8 BIOLOGICAL SCIENCE REPORT USGS/BRD/BSR-1999-0001

development), Jimmy Johnston, and Jorje L. Aravena
(wavelet transform and system design) for technical sup-
port, critique, and discussion of this and related work.
Thanks also are extended to Karen L. McKee for advice
and assistance in the area of S. alterniflora germination,
growth, and maintenance indoors. Much of this work re-
sulted from studies funded by the U.S. Environmental Pro-
tection Agency, National Biological Service, the U.S. Min-
eral Management Service, and private sponsors (Integrated
Resources Inc., New Orleans, Louisiana: Bruno and Bruno
Inc., New Orleans, Louisiana; and Martin Marietta Inc.,
Oak Ridge, Tennessee). There are no particulars that can
summarize the importance of this support or my gratitude
to these supporters.

References

Abraham, R.H., and C.D. Shaw. 1992. Dynamics: the ge-
ometry of behavior. Addison Wesley Publishers. 642
pp.

Anisfeld. S.C.. and G. Benoit. 1997. Impacts of flow re-
strictions on salt marshes: an instance of acidification.
Environmental Science and Technology 31(6):1650-
1657.

Armstrong, W. 1975. Waterlogged soils. Pages 181-218 in
J.R. Etherington, editor. Environmental and Plant Ecol-
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Baas-Becking, L.GM., I.R. Kaplan, and D. Moore. 1960.
Limits of the natural environment in terms of pH and
oxidation-reduction potentials. Journal of Geology
6R(3):243-284.,

Bailey, L.D., and E. Beauchamp. 1971. Nitrate reduction
and redox potentials measured with permanently and
temporarily placed platinum electrodes in saturated soils.
Canadian Journal of Soil Science 51:51-58.

Benedetto, J., and M. Frazier. 1994. Wavelets: mathemat-
ics and applications. CRC Press, Boca Raton, Fla. 575

pp.

Beyers, R.J., and H.T. Odum. 1993. Ecological micro-
cosms. Springer Advanced Texts in Life Sciences,
Springer-Verlag, New York, N.Y. 572 pp.

Bohn, H.L. 1968. Electromotive force of inert electrodes
in soil suspensions. Soil Science Society of America
Journal 32:211-215.

Bohn, H.L. 1969. The emf of platinum electrodes in dilute
solutions and its relation to soil pH. Soil Science Soci-
ety of America Journal 33:639-640.

Bohn, H.L. 1971. Redox potentials. Soil Science 112(1):39-
45.

Box, GE.P., and G.M. Jenkins. 1976. Time series analysis:
forecasting and control. Holden-Day Publishers, San
Francisco, Calif.

Brockwell, P.J., and R.J. Davis. 1991. Time series: theory
and methods. Springer Verlag, New York, N.Y.

Burdick, D.M., LA. Mendelssohn, and K.L. McKee. 1989.
Live standing crop and metabolism of Spartina patens
as related to edaphic factors in a brackish, mixed marsh
community in Louisiana. Estuaries 12:195-204.

Catallo, W.J. 1994. Ecotoxicology of wetland ecosystems.
Current understanding and future research needs. Envi-
ronmental Toxicology and Chemistry 12(12):2209-
2224.

Catallo, WJ. 1996. Transformation of N-, O-, and S- het-
erocycles (NOSHs)in estuarine sediments: effects of re-
dox potential and particle size. Chemosphere:
33(12):2543-2563.

Catallo, W.J., R.J. Gale, R.L. Wong, and M.E. Bender.
1990. The reducible dye 2-(p-iodopheny])-3-(p-
nitrophenyl)- 5-phenyl-2H-tetrazolium chloride (INT)
for use in aquatic toxicology. Notes on chemical struc-
ture, electrochemistry, and toxicity. Pages 222-236 in
W.G. Landis and W. H. van der Schalie, editors. Aquatic
Toxicology and Risk Assessment. ASTM Special Tech-
nical Publication 1096. American Society for Testing
and Materials, Philadelphia, Penn.

Catallo, W.J., and R.P. Gambrell. 1987. The effects of high
levels of polycyclic aromatic hydrocarbons on sediment
physicochemical properties and benthic organisms ina
polluted stream. Chemosphere 16(5):1053-1063.

Catallo, W.J., and R.P. Gambrell. 1994, Fate and effects of
N-, O-, and S- heterocycles (NOSHs) from petroleum
and pyrogenic sources in marine sediments. US. De-
partment of the Interior, Minerals Management Service,
OCS Study MMS 94-0056. 72 pp.

Catallo, W.J.,M. Schlenker, R.P. Gambrell, and B.S. Shane.
1995, Toxic chemicals and trace metals from urban and
rural Louisiana lakes: recent historical profiles and toxi-
cological significance. Environmental Science and Tech-
nology 29(6):1436-1445.

Chen, C.T. 1989. System and signal analysis. Saunders
College Publishing, Orlando, Fla. 543 pp.

Cogger C.G,, and P.E. Kennedy. 1992. Seasonally saturated
soils in the Puget lowland. I, Saturation, reduction, and
color patterns. Soil Science 153(6):421-433.

Cogger C.G,, P.E. Kennedy, and D. Carlson. 1992. Sea-
sonally saturated soils in the Puget lowland. IT, Measur-
ing and interpreting redox potentials. Soil Science
154(1):50-58.

Coleman, D.C., E.P. Odum, and D.A. Crossley, Jr. 1992.
Soil biology, soil ecology, and global change. Biology
and Fertility of Soils 14:104-111.

Cowles, H.C. 1899. The ecological relations of the veg-
etation on the sand dunes of Lake Michigan. Part I.
Botanical Gazette; February 1889. Pages 28-58 in L.A.
Real and J.H. Brown, editors. 1991. Foundations of
Ecology, Classic Papers with Commentaries. Univer-
sity of Chicago Press, Tl.


Contents

Page

Introduction .......
Methods ........
Platinum (Pt) Electrod
Redox Potential Data Loggers
Sed
Static Microcosm
Tidal Mesocosms
Field Site
System Checks:
Results
Static Microcosms
Tidal Mesocosms.
Reproducibility of Redox Potential Measurements
Discussion ..
Acknowledgenicnis

References .....


ARARRAABEGALA<*

1. Light banks with timers 6, Pumps.

2. Plants 7. Cobblestones

3. Sediment 8. Reservoirs

4, Agricultural mesh and burlap —_9. Simulated tide range
5. Oyster shell 10. 300-gallon enclosure

Fig. 2. Schematic diagram of the tidal mesocosms.

at flow rates sufficient to flood the sediment columns after
10h. After 3 hat full flood, the water was withdrawn using
a remotely timed sputter pump. The resulting water level
profile resulted is shown in Fig. 3.

The seawater reservoir was continuously aerated and
the systems were maintained at room temperature. At “hi igh
tide,” the sediment columns were covered with about 2cm
of water, while at “low tide” the columns were completely
drained, with about 5 cm of water remaining at the bottom
of the system. Electrodes were positioned in the top 3-10
cm of sediments and care was taken to avoid positioning
the electrode directly in the root mass of the plants.

120.
100.
80.

60 7,
40.

20.
i)

‘Approximate root zone

Pump floor
(Orain)

Water level (% fill)

0 18 24

12
Time (hours)
Fig. 3. The water profile resulting from flooding of the tidal
mesocosms.

BIOLOGICAL SCIENCE REPORT USGS/BRD/BSR-1999-0001

Duplicate tidal mesocosms were established and poten-
tials were recorded at rates of at least 1 sample/h. Other
data logged at the same or higher rates were water tem-
perature, air temperature, light levels, relative humidity,
pH, and soil temperature. Discrete data were compiled on
output properties, including trace gases leaving sediments
and salinity. System checks included monitoring redox
potential while (a) the system was turned off at high or low
water and (b) the water lines were disconnected but the
pumps and timers were allowed to cycle as usual. “Dummy”
electrodes and grounded leads not in contact with the tidal
system also were monitored during the experiments. These
tests were designed to account for extraneous signals that
might be detected by the system and mistaken for sedi-
ment potential variations.

Field Site

The field measurements were taken in a semi-impounded
tidal saline marsh (salinity16 g/l) in southern St. Charles
Parish, Louisiana. Two calibrated combination redox po-
tential electrodes (Sensorex) were placed at 4-cm depth in
(a) an intertidal flood zone and (b) a nearby flooded back
marsh area. Each electrode had an independent logger sam-
pling at 1/h, and samples were acquired synchronously for
12 days. The electrodes and loggers were recovered at the
end of the sampling and the electrode calibration was again
checked.

System Checks

The possibility of signal aliasing (Box and Jenkins 1976;
Chen 1989; Brockwell and Davis 1991) was examined by
sampling the tidal and static mesocosms and microcosms
at higher frequencies than were used in the experiments
(eg., sample rates between 2 Hz and 15/min). The goal
was to detect any contaminating signals with frequencies
above the Nyquist frequency (i.e., 1/2 the sample frequency)
and amplitudes >5 mV.

As with all instrumental potential measurements, the
process of redox potential data acquisition involved the
transfer of a small amount of current in the sample loop of
the logger (Bohn 1971). In the automated (stand alone)
sampling mode, these currents and associated potential tran-
sients at the electrode surface were small—on the order of
pA and nV, respectively. Nevertheless, it was important to
determine whether repeated measurement caused chemi-
cal changes, for example, redox reactions, in the vicinity
of the electrode being sampled (cf. Zhi-Guang and Tian-
Ren 1984). This determination was made by rapidly sam-
pling an electrode (2 Hz-4/min for 10 min) equilibrated in
a Standard solution of quinhydrone at 25°C. Changes or
drift in the potential readings in the system being rapidly
sampled were taken as evidence of chemical changes near
the electrode resulting from the process of measurement.


Hourly and Daily Variation of
Sediment Redox Potential
in Tidal Wetland Sediments

W. James Catallo
Laboratory for Ecological Chemistry
Toxicology Department, School of Veterinary Medicine
Louisiana State Universi
Buton Kouge, Loutstana 70803

Abstract: Variation of electrochemical oxidation-reduction (redox) potential was examined in surface salt
marsh sediments under conditions of flooding and tidal simulation in mesocosms and field sites. Time series were
generated of redox potential measured in sediment profiles at 2-10 cm depth using combination Pt-Ag/AgCl
(ORP) electrodes. Redox potential data were acquired at rapid rates (1-55 samples/h) over extended periods (3-
104 days) along with similar times series of temperature (water, air, soil) and pH, It was found that redox potential
varied as a result of water level changes and was unrelated to diurnal changes in temperature or pH, the latter of

potential oscillations of 40-300 mV amplitude were

in those sediments exposed to static hydrological conditions. Data coll
site experiencing diurnal tides showed similar fluctuatio
indicated that (a) redox potential can be a dynamic, nonlinear variable in c

P
orded that had the

In tidal salt
ide, redox

a simulated d
ame periodic

were

tu
y as the flood-drain
cycle. Periodic redox potential time series were observed repeatedly in sediments receiving tidal pulsing but not

ted over 12 days from a coastal marsh
in redox potential. Data from these experiments
stal and estuarine wetland sediments

over hourly and daily scales, and the designs of biogeochemical experiments should reflect this, (b) redox

potential can change rapidly and
(c) microbial community proce

nificantly in co

| wetland s
are primary determinants of the time cours
sediments, and elimination or inhibition of microbial activity (¢,
behavior, and (d) fast redox potential dynamics appear to be char

onse to Nooding and draining,
of redox potential in wetland
by pollutants) can significantly alter that
stic of sediments that experience changes

diments in res

in hydrology. Ihe rapid redox potential changes observed in these systems indicated dynamig metabolic and
biogeochemical conditions in the field, and confirmed that hourly and daily redox potential variations should be

resolved in studies of sediment functioning,

Key Words: redox potential, coastal sediments, time series, ecological function

“The ecologist, then, .

Introduction

A central variable of interest in aquatic and wetland
ecosystems is the sediment redox potential, or “Eh” (Baas-
Becking et al. 1960; Ponnamperuma 1972; DeLaune et al.
1976; Gleason and Zieman 1981). In practice, the redox
potential is measured by comparing voltmeter readings
from platinum or other solid electrodes to a reference elec-
trode (Bohn 1968, 1969, 1971). The measured potential
values (typically + 350 mV to -S00 mV vs, the saturated
calomel electrode [SCE]) correspond to the electrochemi-
cal status of the sediment and can be related to important
soil properties and processes. Relationships have been con-
firmed between measured or inferred redox potentials in

must endeavor to discover the laws which govern panoramic changes. Ecology,
therefore, is a study in dynamics

(Cowles 1899).

sediments and (a) biogeochemical cycling of C, H, O, N,
S, P and numerous trace metals (Ponnamperuma 1972;
DeLaune et al. 1976), (b) wetland plant distributions, pro-
ductivity, and physiological status (Howes et al. 1981;
DeLaune and Pezeshki 1991), (c) microbial and meiofaunal
distributions and ecology (Fenchel 1978), (d) transforma-
tion and transport of hydrophobic pollutants (Hambrick et
al. 1980; DeLaune et al. 1981; Catallo and Gambrell 1994)
and trace metals (Khalid 1980; Masscheleyn et al. 1990;
Lindsay 1991; Gambrell 1994), (e) sediment exchange of
radioisotopes (Pardue et al. 1989), and (f) preservation of
“chemical fossils” used in organic chemical source and
diagenetic studies (Meyers and Ishiwatari 1993; Catallo et


Figure

UbeEna

a3

Figures

Effect of equilibration time after placement on redox potential readings from electrodes
Schematic diagram of the tidal mesocosms ..
The water profile resulting from flooding of the tidal mesocosms.....
Time series of redox potential data from the static microcosms upon cessation of aeration (96 h)
Time series redox potential and pH data in surface sediments of a tidal mesocosm, and redox
potential from an identical electrode placed ina drained, nontidal mesecosm

In-phase redox potential oscillations from two tidal mesocosms running synchronously.
Reproducibility of redox potential time series measurements using calibrated electrodes ..
Redox potential time series from ORP electrodes in tidal and back marsh (flooded nontidal)

zones of a saline coastal marsh in Louisiana; spectral analysis of the redox potential time series .....



VARIATION OF SEDIMENT REDOX POTENTIAL IN TIDAL WETLAND SEDIMENTS 3

Giblin and Howarth i984; Cogger and Kennedy i992;
Fessel 1994, Haraguchi 1995; Anisfeld and Benoit 1997),
comparatively little work has been devoted to temporal
variations in a range of processes. The purpose of this work
was to examine the hourly and daily responses of sediment
redox potential under constantly flooded and tidally pulsed
conditions and relate these data to sampling and analytical
design for ecological process monitoring in microcosms
and field settings.

Methods

Platinum (Pt) Electrodes

Combination Pt-Ag/AgCl reference electrodes
(Sensorex, Stanton CA) were calibrated prior to use with
0.05 g quinhydrone in 50 mL of pH 7.0 and pH 4.0 buffer
solutions (Bohn 1968, 1969). Calibration was checked at
the end of each experiment, and agreement between begin-
ning and ending values was required to be within 15% for
data acceptance.

Electrodes placed in flooded soils require substantial
equilibration times in order to provide reliable, stable read-
ings (Fig. 1). Hence, redox electrodes used in the tidal
mesocosms were equilibrated on site for 48 h before sam-
pling. Electrodes used in the field experiments were equili-
brated for 24 h.

Redox Potential Data Loggers

Redox potential was measured by using a data logger
developed for this study. Development of a new data log-
ger was necessary because available commercial potential
loggers were limited by one or more of the following fac-
tors: (a) inappropriate potential ranges (e.g., 0-2 V abso-
lute) for sampling oxidized and reduced sediments, (b) in-
ability to stand alone, that is, without computers and AC
power, and (c) expense and restrictions on field
deployability (size, sensitivity to heat, moisture, etc.). The
unit developed for this work consists of a 12.5 x 10.5 cm
printed circuit board (Precision Etching Inc., Odenton,
MD), seven input channels with single or individual refer-
ence capabilities, auxiliary battery, solar panel, computer
interface, and status LED. The system and electrode leads
were protected from external signals by Faraday cages. With
sampling rates set at I/h, this unit was capable of stand-
alone monitoring of seven independent channels for 30 days.

Sediments and Plants

Sediments for use in mesocosms were collected from a
Spartina alterniflora marsh in Terrebonne Parish, Louisi-
ana (salinity 16-25 g/L; pH 6.6-7.4 + 0.2 wet measurement).
The sediment was wet sieved (1 mm), mixed with quartz
sand (5% w/w) and ground S. alterniflora matter (5% w/
w), and wet with sterile artificial seawater (13-15 g/l).
Readily oxidizable organic matter (OM) contents in both
sediments were on the order of 9%, as measured by using
standard methods (Catallo and Gambrell 1987).

Spartina aiternifiora was grown from seed obtained
commercially (Environmental Concern, Inc., St. Michaels,
Maryland) in greenhouses and in the laboratory under arti-
ficial sunlight. Plants were germinated in commercial seed
starter and then transplanted to the salt marsh soil. When
the plants had attained heights of 6-10 inches, they were
transplanted into mesocosm enclosures and maintained
under artificial sunlight and simulated diurnal tides.

Static Microcosms

The flooding of well-drained. oxidized sediments attenu-
ates gaseous oxygen diffusion to the system and promotes
the development of reducing conditions by microbial res-
piration and accumulation of chemical reductants
(Ponnamperuma 1972; DeLaune et al. 1976). It was of in-
terest to examine the time dynamics of this process and the
effects of “damping” variables, such as biocidal chemicals
and y radiation, on redox potential time series. The salt
marsh sediments were aerated for 96 h and placed in 1-1
glass flasks which were then sealed. The flasks were
equipped with vacuum-tight feed troughs for two calibrated
electrodes. Three treatments were established: (a) clean salt
marsh sediment-water, (b) salt marsh sediment-water con-
taminated with 1% (w/w) coal carbon black, and (c) clean
sediment-water exposed to y radiation (2 MRad) using a
submerged Co® source. The coal carbon black contained
polycyclic aromatic hydrocarbons and heterocycles known
to be toxic to sediment microbes (Catallo and Gambrell
1987; Catallo 1996). Settling of particulates after 24 h in
each microcosm gave approximately a 1:1 sediment to
water ratio (v/v), with an electrode sampling the water and
the sediment phases. Redox potential values in the water
and sediment of the treatments were sampled at a rate of 1/
h for periods of 180-530 h. The pH was measured at the
beginning and end of the experiments. The level of micro-
bial respiration in each treatment was determined at the
beginning and end of the experiments using
iodonitrotetratzolium (INT) reduction (Catallo et al. 1990).
The systems were maintained at room temperature (21 +
2° C) throughout the experiments. Reproducibility of re-
dox potential measurements in these systems was evalu-
ated by establishing duplicate microcosms, each with their
own calibrated electrode and logger.
Tidal Mesocosms

Salt marsh sediments were loaded into 64-cm high PVC
cages (32 x 68 cm) encased in plastic mesh (1 cm?) and
fine landscaping mesh (ca. 0.5 mm’) to a height of 60 cm
(Fig. 2). All sides and the bottoms of these enclosures were
permeable to water, but not to sediments. These enclosures
were situated in 300-gallon injection molded plastic con-
tainers. A diurnal tide was simulated by moving 230-250
gallons of artificial sea water (salinity of 13-15 g/l; pH =
7.0-7.4) into and out of the mesocosms once daily. The
flood tide was delivered via siphon from water reservoirs




6 BIOLOGICAL SCIENCE REPORT USGS/BRD/BSR--1999-0001

For comparison with hydrodynamic situations, an iden-
tical electrode-logger system was positioned in moist, well-
drained sediments but was not exposed to a simulated tide;
this system was monitored synchronously with the tidal
system (Fig. 5). Clearly, the significant redox potential
variations observed in the tidal system were not observed
under static conditions in the nontidal system and could
not be attributed to other factors (i.e., external fields, diur-
nal temperature, measurement artifacts).

Reproducibility of Redox Potential Measurements

It was necessary to evaluate the reproducibility of the
redox potential readings in both the static microcosms and
the tidal mesocosm systems. In order to do this, replicate
static and tide simulation systems were established with
independent redox potential sensors and loggers. These
replicate systems were tested side by side under identical
conditions for several days, with redox potential sampled
hourly. Data in Figs. 6 and 7 are representative of results
obtained: calibrated ORP electrodes exposed to similar
static or dynamic hydraulic conditions in sediments pro-
vided reproducible values for redox potential, particularly
during periods of rapid change in redox potential (e.g., the
first day after flooding). Although there was substantial
variance in individual potential measurements (e.g., 10-
30%), the time series of the replicates are in close agree-
ment. Measurements from tidal mesocosm replicate sen-
sors also tended to vary when considered on a datum by
datum basis (Figs. 6 and 7). Nevertheless, the frequency
and amplitude of the extended time series functions were
in close agreement (Fig. 6). Divergence in these time se-
ries also has been observed, particularly in systems that
have been running for extended periods (several months).
This divergence reflects the development of redox heteroge-
neity in the sediments; the development of heterogeneity is

ror

Power failures.
N

-200

Time (1 interval = 24 h)
-® Mesocosm 1

Fig. 6. in-phase redox potential oscillations from two tidal
mesocosms running synchronously.

— Mesocosm 2

8 3 —e Static System 1
2 -+- Duplicate
2
£ “198 ‘Sample rate = 4/h
z
= 29
3 393
° 10 20 30 40 50 60 7
Time (hours)

3
2
a
i comin
1 2 3 4 3 6
Time (days)

-* Mesocosm 2 E1
-e- Mesocosm 2 E2

— Mesocosm 1 E1
== Mesocosm 1 £2

~#-Mesocosm 3 E1

Fig. 7. Reproducibility of redox potential time series measurements
using calibrated electrodes. Top: Redox potential values from
duplicate sealed sediment slurry microcosms (as in Fig. 3).
Bottom: Redox potential time series data from surface electrodes
(7 cm) in triplicate tidal mesocosms. Systems were tested
synchronously with respect to all input variables (e.g., tides, light)
and sampling times were approximately synchronous, E1 and E2
are duplicate electrodes for each mesocosm. E2 for mesocosm 3
was damaged and therefore data were not acquired.

an expected part of the evolution of many dynamic sys-
tems (Abraham and Shaw 1992). Nevertheless, as shown
in Fig. 6, even separate tidal systems tested and sampled
synchronously displayed redox potential oscillations that
remained in phase for weeks, with good agreement between
the time series of the signals over many tide cycles. It is
important to point out that the reproducibility of the mea-
surements of redox potential under hydrostatic and dynamic
conditions applied to contemporaneous measurements. No
attempt was made to determine reproducibility between
measurements taken in different systems (e.g., field vs.
mesocosms) or measurements made in the same kind of
system at different times in the year or between laboratories.

Limited data from a semi-impounded tidal wetland in
St. Charles Parish, Louisiana, indicated diurnal fluctuations
of redox potential in the surface sediments of an intertidal
zone (Fig. 8). Comparison of these data with those col-
lected in a flooded, nontidal area nearby showed no diur-
nal changes in redox potential, even though the electrode
was positioned similarly and would have been equally sus-
ceptible to thermal or other possible confuting phenom-
ena. These data agree with those generated in mesocosms,
that is, tidal flushing was a major cause of the redox poten-
tial fluctuations.


VARIATION OF SEDIMENT REDOX POTENTIAL IN TIDAL WETLAND SEDIMENTS 5

Results

Static Microcosms

The purpose of these systems was to simulate condi-
tions of aerobic sediment being flooded or a flooded sedi-
ment being deprived of regular flushing with oxygenated
water. The goal was to observe the rate and nature of the
redox potential response versus systems that had been con-
taminated or sterilized, thus inhibiting microbial respira-
tion. Fig. 4 shows the redox potential versus time profile
for clean, coal tar-contaminated, and y-irradiated salt marsh
sediment slurries in static microcosms. Upon elimination
of oxygen supply to these systems (i.e., cessation of aera-
tion), the clean, untreated sediments developed rapidly re-
ducing redox potentials (>370 mV/48 h). The overlying
water remained more oxidized until about 300 h, after which
the potential decreased to the same level as the sediment.
At this point, the sediment and water darkened, and sul-
fides were detected in head space samples. By the end of
the test, the pH had dropped from 6.6 to 5.9. Conversely,
the coal tar-contaminated and -ray-sterilized sediments
did not display rapid redox potential decreases, color
changes, pH decreases, or evolution of sulfides. The con-
taminated sediment remained electrochemically unchanged
until minor redox potential decreases were observed after
400 h. It was found that INT reduction (proportional to
microbial respiratory electron transport) was approximately
tenfold greater in the clean sediments than in the contaminated

3
2 \
zg
3
§ 335
é ioe
435
535 Sample Rate = 1/h

0 48 96 144 192 240 288 336 384 432. 480 528
Time (Hours)
% Killed control water © Marsh surface water (contaminated)

© Killed control sediment = Marsh surface water (clean)
‘4 Marsh sediment (contaminated) ~ ¢ Marsh sediment (clean)

Fig. 4. Time series of redox potential data from the static
microcosms upon cessation of aeration (96 h). “Contaminated”
sediments received 1% coal carbon black, and ‘killed contro!”
slurries were sterilized with 2Mrad y radiation prior to the run.

sediments at the end of incubation (530 h). The y-irradi-
ated sediments showed no pH changes or INT reduction,
that is, there was no evidence of microbial respiration.

Tidal Mesocosms

The tidal mesocosms were used to examine the ef-
fects of periodic water level fluctuations on redox poten-
tial time dynamics in sediment-plant systems. It was clear
from the static microcosm results that measured redox po-
tential values could change dramatically in short periods;
assuming that the electrochemical processes at the elec-
trode were reversible or nearly so, regular variations on
the order of >75 mV were expected to be realized within a
tide period (based on the linear part of data shown in Fig.
4), Previous sampling studies had shown that (a) there were
no signals in the tidal simulation (or static microcosms)
with frequencies higher than, or close to, the sampling fre-
quency (i-e., 1/h), and (b) the process of automated sam-
pling did not perturb the chemical environment around the
electrode so as to stabilize subsequent potential readings.
Fig. 5 shows the high resolution redox potential and pH
time series from a surface electrode in salt marsh sediment
enclosures containing S. alterniflora in a tidal mesocosm.
It was clear that the redox potential electrode registered
potential fluctuations that followed the simulated tides and
that measured pH changes were too small to account for
this variation (Fig. 5). Fast Fourier transform (FFT) and
multiresolution “wavelet” analyses (Benedetto and Frazier
1994) confirmed the presence of strong diurnal variations
in the redox potential series from the tidal systems, with
the wavelet transform providing a mean value period of
23.78 + 2.10 h.

(ove Ana

Fig. 5. Time series redox potential (A) and pH (B) data in surface
sediments (7 cm) of a tidal mesocosm, and redox potential from
an identical electrode placed in a drained, nontidal mesocosm
(C)


2 BIOLOGICAL SCIENCE REPORT USGS/BRD/BSR-1999-0001

al. 1995). Other studies have related redox potential to
wetland soil function and taxonomy (Faulkner et al. 1989),
groundwater chemistry (Grenthe et al. 1992), and
ecotoxicology of sediment microbes (Koepfler and Kator
1986; Catallo and Gambrell 1987). In many cases the re-
dox potential can be considered a “master variable” in sedi-
ment-water systems, and its behavior in response to natu-
ral and anthropogenic perturbation is of continuing inter-
est (Lindsay 1991). The behavior of redox potential and
related variables in different sediments under conditions
of environmental stress and natural change have global
ecological significance (Coleman et al. 1992).

The redox potential measured in the field reflects inter-
actions between hydrology (e.g., flood duration and peri-
odicity), microbiological activity, plant root processes, sedi-
ment factors (e.g., clay content, chemical equilibria), and
budgets of organic matter and nutrients (Ponnamperuma
1972; Armstrong 1975; Cogger and Kennedy 1992; Cogger
et al. 1992, DeLaune et al. 1983). Several studies have
shown that fluctuating redox potentials arising from alter-
nate wetting and drying of sediments are conducive to faster
rates of C and N transformations than are more static con-
ditions (i.e., those that are constantly drained or flooded;
Patrick and Wyatt 1964; Sorensen 1974; Reddy et al. 1978;
Smith and Patrick 1983; Odum 1983a; Odum 1983b). In
some work, strong associations between wetland hydro-
logic conditions, sediment redox potentials, and bio-
geochemical-ecological processes have been described
(Burdick et al. 1989; Nyman and DeLaune 1991).

Generally, redox potential is measured infrequently in
natural settings and microcosms, that is, with sample inter-
vals on the order of days to weeks. This frequently results
from inaccessibility of field sites (Faulkner et al. 1989)
and technological inadequacies (i.e., lack of field
deployable automated samplers). Ongoing work has shown
that, even in cases of static hydrological conditions (con-
tinuous flooding) and feedback-control of redox potential
in microcosms (Patrick 1966; Patrick et al. 1973), struc-
tured changes in redox potential can be large in natural
sediments and in microcosms and mesocosms over hourly
and daily scales (Catallo unpublished data, 1996).

From a signal analysis perspective (Chen 1989;
Benedetto and Frazier 1994), these studies showed that
experimental and analytical variables interact to determine
the magnitude and response of redox potentials measured
in the field. Important factors included electrode type (size,
configuration, composition, junctions), potentiometric cell
construction (distance between working and reference elec-
trodes, use of salt bridges), physicochemical effects on ref-
erence. electrodes. or salt bridge behavior (e.g., colloidal
organic matter, temperature), electrode placement (perma-
nent vs. temporary; zone of placement), and electrode
equilibration and passivation rates after placement. A lim-
ited example of the effect of electrode equilibration on

redox potential measurement indicates that variation in mea-
sured redox potential may result from experimental ma-
nipulation rather than from changes in natural conditions
(Fig. 1). The data are from a redox potential time series
from a combination electrode inserted approximately 10
cm into saturated, anaerobic, salt marsh mud. The intro-
duction of the electrode (accompanied by outgassing of
sulfides) apparently caused relative oxidation of the sam-
pling zone which resulted in artificially high potential val-
ues for many hours after placement.

After nearly 24 h, the measurement zone equilibrated
with the bulk phase and stable redox potential values were
obtained. The exponential character of this equilibration
was close to what was predicted using a simple model of
aqueous sulfide diffusion across the sediment volume
sampled by the electrode (Armstrong 1975). Thus, discrete
potential values measured for many hours after electrode
placement reflected conditions resulting from experimen-
tal manipulation of the system rather than the true system
properties of interest. This problem would be compounded
if the experimental design featured repeated use of tempo-
rary electrodes after insufficient equilibration times, or if
the equilibration times were different in different parts of
the system (e.g., streamside vs. marsh interior) or at differ-
ent times of the day or year (moming vs. noon, winter vs.
summer).

These and other data suggested that rapid, periodic fore-
ing (e.g., diurnal tides) could elicit significant variation in
redox potentials at hourly or daily scales. While there has
heen substantial effort devoted to providing data on bio-
geochemical process variability in sediments and organic
mats (Patrick and Wyatt 1964; Bailey and Beauchamp
1971; Sorensen 1974; Whisler et al. 1974; Gleason and
Zieman 1981; Howes et al. 1981; Smith and Patrick 1983;

210 — =

Sample Rate = 55h

Equltriam Potent:
57 mv

Potential (Vo va AGIARC

220 ae

° 8 Time h) em 2

Fig. 1. Effect of equilibration time after placement on redox
potential readings from electrodes.. Samples were logged at 56
samples/h and 1800 data points are shown (thick line). Also given
is a negative exponential fit of the data points (thin line). The
electrode was a calibrated ORP electrode (combination Pt-Ag/
‘AgC! reference). Sampling in our studies was begun after a 24 (or
48) h equilibration period.


Suggested citation
Catallo, W.J. 1999. Hourly and daily variation of sediment redox potential in tidal wetland sediments. U.S. Geological
Survey, Biological Resources Division Biological Science Report USGS/BRD/BSR-1999-0001. 10 pp.


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4, TITLE AND SUBTITLE 5. FUNDING NUMBERS

Hourly and Daily Variation of Sediment Redox Potential in Tidal Wetland Sediments

6. AUTHOR(S)
W. James Catallo

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESSES 8. PERFORMING ORGANIZATION
U.S. Department of the Interior REPORT NUMBER
U.S. Geological Survey USGS/BRD/BSR--1999-0001

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13, ABSTRACT (Maximum 200 words)

Variation of electrochemical oxidation-reduetion (redox) potential was examined in surfa under conditions of flooding and tidal
simulation in mesocosms and field sites, Time series were generated of redox potential measured in sediment profiles at 2-10 em depth using combination Pt-
Ag/AgCl (ORP) clectrodes. Redox potential data were acquired at rapid rates (1-55 samples/h) over extended periods (3-104 days) along with similar times
series of temperature (water, ait, soil) and pHl. It was found that redox potential varied as a result of water level changes and was unrelated to diurnal changes
in temperature or pH, the latter of which changed by <0.5 units over the tide cycles. In closed, hydrostatic microcosms isolated from atmospheric oxygen,
development of negative redox potentials proceeded rapidly (370 mV redox potential decrease in under 48 h). Attenuation of microbial activity by d-radiation
and toxic chemicals eliminated this response. In tidal salt marsh mesocosms where the sediment-plant assemblages were exposed to a simulated diurnal tide,
redox potential oscillations of 40-300 mV amplitude were recorded that had the same periodicity as the flood-<train eycle. Periodic redox potential time series
were observed repeatedly in sediments receiving tidal pulsing but not in those sediments exposed to static hydrological conditions, Data collected over 12 days
from a coastal marsh site experiencing diumal tides showed similar fluctuations in redox potential. Data from these experiments indicated that (a) redox
potential can be a dynamic, nonlinear variable in coastal and estuarine wetland sediments over hourly and daily scales, and the designs of biogeochemical

experiments should reflect this, (b) redox potential can change rapidly and significantly in coastal wetland sediments in response to flooding and draining, (c)
.dox potential in wetland sediments, and elimination or inhibition of microbial
rience

salt marsh sediment

microbial community processes are primary determinants of the time course of
activity (e.g, by pollutants) can significantly alter that behavior, and (d) fast redox potential dynamics appear to be characteristic of sediments that exper
changes in hydrology. The rapid redox potential changes observed in these systems indicated dynamic metabolic and biogcochemical conditions in the field,
and confirmed that hourly and daily redox potential variations should be resolved in studies of sediment functioning,

15, NUMBER OF PAGES

14, SUBJECT TERMS (Keywords) iv +10
iv +10 pp.

redox potential, coastal sediments, time series, ecological function
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Hourly and Daily Variation of
Sediment Redox Potential in
Tidal Wetland Sediments

Biological Science Report
USGS/BRD/BSR-1999-0001
March 1999



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VARIATION OF SEDIMENT REDOX POTENTIAL IN TIDAL WETLAND SEDIMENTS 7

70} A
3 Samp atest
z
2 an
2
pan > Surface Electrode:
i Boctiane
. —* Surface Electrode:
a a a
Tine ours
2 no ser
B
ae psa
. Nou
Fa i| ae
é I ser
i [tem
4 ‘stuns
Wea (ramonie) 06
° —s 0
Ped

Fig. 8. A. Redox potential time series from ORP electrodes in
tidal and back marsh (flooded nontidal) zones of a saline coastal
marsh in Louisiana. B. Spectral analysis of the redox potential
time series shown in part A.

Discussion

Data from the experiments summarized above indicated
that (a) redox potential can be a dynamic, nonlinear vari-
able in wetland sediments over hour and day time scales,
and sampling/data handling strategies should take this vari-
ability into account, (b) periodic and other nonlinear
changes in redox potential can be detected reproducibly
by using the potentiometric systems described here, (c)
redox potential can change quickly and significantly in re-
sponse to tidal flood-drain cycles (e.g., 40-300 mV/12 h)
and events that attenuate oxygen supply to the sediment
(e.g., static flooding, >350 mV/48 h), and (d) microbial
processes and hydrologic variability are primary determi-
nants of the rapid changes in sediment potential, and elimi-
nation or inhibition of microbial activity (e.g., by pollut-
ants) or alteration of hydrology can significantly alter that
behavior. The importance of microbial respiratory processes
to redox potential agrees with previous field work show-
ing significant alteration of potential in contaminated ver-
sus pristine sediments resulting from toxicity to sediment
microflora and meiofauna (Koepfler and Kator 1986;
Catallo and Gambrell 1987).

The rapid potential changes observed in the microcosms
and mesocosms were significant from a biogeochemical
perspective. For example, the static microcosms were aero-
bic at the beginning of the runs, but they became anaerobic
within several hours, and highly reducing soon thereafter.
Thus, the observed potential change corresponded to rapid
changes in microbial metabolic status and bioenergetics
(i.e., aerobic vs. anaerobic metabolism) and the oxidation
states of metabolic endpoints (i.e., H,O/CO, in aerobic

processes vs. organic acids and sulfides in anaerobic pro-
cesses). This transition from aerobic to highly reducing con-
ditions occurred in under 48 h. In tidal mesocosms, the
magnitude of change (40-160 mV) was sufficient to alter
the biogeochemical processes. Changes of 50 mV can cor-
respond to changes in the processing rate and disposition
of organic matter, nutrients, and toxic sulfur species
(DeLaune et al. 1976). The equilibration time for these bio-
geochemical changes can be rapid; for example, upon flood-
ing and development of anaerobic conditions, denitrifica-
tion half-times for NO,-N (107-163 mg/g) were on the or-
der of 15-50 h in numerous soils and sediments (Reddy et
al. 1978). Thus, the oscillating redox potential profiles
observed in the tidal simulation experiments can provide
for biogeochemical conditions not observed in more static
systems, for example, rapid redox cycles of elements.
Clearly the redox potential dynamics in these experimen-
tal systems would not have been observed by using dis-
crete sampling designs (e.g., | sample/day or 1/week), nor
would the data be appropriately handled using general lin-
ear statistical models. These observations bear directly on
the design and sampling strategy of ecological microcosms,
creation/remediation of wetland environments, and field
studies for biogeochemical and pollutant fate evaluation.

The life cycles, ecological functioning, and productiv-
ity of species in many ecosystems are influenced by physi-
cochemical cycles operating over a wide range of tempo-
ral scales (Ziser 1978; Odum 1983a; Odum 1983b; Sklar
1985; Lloyd and Stupfel 1991; Mann and Lazier 1991;
Bevers and Odum 1993). Process “pulsing” at micro to
global scales is thought to be central to ecosystem integra-
tion and homeostasis (Odum 1983b). Much work in mi-
crocosms and mesocosms (Beyers and Odum 1993) em-
phasizes the importance of periodic inputs (temperature,
light, physical disturbance) as drivers of biological pro-
cesses, with nonlinear or oscillating process outputs typi-
cally resulting (e.g., diurnal 0,/CO, fluxes). Periodic and
other nonlinear disturbances are major determinants of bio-
logical community structure and diversity in natural sys-
tems (Odum 1983b; Reice 1994; Schmidt-Nielson 1994).
Therefore, field and microcosm studies and environmen-
tal restoration attempts must address these physicochemi-
cal cycles within experimental designs and models, par-
ticularly if critical process rates or endpoints are directly
influenced. Thus, endeavors including design of an effec-
tive microcosm or created ecosystem would require know|-
edge of, and the ability to provide, the appropriate forcing
needed to influence process in ways characteristic of natu-
ral systems (Reddy 1993).

Acknowledgments
The author is indebted to Robert P. Gambrell, Ronald
D. DeLaune, Gene Tumer, Irving A. Mendelssohn, Pat
Crawford, H. L. (Dharme) Dharmasena (data logger


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