
6 CONCLUSION ........................................................................................................ 47
LIST OF REFERENCES ............................................................................................... 48
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BIOGRAPHICAL SKETCH ............................................................................................ 53
7
LIST OF TABLES
Table page 4-1 Flow weighted residence time results ................................................................. 31
4-2 pH cross correlations .......................................................................................... 36
4-3 Ca2+ cross correlations ....................................................................................... 36
5-1 Comparison of estimated Ca2+ lost to precipitation at the monitoring sites. ........ 46
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LIST OF FIGURES
Figure page 1-1 Diagram showing key diel biogeochemical processes affecting aqueous
chemistry of streams. ......................................................................................... 15
2-1 Location maps of the Santa Fe and Ichetucknee rivers ...................................... 20
3-1 Conceptual figure for flow weighted residence time calculation.......................... 27
4-1 Ratio of cumulative spring discharge to river discharge ..................................... 31
4-2 Time-series measurements ................................................................................ 32
4-3 A. Winter and B. summer diel cycles .................................................................. 33
4-4 Lag time from solar radiation versus flow weighted residence time. ................... 34
4-5 Average values and amplitudes .......................................................................... 35
5-1 Conceptual figure for the proposed processes controlling longitudinal variations in diel biogeochemical cycling with increasing residence time ........... 45
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LIST OF ABBREVIATIONS
AS Area of nth segment AT Area at sampling site LS Length of nth segment QS Discharge at nth segment QT Total discharge at sampling site] ƮS Residence time of nth segment ƮS,S Residence time of nth segment, occupying discharge from nth spring ƮT Flow weighted residence time VS Volume of nth segment, occupying discharge from nth spring
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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
LONGITUDINAL AND SEASONAL VARIATIONS IN AMPLITUDE AND PHASE OF
DIEL CARBONATE CYCLING IN CLEAR, SPRING-FED RIVERS
By
Carolyn Elizabeth Ball
August 2012
Chair: Jonathan B. Martin Major: Geology
Photosynthesis and respiration cause diel cycles in water variables of karst river
systems, including dissolved oxygen, pH, and the equilibrium state of calcite. Four 28-
48-hr water-sampling surveys were completed at three locations on the Ichetucknee
and Lower Santa Fe rivers, north-central Florida to understand controls on downstream
variations in these diel cycles. Diel cycles of water chemistry at different locations
downstream of the headwaters of spring-fed streams were compared using calculated
flow-weighted residence times. Diel cycles of carbonate-related variables pH, Ca2+ and
DIC concentrations, alkalinity, δ13CDIC, and SIcalcite increasingly lagged solar radiation,
which drives the cycles, with increasing residence time (i.e. distance downstream).
Amplitudes of cycles also decreased with increasing residence time. Assuming that diel
cycling of Ca2+ is related to calcite precipitation, the amount of Ca2+ lost to calcite
precipitation decreased by about 0.32 mM/day after 9 hr of water travel time. The
increased lag and decreased amplitude could be controlled by at least three processes,
including downstream diagenetic reactions as flow is retarded by transient storage,
limitation of biological productivity with decreased light availability, and downstream
accumulation of reaction products of biological metabolism. The primary process
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appears to be asynchronous accumulation of metabolic reaction products as water
flows downstream. The longitudinal variations in amplitude and phase in diel cycles
indicate that timing and location of water quality measurements need to be considered
for long-term monitoring schedules designed to estimate fluxes of materials through
watersheds.
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CHAPTER 1 INTRODUCTION
Biogeochemical Cycling
Biogeochemical processes, such as photosynthesis and respiration, affect the
chemical composition of streams at diel frequencies as a consequence of changes in
sunlight and air temperature (Falkowski and Raven, 1997; Neal et al., 2002; Drysdale et
al., 2003; Parker et al., 2007; Nimick et al., 2010) (Figure 1-1). During the day,
photosynthesis consumes CO2 and produces O2, whereas respiration consumes O2 and
yields CO2. Only respiration operates by night, consuming O2 and producing CO2
(Odum, 1956; Simonsen et al., 1978; Aucour et al., 1999; Clarke, 2002; Parker et al.,
2007). Changes in CO2 concentrations control pH, resulting in associated diel variations
in the saturation states of minerals such as calcite (SIcalcite) (Spiro and Pentecost, 1991;
Hartley et al., 1996; Guasch et al., 1998; Cicerone et al., 1999; de Montety et al., 2011).
Diel changes in dissolved oxygen (DO) concentrations control the redox potential of the
water and thus concentrations of redox-sensitive elements such as Fe (Stumm and
Morgan, 1996; Loperfido et al., 2009; Nimick et al., 2010; Kurz, in review). Therefore,
plant metabolism ultimately affects the chemical composition of stream water, the
frequency of compositional variations, and equilibrium between stream water and
mineral phases, particularly soluble minerals such as calcite (Findlay, 1995). Although
these diel variations ultimately stem from variations in solar radiation, not all cycles are
in phase with solar radiation and the magnitude of chemical diel cycles may vary with
longitudinal distance along the river channel. Diel variations of water chemical
composition may also vary seasonally along stream channels because daylight hours
and primary production are lower in winter and higher in summer.
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Transient Storage
Longitudinal variations in magnitudes and phases of diel cycles may relate to the
amount of time required for biological processes to alter stream water chemistry, which
can be represented by the average residence time of water in the stream channel.
Residence time is controlled by river velocity, channel size, and transient storage
(Bencala and Walter, 1983). Transient storage occurs where flow is stagnant relative to
the flow in the main channel, for example where sub-aquatic plants increase bottom
roughness and within the hyporheic zone of the stream-bed sediments. Transient
storage would impact concentrations of bio-reactive elements by increasing the amount
of time for biogeochemical reactions. A common reaction that may occur in the
transient storage zone is remineralization of organic carbon (Boulton et al., 1998;
Findlay, 1995), which increases concentrations of CO2, thereby lowering pH and
reducing the saturation state of carbonate minerals.
Light Limitations
Light limitation diminishes photosynthesis by sub-aquatic vegetation and thus may
contribute to changes in the magnitude of diel signals along the length of stream
channels. Light limitation can result from riparian plants that shade the stream. Light
limitation may also result from high concentrations of dissolved color (i.e. dissolved
organic carbon[DOC]) and inorganic turbidity. Turbidity should increase downstream as
increased amounts of fine-grained sediment are entrained in the water column. Thus,
increased turbidity should correlate roughly with residence time (Brown and Ritter,
1986; Lenhart et al., 2010).
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Hypothesis
Variations in diel cycles of dissolved solutes longitudinally along a stream could
reflect the amount of carbonate minerals that precipitate or dissolve within a stream
channel. Dissolution and precipitation may alter channel morphology (Pentecost, 1992)
and play a role in the global carbon cycle (Berner et al., 1983; Oki, 1999; Aucour et al.,
1999; Brunet et al., 2005). Because of diel cycling, synoptic monitoring schemes for
stream water chemistry are affected by the timing of sampling, and if longitudinal
variations are large, the location of sampling sites along the stream length will also
affect sampling results. Differences in diel cycling with distance downstream have
major implications for estimates of whole-stream metabolism using diel cycling of
metabolic products such as DO and NO3- concentrations (Odum, 1956; Heffernan and
Cohen, 2010). This study focuses on how diel cycles vary downstream by comparison
of diel cycles and estimates of water residence time in the stream channel.
Understanding how diel cycles vary spatially and relate to residence time may improve
the understanding of controls on the diel cycling of stream water chemistry and thus the
ultimate composition of stream discharge. This work focuses primarily on the diel
variations in factors related to carbonate mineral diagenesis in two rivers, the
Ichetucknee and the Lower Santa Fe, that flow across carbonate terrains in north-
central Florida.
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CO2 O2
P > R R > P
CO2 O2pH
DO
Twater
pH
DO
Twater
ET
Tair
ETTair
Potential Carbonate Precipitation Potential Carbonate Dissolution
Figure 1-1. Diagram showing key diel biogeochemical processes affecting aqueous
chemistry of streams with neutral to alkaline pH. During the day photosynthesis (P) is a more important process than respiration (R), and at night the opposite is true, which alter CO2 and O2 concentrations at diel frequencies (modified from Nimich et al., 2010).
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CHAPTER 2 STUDY SITE
Geology and Hydrogeology
The Ichetucknee and Lower Santa Fe rivers are located in north-central Florida,
USA (Figure 2-1), which is underlain by the carbonate Floridan Aquifer of Eocene and
Oligocene age (Lane, 1986; Scott, 1992). The Floridan Aquifer is confined where the
Miocene Hawthorn Group is > 30 m thick and semi-confined where the Hawthorn Gp is
0 to 30 m thick (Figure 2-1A). The Floridan Aquifer is unconfined and mantled by a thin
veneer of undifferentiated Pleistocene sands where the Hawthorn Gp is missing. The
boundary between the confined and unconfined portions of the Floridan Aquifer is a
marine terrace that represents the erosional edge of the Hawthorn Gp (Scott, 1992).
This feature is called the Cody Scarp (Hunn and Slack, 1983) and trends northwest to
southeast through north-central Florida (Figure 2-1A).
The Ichetucknee and Lower Santa Fe rivers are located in the unconfined western
portion of the watershed, whereas the Upper Santa Fe River is located in the confined
to partly confined eastern portion of the watershed. The Upper Santa Fe River is
completely captured by a sinkhole (the River Sink) at the Cody Scarp, and the Lower
Santa Fe River reemerges as a 1st magnitude spring called the River Rise
approximately 6 km from the River Sink (Katz et al., 1997; Martin and Dean, 2001;
Scott, 2004). The Upper Santa Fe River drains wetlands perched on the confining unit,
and productivity of the wetlands and surrounding forests causes the river to contain
elevated DOC concentrations, in the form of tannic, humic, and fulvic acids, which make
the stream waters high in dissolved color. The Lower Santa Fe River differs from the
Upper Santa Fe River because during baseflow, the Lower Santa Fe River originates
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from springs that discharge from the Floridan Aquifer, and thus has low DOC
concentrations and clear water (Hunn and Slack, 1983). During flooding, DOC-rich
water passes through the sink-rise system, increasing color in the Lower Santa Fe
River.
Hydrology
The Santa Fe River flows 120 km west across north-central Florida from its head
waters in the Santa Fe Swamp to its confluence with the Suwannee River. Its
watershed drains more than 3500 km2, including the Ichetucknee Springshed (Hunn
and Slack, 1983). The Ichetucknee River flows 8 km south from its head spring to its
confluence with the Lower Santa Fe River. Unlike the Santa Fe River, the Ichetucknee
River receives minimal surface runoff from the confining unit, but drains approximately
960 km2 of the Floridan Aquifer (Champion and Upchurch, 2006). From 2007-2012,
the Ichetucknee and Lower Santa Fe rivers discharged an average of 9 m3/s and 38
m3/s, respectively, according to data from USGS gauging sites 02322700 and
02322500, (U.S. Geological Survey, http://waterdata.usgs.gov2012, Figure 2-1B). The
Ichetucknee River has an average depth of 2.15 m (Hensley and Cohen, 2012) and an
average width of 18 m (Google Earth, 2012). The Lower Santa Fe River has an average
depth of 3.04 m (Grubbs and Crandall, 2007) and an average width of 33 m (Google
Earth, 2012).
The Ichetucknee River receives hydrologic input from eight named springs and the
Lower Santa Fe River receives inputs from 18 named springs. All springs discharge
from the Floridan Aquifer and the inter-spring water discharge rates range from 0.27 to
6.29 m3/sec, making them 1st to 3rd magnitude springs (Meisner, 1927). First magnitude
springs on the Ichetucknee River are Blue Hole and Mission springs, which had an
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annual mean discharge of 3.7 and 2.7 m3/s, respectively between 2002 and 2008 (U.S.
Geological Survey, 2012). The Ichetucknee River is also sourced by several 2nd
magnitude springs, including: Head, Devil’s, Grassy, and Mill Pond springs, and 3rd
magnitude springs, including Cedar and Coffee (Figure 2-1B; Scott, 2004). First
magnitude springs on the Lower Santa Fe River are Treehouse, Devil’s Complex, and
July springs. These springs discharge an average 6.3, 5.9, and 2.8 m3/s, respectively
(Scott, 2004). The Lower Santa Fe River also has numerous 2nd magnitude springs
including Deer, Dogwood, Ginnie, Gilchrist Complex, Pickard, Lilly, Poe, Darby,
Columbia, and River Rise springs; and 3rd magnitude springs including Twin, Sawdust,
Rum, Jonathan, and Hornsby (Figure 2-1B; Scott, 2004). In addition to the named
springs, unnamed and ungauged springs, boils, and seeps contribute to the flow of both
rivers, but the magnitude of those sources is unknown.
During baseflow, the River Sink captures less water than discharges from the
River Rise (Martin and Dean, 2001; Screaton el al., 2004). During droughts, all flow in
the Upper Santa Fe River is captured by a sinkhole approximately 1.5 km upstream
from the River Sink, although water continues to discharge from the River Rise. At
these dry times, the River Rise discharges water primarily from the Floridan Aquifer,
including a source from around 400 m below the land surface that is enriched in Na+,
Mg2+, K+, Cl-, and SO2-4 (Martin and Dean, 2001; Moore et al., 2009). Mass balance
calculations made by Moore et al. (2009) suggest that during baseflow >50% of the
water discharging from River Rise is deep water upwelling from the Floridan Aquifer.
Vegetation
Subaquatic vegetation in the Ichetucknee River is mainly native submerged C3
macrophytes, such as strapleaf sagittaria (Sagittaria kurziana) and tapegrass or
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eelgrass (Vallisneria americana) (Heffernan et al., 2010). These taxa are common in
Florida springs (Odum 1957). Other species such as wild rice (Zizania aquatica) and
emergent (Cicuta maculata) and floating species (non-native Pistia stratiotes) are
present in the Ichetucknee River (Heffernan et al., 2010). Epiphytic and benthic algal
mats are also commonly observed in most springs in north-central Florida
(Frydenbourg, 2006; Heffernan et al., 2010).
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Figure 2-1. Location maps of the Santa Fe and Ichetucknee rivers. (A) Distribution of the confined and unconfined Floridan Aquifer in North Florida (from DEP, modified). (B) Detailed map of the Santa Fe and Ichetucknee rivers (white lines), the tributary springs (white dots), and the three sampling locations (red stars). The site at US 27 Bridge and site 2500 are the locations of USGS gauging stations 02322700 and 02322500.
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CHAPTER 3 METHODS
To understand how longitudinal and seasonal changes control the magnitude and
phase relationship of biogeochemical cycles with respect to solar radiation, four 28-to-
48-hr water-sampling surveys were completed at three locations, two on the
Ichetucknee River and one on the Santa Fe River. Each site was located at a different
distance from the river’s Head Spring, yielding sampling localities in the channel with
different water residence times.
Sites
The farthest upstream site on the Ichetucknee River was at site US 27 Bridge,
collocated with USGS gauging station 02322700 (Figure 2-1B). A second site,
approximately 3 km downstream of site US 27 Bridge, was established at site Three
Rivers Estates at the confluence of the Ichetucknee and Lower Santa Fe Rivers. A third
site was established on the Lower Santa Fe River (Site 2500) and is collocated with
USGS gauging station 02322500. The sites at US 27 Bridge and Three Rivers Estates
are located 5080 and 8065 m, respectively from the Head Spring on the Ichetucknee
River. Site 2500 is located 21,513 m from River Rise, which we consider to be the
headwaters of the Lower Santa Fe River (Figure 2-1B).
Field Sampling and Analytical Methods
Samples were collected on four sampling surveys, once each at site Three Rivers
Estates and Site 2500 and twice at site US 27 Bridge. While water samples were being
collected, field data were recorded at 15-minute intervals using a YSI 6920 sonde, for
temperature, specific conductivity (SpC), pH, and dissolved oxygen (DO). NO3-
concentrations were obtained with a SUNA in-situ nitrate sensor.
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Stream discharge, solar radiation, precipitation, and evapotranspiration data were
compiled for each sampling time. Information about discharge was compiled from the
National Water Information System (NWIS), which is maintained by the U.S. Geological
Survey (USGS) at 15-minute resolution (http://www.srwmd.state.fl.us/). Information
about solar radiation, precipitation, and evapotranspiration was compiled from data
measured by the Florida Automated Weather Network (FAWN; http://fawn.ifas.ufl.edu)
at 15-minute intervals in the town of Alachua, about 15-20 km from all study sites.
Water samples were collected at site US 27 Bridge site on 2 November 2009 and
28 May 2010, at site Three Rivers Estates on 8 November 2011, and at Site 2500 on 1
June 2011. The sampling period in November 2009 was about 28 hours and all other
sampling periods were more than 36 hours. Water samples were collected at least 1 m
below the surface using an ISCO autosampler and 1-L bottles. After collection, samples
were split and preserved for laboratory analyses. Samples for measurement of DIC
concentration and δ13CDIC were collected unfiltered and preserved with mercuric
chloride; samples for measurement of DOC concentration were filtered and preserved
using hydrochloric acid; samples for measurement of major cations were filtered and
preserved with nitric acid; and samples for measurement of major anions and alkalinity
were filtered, but not preserved. Samples were kept on ice while in the field and either
refrigerated at 4°C or frozen (nutrient samples) until analyzed.
All samples were analyzed at the Department of Geological Sciences, University
of Florida for major element (Na+, K+, Mg2+, Ca2+, Cl- and SO42-), nutrient, DOC, and DIC
concentrations, δ13CDIC, and alkalinity. Alkalinity was titrated to the second end point
(i.e. pH=X.X) of the carbonate system within 36 hr of each survey, using 30 mL of
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sample and 0.1 N HCl, and calculated using the Gran function (Stumm and Morgan,
1996). Error in the alkalinity measurements was estimated to be ±0.05 meq/L from
measurement of a NaHCO3 standard (n = 8). Major element concentrations were
measured within 2 months of sampling using an automated Dionex DX500 Ion
Chromatograph. Analyses had a precision of <3%, i.e. the relative standard deviation of
internal standards (n = 16) measured along with the samples. Charge balance errors
were <5%, with the exception of two samples. DOC and DIC concentrations and
δ13CDIC values were analyzed within 1-2 months of collection. DIC concentrations were
measured with a UIC 5011 carbon coulometer coupled to an AutoMate Prep Device.
Results were standardized by measurement of known concentrations of dissolved
KHCO3. The average error was estimated to be ±0.02 mM. Dissolved CO2 in water was
extracted with a Thermo-Finnigan Gasbench II connected directly to a Thermo-Finnigan
Delta-PlusXL isotopic ratio mass spectrometer, which was used to measure δ13CDIC
values. Dissolved KHCO3 with a known δ13CDIC value was used for standardization and
the average error was estimated to be ±0.17‰. Isotopic data are reported in
conventional delta notation (‰) versus V-PDB. Saturation indices (SI = log [IAP/Ksp],
where IAP is the ion activity product and Ksp is the solubility constant for individual
mineral phases) of the major carbonate minerals were calculated using the geochemical
modeling program PHREEQC (Parkhurst and Appelo, 1999) with thermodynamic
constants in the phreeqc.dat database.
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Model Estimates and Diel Cycling
A model was developed to estimate if the observed data fit a sine function with a
24-hr periodicity (Kurz et al., in prep)
(3-1)
where CE is the concentration estimated by the model, MM is the mean of the measured
data, AM is the amplitude of the diel cycle, PM is the phase of the measured diel cycle,
and t is the time. The goodness of fit for this model was based on the Akaike
Information Criterion, AIC (Akaike, 1974) according to
(3-2)
where K is the number of parameters, ND is the number of data points, CM is the
measured concentration, and CE is either the concentration estimated by Equation 3-1
or the mean value of the data. I considered the measured values to be better
represented by a sinusoidal cycle, if the AIC value fit the observed data better using CE
based on the results of Equation 3-1 rather than CE based on the mean of the data.
Phase shifts between site US 27 Bridge (May 2010), site US 27 Bridge
(November 2009), Three River Estates (November 2011), and Site 2500 (June 2011)
were compared through cross-correlation analysis as an alternate approach to assess
the lag between each site for the pH and Ca2+ concentration cycles.
Flow Weighted Residence Time
Mean residence times at each sampling location were estimated by summing the
fraction of discharge from each spring contributing to the discharge at the sampling
point (Figure 3-1). Cross-sectional area of required flow from each spring discharge
(AS) was calculated by
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(3-3)
where QT is the total discharge at the sampling site, QS is the discharge at an individual
spring, and AT is the cross section area at the sampling site. AT was calculated using
stream width measured with Google Earth and stream depth estimates found in Hensley
and Cohen (2012) and Grubbs and Crandall (2007) for the Ichetucknee and Lower
Santa Fe Rivers, respectively. QT was acquired from USGS NWIS on the day of
sampling. Values for QS were average discharge values for the springs reported in Scott
(2004). The springs are ungauged and thus their discharges are unknown for the
sampling periods. The average discharges were normalized to the stream flow during
each sampling period. Sampling occurred during low flow periods and thus typically the
river discharges at the sampling sites were lower than average river flows and
consequently, averages for each spring discharge were also proportionately lower. The
volume of the channel occupied by discharge from each spring (VS) was estimated by
(3-4)
where LS is the length of the channel from the spring to the sampling location. The
length of each segment was measured using Google Earth. Individual residence times
for each spring input (ƮS) was estimated by
(3-5)
The residence time for each spring discharge were then flow-weighted (ƮS,S) by
(3-6)
and these weighted residence times were summed for each flow to estimate the flow-
weighted residence time for water at each sampling location (ƮT)
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(3-7)
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Sampling SiteQT
Q3
V3
V2
V1
A2
Q2
Q1
A1 A3
L3
L2
L1
Figure 3-1. Conceptual figure for flow weighted residence time calculation
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CHAPTER 4 RESULTS
Flow Weighted Residence Time
Estimated residence times differ at each of the sampling locations because of
differences in the distance between the head springs and sampling points and the
number of springs and discharges. Most springs discharging to the Ichetucknee River
occur near the headwaters of the river. Nearly 40% of the Lower Santa Fe River flow
originates from five springs about 15 km from Site 2500, which is almost twice the
distance of the Ichetucknee River (Figure 4-1). No springs occur between site US 27
Bridge and site Three Rivers Estates, and thus site Three River Estates has a
proportionately longer residence time than site US 27 Bridge. Based on the spring
discharges and the distance between the springs and the sampling locations, flow-
weighted residence times for water at the sampling locations were found to be 5.2, 9.0,
and 14.1 hrs for site US 27 Bridge, site Three Rivers Estates, and Site 2500,
respectively. Site 2500 had a 60% and 271% longer residence time than at site Three
Rivers Estate and site US 27 Bridge, respectively (Table 4-1 and Figure 4-1).
Diel Cycles
Diel cycles occur in DO concentration, pH, Ca2+ concentration, SIcalcite,
temperature, alkalinity, DIC concentration, and δ13CDIC values at all sites (Figure 4-2).
These diel cycles decreased in amplitude from site US 27 Bridge to site Three Rivers
Estates to Site 2500. At site US 27 Bridge and site Three Rivers Estates, pH, SIcalcite,
and DO concentration diel cycles were asymmetrical and had constant values for about
6 and 3 hours during the night, respectively. At Site 2500, the pH and SIcalcite were
symmetrical over the 24 hour cycle. The DO cycle was also asymmetrical at Site 2500,
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but never became constant at nighttime and instead displayed a gradual decrease
through the night with a rapid rise during the day.
All diel cycles lagged solar radiation but by different amounts. All measured
components, other than Cl- concentrations, show significant diel cycles according to
Equations 3-1 and 3-2 (Figure 4-3). Diel cycles of DIC concentration, alkalinity, and
Ca2+ concentrations lag the solar radiation cycle by different amounts depending on the
season they were collected. During winter (November 2009 and November 2011
sampling times), the diel cycles of DIC concentration, alkalinity, and Ca2+ concentrations
lag solar radiation by 12-18 hrs (Figure 4-3A). During summer (May 2010 and June
2011 sampling times), DIC concentration, alkalinity, and Ca2+ concentrations lag solar
radiation by 18-24 hrs, or about a 4 hours longer lag than in winter (Figure 4-3B).
Regardless of these seasonal variations, diel cycles of all components consistently
lag solar radiation with increased residence time (Figure 4-4). Lag time in DO
concentration, pH, Ca2+ concentration, SIcalcite, temperature, alkalinity, DIC
concentration, and δ13CDIC values increased from site US 27 Bridge to site Three Rivers
Estates to Site 2500. At site US 27 Bridge, with an estimated residence time of about
5.2 hrs, SIcalcite, DO concentration, temperature, and pH lag solar radiation by about 4.2
hrs in May 2010, while at Site 2500, with an estimated residence time of 14.1 hrs, the
same components lag solar radiation by about 5.9 hrs in June 2011 (Figure 4-4). DIC
and Ca2+ concentrations at Site 2500 lag components at site US 27 Bridge by about 1.8
hrs. SIcalcite, DO concentration, temperature, pH, DIC concentration, δ13CDIC value, and
Ca2+ concentration at site Three Rivers Estates in November 2011 lag these values at
site US 27 Bridge in November 2009 by about 1 hr.
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Cross correlation values support the observations that lag times increase with
increasing distance from the head springs. The cross correlation analyses, although
limited to a resolution of 1 hr because of the sampling interval, indicate that pH has a
lag of ≤ 1 hr between sites, while Ca2+concentrations have lags of ≤ 2 hrs between sites.
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Figure 4-1. Ratio of cumulative spring discharge to river discharge (at US 27 Bridge,
Three Rivers Estates, and Site 2500) compared to the distance from each spring to the sampling location. US 27 Bridge and Three Rivers Estates have identical slopes because they are located on the same river with identical spring inputs. Stars indicate sampling location distance for US 27 Bridge (light gray), Three Rivers Estates (dark gray), and Site 2500 (black).
Table 4-1. Flow weighted residence time results
River Site Location Sampling Time Weighted Residence Time
Ichetucknee US 27 Bridge May 2010 5.2 hr Ichetucknee US 27 Bridge November 2009 5.2 hr Ichetucknee Three Rivers Estates November 2011 9.0 hr Lower Santa Fe Site 2500 June 2011 14.1 hr
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Figure 4-2. Time-series measurements of (A) DO concentration, (B) pH, (C) calcium concentration, (D) the saturation index of calcite, (E) temperature, (F) alkalinity, (G) DIC concentration, and (H) δ13CDIC values from all four sampling surveys. Open points were sampled during the summer; filled points were sampled during the winter. Triangles are data on the Ichetucknee River, and circle data are on the Santa Fe River.
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Figure 4-3. A. Winter and B. summer diel cycles. The upper vertical line represents the
phase of solar radiation and all other data are plotted clockwise around the circle depending on their phase lags relative to solar radiation. The circles radiating out from the center reflect the goodness of fit estimated based on Equation 3-2. Cl- shows no significant diel variation as shown by its location at the center of the figure.
34
Figure 4-4. Lag time from solar radiation versus flow weighted residence time for (A) DO concentration, (B) pH, (C) calcium concentration, (D) the saturation index of calcite, (E) temperature, (F) alkalinity, (G) DIC concentration, and (H) δ13CDIC value. Open data were sampled during the summer, closed data were sampled during the winter, triangles are data on the Ichetucknee River, and circle data are on the Santa Fe River.
35
Figure 4-5. Average values and amplitudes of DO concentration, pH, calcium concentration, and the saturation index of calcite compared to flow weighted residence time.
36
Table 4-2. pH cross correlations
pH US 27 Bridge May (5.2h)
US 27 Bridge Nov (5.2h)
Three Rivers Estates Nov (9.0h)
Site 2500 Jun (14.1h)
US 27 Bridge May (5.2h)
X 0.965 0.926 0.883
US 27 Bridge Nov (5.2h)
1 X 0.949 0.902
Three Rivers Estates Nov (9.0h)
0 0 X 0.944
Site 2500 Jun (14.1h)
-1 -1 0 X
Table 4-3. Ca2+ cross correlations
Ca2+ US 27 Bridge May (5.2h)
US 27 Bridge Nov (5.2h)
Three Rivers Estates Nov (9.0h)
Site 2500 Jun (14.1h)
US 27 Bridge May (5.2h)
X 0.631 0.65 0.791
US 27 Bridge Nov (5.2h)
2 X 0.652 0.341
Three Rivers Estates Nov (9.0h)
0 -2 X 0.463
Site 2500 Jun (14.1h)
-1 -2 -1 X
* Bottom results are lag times and top results are r2 values
37
CHAPTER 5 DISCUSSION
It is important to understand longitudinal variations in diel cycling because they
affect whole-stream estimates of metabolic products, such as DO and NO3-
concentrations. Depending on where measurements are taken along streams,
estimates of whole stream metabolism may vary because of the downstream changes
in amplitude and phase. In addition to DO and NO3- concentrations, using whole-
stream estimates of calcite mineral precipitation may have major implications for
estimates of short-term climate change (Liu et al., 2010). Accurate quantification of
calcite precipitation in streams is important because it effects the bicarbonate
concentration within the river and thus affects atmospheric CO2. Liu et al. (2010) argue
that calcite precipitation can affect short-term climate change as well, by the removal of
CO2 from the atmosphere. Consequently, we focus on how diel cycles vary
downstream through a comparison of the diel cycles and estimates of residence time of
water in the stream channel. Understanding how diel cycles vary spatially and relate to
residence time may improve the understanding of controls on the diel cycling of stream
water chemistry and thus the ultimate composition of stream discharge.
Estimates of Residence Time
Estimates of residence time based on Equations 3-3 through 3-7 match closely
with results from a single tracer study of residence time on the Ichetucknee River
reported in Hensley and Cohen (2012). This tracer study found residence time to be
about 6 hrs from Blue Hole Spring to site US 27 Bridge when the river was discharging
approximately 6.5 m3/s. This measured residence time is similar to our estimate of 5.2
hrs at site US 27 Bridge when discharge was 7.8 and 8.2 m3/s on November 2009 and
38
May 2010, respectively. Residence time would be expected to be shorter during
elevated flow, which may explain the difference between the residence time reported in
Hensely and Cohen (2012) and my results. In addition, Hensley and Cohen (2012)
suggested that approximately 20% of the residence time they measured resulted from
transient storage, including hyporheic exchange and retardation of flow by subaquatic
vegetation. My estimates of residence time do not include transient storage and thus
would be expected to be shorter than those found by tracer studies. Although transient
storage is neglected in my estimates of residence time presented here, similarity of my
results to Hensley and Cohen’s (2012) measured residence time suggests Equations 3-
3 to 3-7 provide reasonable estimates for residence time.
The residence time at the sampling location appears to control the asymmetry of
diel cycles of DO concentration, pH, and SIcalcite values at site US 27 Bridge and site
Three River Estates (Figure 4-2). These solutes exhibit minima at night that remain
constant for approximately 6 hrs at site US 27 Bridge and about 3 hrs at site Three
Rivers Estate. These sites have residence times shorter than the length of night, thus
allowing water to flow to the site from the springs before sub-aqueous plants would
begin photosynthesis and associated alteration to the stream water chemistry.
Consequently, the water would retain the composition of the spring water, which has
little DO, low pH and SIcalcite values (Martin and Gordon, 2000; Champion and
Upchurch, 2006; Heffernan et al., 2010). Similar periods of constant minima do not
occur at Site 2500 because the residence time there allows at least a fraction of the
water flowing past the site to have been modified by photosynthesis. This relationship
between the constant minima in water composition and the residence time suggests
39
that the shapes, and possibly the amplitudes, of diel cycles are impacted by the
residence times of the water through accumulation of reaction products during flow from
the head waters to the sampling sites.
Seasonal Effects of Diel Cycles
Seasonal variations in precipitation or evapotranspiration could affect downstream
variations in diel cycles by changing the water budget and thus the concentrations of
solutes and lag in the cycle relative to solar radiation (e.g., Lundquist and Cayan, 2002;
Czikowsky and Fitzjarrald, 2004). In the field area, precipitation is highly seasonal, with
about 50% of the annual rainfall occurring during the months of June to September
(Jordan 1985; Chen and Gerber 1990). During the rainy season, evapotranspiration is
elevated and thus most high discharge events occur during the passage of cold fronts
during the winter dry season and occasionally during the passage of tropical storms,
most commonly in August and September (Jordan, 1985; Pentecost, 1992; Martin and
Gordon, 2000).
Seasonal differences in precipitation and evapotranspiration appear to have little
effect on variations in chemical composition. No precipitation fell during May 14th, 2010,
or November 2nd, 2009 sampling trips, but evapotranspiration was 0.43 cm on May 14th,
2010, but only 0.08 cm in November 2nd, 2009. Regardless of the differences in
evapotranspiration, Cl- concentrations were constant at 0.3 mM during both sampling
times. Because Cl- is conservative in this setting (Martin and Gordon, 2000), its
constant value indicates evapotranspiration has little to no impact on the concentration
of water chemistry.
DIC concentration, Ca2+ concentration, and alkalinity lag solar radiation at site US
27 Bridge by ~4 hrs more in May 2010 than November 2009 sampling times (Figure 4-
40
3). This greater lag may result from seasonal variations in plant metabolic processes.
The combination of vigorous growth of plants during the summer and the longer period
of daylight during the summer could cause a greater lag than during winter because the
timing of plant productivity. Seasonal variations in lag times of temperature, pH, SIcalcite,
DO concentration, and δ13CDIC values are minimal and less consistent than longitudinal
variations.
Residence Time Controls on Diel Cycles
Longitudinal variations in phase and amplitude of diel carbonate cycling appear to
be largely controlled by residence times (Figures 4-4 and 4-5). These changes could be
controlled by at least three processes, including downstream diagenetic reactions as
flow is retarded by transient storage, limitation of biological productivity with decreased
light availability, and downstream accumulation of reaction products of biological
metabolic processes (Figure 5-1).
Transient storage, including hyporheic exchange, has previously been proposed to
cause a time lag in temperature with increasing residence time (Loheide and Gorelick,
2006; Vogt et al., 2010). Transient storage may be similar for both the Ichetucknee and
Lower Santa Fe rivers because they both flow over unconfined Ocala Limestone, which
retains elevated primary depositional porosity (i.e. the eogenetic karst aquifers of
Vacher and Mylroie, 2002) and thus has elevated hydraulic conductivity, allowing
extensive exchange between surface and groundwater (Martin and Dean, 2000;
Screaton et al., 2004; Ritorto et al., 2009). The Ichetucknee River has a thicker layer of
sediment overlying the Ocala Limestone than the Santa Fe River, which may enhance
or limit transient storage there. Transient storage should increase the lag of the diel
cycles relative to solar radiation by increasing residence time of the water and thus
41
changing the phase of the diel cycle. Transient storage could also either increase or
decrease the amplitudes of the diel cycle with increasing residence time, depending on
whether exchange acts as a source or sink of biogeochemical elements. In particular,
remineralization of organic carbon within the hyporheic zone would increase the DIC
concentration and decrease the DO concentrations of the pore-water. The lag in diel
cycles relative to solar radiation increases rather than decreases with distance
downstream, indicating that transient storage is a minor effect on the diel cycles (Figure
5-1A). Nonetheless, with hyporheic exchange this DO-poor and CO2-rich water would
impact the CO2 and O2 concentrations within the water column, and the CO2
concentration will alter the saturation state of the river with respect to carbonate
minerals.
Light limitations, resulting from turbidity, plant cover, or from increased
concentrations of DOC, can decrease photosynthesis and thus decrease the reaction
products from primary production as well as magnitudes of diel cycles (Tilzer, 1973;
Jeydrysek, 1998; Loperfido et al., 2010; de Montety et al., 2011). Flow conditions were
low during the three sampling periods and thus DOC concentrations were also low and
unlikely to impact the diel cycles during this study. Although the water was clear,
turbidity is known to increase with distance downstream (Brown and Ritter, 1986;
Lenhart et al., 2010). If a similar effect occurs in the Ichetucknee and Santa Fe rivers,
the limitation of light may reduce diel variations downstream (Figures 4-4 and 4-5).
Because of downstream reduction in light, in the magnitude of diel cycles should
decrease in a downstream direction (Figure 5-1B). This process should not cause a
shift in the phase of the signal.
42
The most likely control on lag and the decrease in amplitude of the diel cycles is
from downstream accumulation of reaction products of the biological metabolic
processes. The timing of photosynthesis and respiration is controlled by the amount of
solar radiation that hits the river surface. These metabolic processes vary with distance
downstream causing longitudinal changes in whole stream ecology. The magnitude of
these processes will occur simultaneously along the river channel but will depend on
time of day. Water emerging from the springs will obtain a diel cycle in chemical
composition that corresponds to the immediate variation in light. As water flows
downstream, chemical changes caused by primary productivity will be out of phase with
the cycle in the section of the river immediately upstream (Figure 5-1C). Consequently,
as water flows downstream, successive shifts in the timing of the metabolic cycle will
continuously reduce the magnitude and increase the lag of the diel signal at the
sampling location. The cumulative effect of these shifts results in the observed
decreases in amplitude (Figure 4-5) and increases in lag time (Figure 4-4) with
increasing residence time (Figure 5-1C). Although each of these processes (i.e.
transient storage, light limitation, and downstream accumulation) produces variations in
the downstream variation in the diel cycles, none of them acts alone and combined they
are likely to enhance the observed signal (Figure 5-1D).
Longitudinal Variations in Ca2+ Concentrations and Carbonate Mineral Diagenesis
Average Ca2+ concentration increases with residence time in the Ichetucknee and
Santa Fe rivers (Figure 4-5). The longitudinal increase in Ca2+ may have implications
for whole-stream calcite precipitation budgets, and thus affect short-term climate
change predictions. This increase in Ca2+ is somewhat surprising considering that water
in both the Santa Fe and Ichetucknee rivers is continuously supersaturated with respect
43
to calcite and raises the question of what could be the source of the Ca2+ (Figure 4-2).
Pore- water in sediments in the channel of the Ichetucknee River has been found to
have Ca2+ concentrations of 1.40 mM, which is about 0.10 mM greater than the river
water (Kurz et al., 2011). Pore-water chemistry is dominated by the remineralization of
organic carbon which in turn drives changes in carbonate saturation state making the
pore-waters a potential source of calcite-undersaturated water to the river. Hydraulic
head gradients are oriented from the pore-water to the stream suggesting the river
gains Ca2+-rich pore water (Kurz et al., 2011). Furthermore, hyporheic exchange would
allow river water to react with the bottom sediments and increase its Ca2+ concentration.
Consequently, with increased residence times, more Ca2+-rich water could enter the
system, causing the observed relationship between increased Ca2+ concentration and
the estimated residence time (Figure 4-5).
Regardless of the overall increase in Ca2+ concentration with distance
downstream, the diel decreases in Ca2+ concentrations reflect precipitation of calcite (de
Montety et al., 2011) considering the river water is continuously supersaturated with
respect to calcite (Figure 4-2D). Whatever calcite precipitates is likely to be flushed
from the system since no deposits of massive calcite occur in the rivers. de Montety et
al. (2011) suggests that fine-grained authigenic calcite may remain in suspension and
be exported out of the Ichetucknee River in colloidal or fine-grained particulate form.
High-pH microenvironments, such as algae, are able to have localized carbonate
precipitation due to small scale photosynthesis-driven cycles (Hartley et al., 1996;
Shiraishi et al., 2010). Consequently, calcite could also precipitate on subaquatic
44
vegetation or on the surface of biofilms, which would also be exported from the system
(e.g., Hoffer-French and Herman, 1989; de Montety et al., 2011).
Assuming that the diel cycling of Ca2+ results from calcite precipitation, we can
estimate the amount of calcite precipitated longitudinally along the rivers based on the
loss of Ca2+, assuming the difference between peak Ca2+ concentration and the
measured Ca2+ concentration over a 24-hour period at each sampling location
represents the amount of calcite precipitated (de Montety et al., 2011). With this
assumption, 0.66 mM/day of calcite precipitated on May 14th, 2010 at site US 27 Bridge,
0.60 mM/day of calcite precipitated on November 9th, 2011 at site Three Rivers Estates
and 0.34 mM/day on June 1st, 2011 at Site 2500 (Table 5-1). This pattern suggests
that less calcite precipitates as residence time of the water increases. In contrast with
this systematic change, only 0.16 mM/day of calcite precipitates at site US 27 Bridge on
November 2nd, 2009.
45
Figure 5-1. Conceptual figure for the proposed processes controlling longitudinal variations in diel biogeochemical cycling with increasing residence time. (A) transient storage, (B) light limitation and (C) accumulation. Panel D shows the accumulative effects of all processes. The right hand side of the diagram shows the sum of the variations in cycles. The cumulative effect results in decreased amplitudes and increased lags relative to solar radiation downstream.
46
Table 5-1. Comparison of estimated Ca2+ lost to precipitation at the monitoring sites.
US 27 Bridge May 2010
US 27 Bridge Nov. 2009
Three Rivers Estates Nov. 2011
Site 2500 June 2011
(Ca2+) (Ca2+) (Ca2+) (Ca2+)
Loss to precip. (mM/day) 0.66 *0.16 0.60 0.34
*Possible outlier
47
CHAPTER 6 CONCLUSION
To understand the control of residence times on the amplitude and timing of
biogeochemical cycles, four 28 to 48-hr water sampling surveys were completed at
three locations on the Ichetucknee and Lower Santa Fe rivers, north-central Florida.
Estimates of residence time at each of location increased with distance from the head
springs from 5.2 to 14.1 hrs. Diel cycles of carbonate components, Ca2+ concentrations,
DIC concentrations, alkalinity, pH, δ13CDIC values, and SIcalcite lagged solar radiation by
more than 4 hours with increasing residence time. Additionally, amplitudes of the diel
cycles decreased with increasing residence time. Loss of Ca2+ due to calcite
precipitation decreased with increasing residence time. The observed lags, decreases
in amplitude, and loss of Ca2+ appear to result from several processes in the river. The
primary process appears to be the asynchronous accumulation of metabolic reaction
products as water flows downstream. Decreasing amounts of light with increased
turbidity limits the amount of photosynthesis and respiration, which decreases the
amplitude of the diel cycles, but is unlikely to cause a lag relative to solar radiation.
Transient storage, which will increase the residence time as well as alter the chemical
composition of the water, but the changes in concentrations could either increase or
decrease depending on whether the pore-waters represent a source or sink of material
to the river. Observations of longitudinal variations in diel cycles may provide
information on carbonate mineral diagenesis and its effects on channel morphology and
the global carbon cycle. In addition, differences in diel cycling with distance
downstream have major implications for estimates of whole-stream metabolism, shown
by diel shifts in concentrations of metabolic products such as DO and NO3-.
48
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53
BIOGRAPHICAL SKETCH
Carolyn Ball received her bachelor’s degree in geological science from the
University of Florida in 2011. She finished her master’s degree, in only one year after
her bachelor’s degree, in August 2012. She will be working for Shell Oil Company in
August 2012 as an Exploration GeoScientist in Houston, Texas.
