Table Of ContentInterstitial Dissolved Organic Carbon in Sediments of a Southern Appalachian Headwater Stream
Author(s): M. Tad Crocker and Judy L. Meyer
Source: Journal of the North American Benthological Society, Vol. 6, No. 3 (Sep., 1987), pp.
159-167
Published by: Society for Freshwater Science
Stable URL: http://www.jstor.org/stable/1467507
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J. N. Am. Benthol. Soc., 1987, 6(3):159-167
? 1987 by The North American Benthological Society
Interstitial dissolved organic carbon in sediments of a
southern Appalachian headwater stream
M. TAD CROCKER AND JUDY L. MEYER1
Zoology Department and Institute of Ecology, University of Georgia,
Athens, Georgia 30602 USA
Abstract. This study had two objectives:( 1) to compare seasonal and spatial patterns of water-
column dissolved organic carbon (DOC)a nd sediment interstitialD OC (IDOC)c oncentrationsa t a
headwater spring seep, and (2) to explore interactionsb etween sediment organic matter content,
benthic bacterialb iomass and production, water column DOC concentration,a nd sediment IDOC
concentrationa nd composition (high versus low molecularw eight components).
Fort he first objective,s ediment organic matterc ontent, IDOCc oncentration( sampledf rom small
wells by dialysis or by pipet), water column DOC concentration,a nd benthic bacterialb iomassw ere
measureda t a spring seep in Coweeta Hydrologic LaboratoryM, acon Co., North CarolinaU SA, for
two years. For the second objective, organic matter content was increased or decreased in experi-
mental sediments. The same variablesl isted above were measuredw ithin three weeks of sediment
manipulationa nd again after five and a half months. Benthic bacterialp roduction was measured
one week after sediment manipulation.
Results indicate benthic organic matteri s a source of IDOC,t he concentrationo f which depends
on sediment organic matter content, the relative proportion of high and low molecular weight
IDOCc ompounds,a nd the exchange of interstitiala nd water column DOC. High molecularw eight
IDOC (>50,000) accumulatesi n sediments in proportiont o sediment organic matterc ontent. Low
molecular weight IDOC (<50,000) concentrations are lower than high molecular weight IDOC
concentrationsa nd are independent of sediment organic matterc ontent. However, low molecular
weight IDOC concentrationsa re generally greater than water column total DOC concentrations.
Low molecular weight IDOC concentrationsa re greater during the dormant season than during
the growing season.
Keyw ords: streams,D OC,h yporheos, organic matter,d ecomposition,b acterialb iomass,b acterial
production,d ialysis, Coweeta Hydrologic Laboratory.
Studies of the distribution and dynamics of al. (1981) and Hynes (1983) suggested that
dissolved organic carbon (DOC) in streams have groundwater DOC inputs may be trophically
considered the following aspects of water col- important through microbial metabolism of
umn DOC: its fluvial patterns, mass balance in these compounds.
stream reaches and watersheds, and within- Another potentially important source of DOC
stream transformations (e.g., Fisher and Likens to this interstitial DOC pool is the gradual de-
1973, Kaplan et al. 1980, McDowell and Fisher composition of particulate organic matter (POM)
1976). Stream sediments have been shown to buried in stream sediments (Meyer et al. 1987b).
influence water column DOC by physical ad- The existence of DOC concentration gradients
sorption and microbial utilization of DOC ranging from <1 mg/L in the water column
(Dahm 1981) and by photosynthetic production (e.g., Moeller et al. 1979), to >10 mg/L within
of DOC (Kuserk et al. 1984). Recent develop- anaerobic leaf accumulations (C. N. Dahm, Uni-
ments in the study of hyporheic communities versity of New Mexico, personal communica-
have attracted attention to the distribution and tion), to >1000 mg/L within decaying and an-
dynamics of DOC in sediment interstitial spaces. aerobic leaf tissues (D. L. Lawson, Michigan State
One potentially important source of interstitial University, personal communication) supports
DOC to stream sediments is groundwater. Based the idea that long-term particulate organic mat-
on DOC concentration differences between ter decomposition is a source of DOC. Linkages
groundwater and seep water and the impor- between POM decomposition, DOC produc-
tance of groundwater to streamflow, Wallis et tion, and microbial metabolism have been made
in several recent studies in a variety of systems
1T o whom correspondences hould be addressed. (Cole 1982, Cole et al. 1982, Kato 1984, Kepkay
159
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160 M. TAD CROCKERA ND JUDY L. MEYER [Volume 6
and Andersen 1985, Murray and Hodson 1985). bacterial biomass and production, water col-
Thus, the production of DOC within interstitial umn DOC concentration, and IDOC concentra-
spaces of sediments by the decomposition of tion and composition (high and low molecular
buried POM may be an important trophic link weight components) by altering sediment or-
between the riparian vegetation and the ben- ganic matter content and observing subsequent
thos, and an important within-stream source of changes in IDOC concentration and composi-
water column DOC, especially in headwater tion, bacterial biomass, and bacterial produc-
streams with extensive organic matter stores. tion.
If decomposition of stored POM is a source
of interstitial DOC (IDOC), then the rate of IDOC Study Site
production should be directly coupled to the
rate of POM decomposition. If IDOC generation We studied Cold Spring, a headwater seep of
from stored POM is rapid relative to its fluvial, Dryman Fork, located within an oak-hickory
chemical, and biological loss, then the concen- forest at the Coweeta Hydrological Laboratory
tration of IDOC compounds in sites where POM in Macon County, North Carolina USA. Ele-
has accumulated should be higher than stream vation at Cold Spring is ca. 1245 m and wa-
water DOC concentrations. The microbial com- tershed area is 3.4 ha (Wallace et al. 1982). This
munity will take up some fractions of DOC pref- site was described in detail by Ross and Wallace
erentially, and if microbial uptake is rapid rel- (1982) and Wallace et al. (1982). Study sites were
ative to fluvial IDOC losses, IDOC should located at 0, 7, 12, and 32 meters from the head-
contain a predominance of refractory com- water seep. Stream width at these sites was 2.5,
pounds. To the extent that these conditions are 2.6, 2.8 and 2.0 m, respectively. Stream depth
met, IDOC will represent a within-stream pool was usually 3-5 cm at all sites. Sediments were
of predominantly refractory DOC being gen- composed of gravel and organic matter, includ-
erated in proportion to the amount of stored ing leaves, sticks, branches, and logs. Sediments
organic matter and being supplied to the water in the seep also contained dark, fine soil. Al-
column by diffusion, convection, and physical though interstitial oxygen concentrations were
and biological perturbation of the habitat. The not measured, we observed no indication of an-
observed concentration of IDOC will reflect the aerobic conditions. Dye studies in Coweeta
net difference between rates of generation (POM streams indicate rapid exchange of water be-
leaching) and loss (microbial uptake and export tween the water column and sediments (Nancy
to the water column). Munn, University of Georgia, unpublished data).
Meyer and Tate (1983) observed water col- The seep site (0 m) is referred to here as "the
umn DOC concentrations to be greater during upstream site". The 7 and 12 m sites rarely dif-
the growing season than during the dormant fered for the variables measured (Crocker 1986),
season in a southern Appalachian stream. Sea- and we refer to them collectively as "the down-
sonal differences in temperature, stream flow, stream site". The site at 32 m was used for ex-
organic matter inputs and decomposition influ- perimental studies.
enced the production, metabolism, and trans- Stream discharge, measured at a culvert 62 m
port of DOC. These factors are also expected to from the seep, ranged from 0.02 L/s during Oc-
influence rates of generation and loss of IDOC tober to 2.10 L/s during May. Seasonal variation
and, hence, IDOC concentrations. in water column temperature was least at the
This study addressed temporal and spatial as- upstream site (7.9-11.0?C) and greatest at the
pects of water column DOC and sediment IDOC experimental site (8.0-17.6?C). No detectable dif-
concentration with reference to mechanisms re- ferences between sediment (0-10 cm) and water
sponsible for concentration patterns. It includ- column temperatures occurred on the two oc-
ed seasonal and spatial measurements of water casions when we measured both.
column DOC and IDOC concentration, water
column and sediment temperature, and sedi- Methods
ment organic matter content and bacterial bio-
mass in a small headwater spring seep. Also, DOC and IDOC samples were taken period-
we experimentally explored interactions be- ically from October 1982 through September
tween sediment organic matter content, benthic 1984. DOC samples were taken by pipet from
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1987] INTERSTITIALD OC IN A STREAM 161
running water overlying a glass slide placed on pore size in a laboratory study using non-sterile
the sediments. Samples (25 ml) were stored in 2000 and 250,000 molecular weight polyacrylic
pre-ashed (450?C, 12 hr) glass vials at 4?Cu ntil acid solutions (Crocker 1986). For this study, we
analysis. Samples were filtered through pre- use the phrase "low molecular weight" to refer
ashed Gelman AE glass fiber filters (nominal to compounds which entered the dialysis bags,
pore size 0.3 um) and analyzed for DOC con- and the phrase "high molecular weight" to re-
centration usually within two days of collec- fer to compounds excluded by dialysis.
tion. DOC and IDOC concentrations were mea- Leaching from dialysis bags was negligible.
sured by persulfate catalyzed UV-oxidation with After 18 d, DOC concentration was the same in
a Dohrman DC 54 carbon analyzer. beakers of filtered stream water with or without
IDOC samples were taken by dialysis and dialysis bags, and DOC concentration inside the
analyzed as above, but were not usually filtered bags was less than outside. On one occasion, we
through glass fiber filters. Dialysis samplers stained dialysis membranes that had been in
(modified from designs by Hesslein 1976 and the field for two weeks to qualitatively observe
Mayer 1976) consisted of glass tubes, each con- the extent of bacterial colonization. Gram pos-
taining a dialysis bag, pressed 8-10 cm into the itive and gram negative forms were found on
sediments and capped with a foil-covered stop- the membranes.
per. Interstitial water entered the glass tubes Sediment samples for organic matter analysis
through several 1 mm pores in the lower 2 cm and bacterial counts were taken with a plastic
of each glass tube. Dialysis bags, filled with 15 corer (12 mm diam.) to a depth of 2 cm. Several
ml deionized water, were made from Spectra/ core samples were pooled and mixed in a bea-
Por 6 cellulose dialysis tubing (Spectrum Med- ker. Five sub-samples for organic matter con-
ical Industries, Inc., Los Angeles, California tent and three or four sub-samples for bacterial
USA). Dialysis membranes were boiled for 10 abundance were removed with a plastic 1.3-ml
minutes in a sodium bicarbonate/EDTA solu- spatula. Sediment organic matter content was
tion and for 10 minutes in deionized water. The estimated as dry mass lost upon combustion
inside and outside walls of the cleaned tubing (450?C, >12 hr).
were rinsed in deionized water. Sediment bacterial abundance was estimated
Dialysis bags were held in the glass tubes by by epifluorescent direct counts of cells dis-
a glass rod the same diameter as the tubes. The lodged from sediments with a tissue homoge-
rod also prevented the creation of a new en- nizer (Findlay et al. 1986) and stained with
vironment within the well near the bag. After acridine orange (Hobbie et al. 1977). Cell abun-
an equilibration period of 6 to 14 d, water with- dances were converted to biomass (mg Bact-C/
in the dialysis bags (10-15 ml) was removed m2) based on a weighted average cell volume
with a glass hypodermic syringe. IDOC samples of 0.13 ,um3 for Coweeta sediment bacteria (J. L.
were filtered only if particles were visible in Meyer, unpublished data) and 2.2 x 10-13 g
the dialysis bags (about 5% of the time). Bact-C/lMm3 (Bratbak 1985).
The length of time necessary for the sampler To evaluate the relationships between organ-
to reach equilibrium with the surrounding water ic matter, the microbial community, and IDOC,
was determined in both field and laboratory we manipulated sediment organic matter con-
incubations. In the laboratory, samplers equil- tent at the experimental site during July 1983
ibrated with the surrounding stream water in and March 1984. Sediments from a 1-m2 plot in
2 d (Crocker 1986). In sediments of Hugh White a depositional area on the upstream side of a
Creek (Coweeta Hydrologic Laboratory), equil- fallen tree were removed to a depth of 10 cm
ibration was observed after 6 d (Crocker 1986). or to the bedrock substrate. The cavity was di-
Seven days was generally used as the equili- vided into three adjacent 33 x 100 cm sub-sites.
bration period for this study. One third of the sediments was mixed by hand
The nominal molecular weight cut-off of the and returned to the upstream sub-site serving
dialysis tubing was 50,000 (Spectrum Medical as the medium organic content treatment.
Industries, Inc.). Recognizing the low resolu- Another third (low organic content) was com-
tion of filtration by dialysis, especially of bio- busted at 525?C for nine hours and replaced
degradable membranes used under non-sterile downstream of the medium organic content
field conditions, we confirmed this approximate sediments. To the last third (high organic con-
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162 M. TAD CROCKERA ND JUDY L. MEYER [Volume 6
TABLE 1. Sediment organic matter content (% AFDM of total sediment dry mass), sediment bacterial
biomass,D OCc oncentration,a nd IDOCc oncentrationa t the unmanipulatedu pstreama nd downstreams ites.
Growing season is May-Octobera nd dormants eason is November-April. Values are mean + 95%C I with n
in parentheses.
Upstream Downstream
%A FDM 4.6 ? 0.4 (40) 1.5 + 0.1 (49)
Bacterialb iomass (mg C/m2) 249 + 46 (19) 177 + 13 (24)
DOC (mg C/L)
Growing season 0.39 + 0.17 (5) 0.46 + 0.18 (7)
Dormants eason 0.44 + 0.19 (7) 0.58 + 0.30 (6)
IDOC (mg C/L)
Growing season 0.77 + 0.13 (26) 0.98 ? 0.12 (42)
Dormants eason 1.69 ? 0.41 (27) 1.31 ? 0.27 (15)
tent) we added dried, crushed and leached leaf In August 1984, five and a half months after
material (predominantly Quercus, Carya, and the initial treatment, we sampled interstitial
Aesculus) collected from the nearby forest floor. water from the glass tubes by pipet just before
IDOC glass sampling tubes (four during the July inserting dialysis bags and again after removing
experiment and three during the March exper- the bags one week later. These samples were
iment) were placed in each treatment plot to treated as described above for water column
sample IDOC at 6-8 cm depth. These tubes re- samples. They constitute "total IDOC" which
mained in place for the duration of each ex- includes compounds sampled by dialysis and
periment. In addition to IDOC concentration, compounds excluded by dialysis. Sediment or-
we measured discharge, water temperature, ganic matter content and benthic bacterial bio-
sediment organic matter content, and bacterial mass were also measured in samples from the
biomass in the top 2 cm as described above. 6-8 cm core band (the level of IDOC samples).
Samples were collected for two to three weeks Statistical significance was defined at the 0.05
after treatment during the July experiment, de- level throughout this work.
pending on the sample type.
The March experiment was conducted in two Results
phases. At the end of the first week post treat-
ment, the dialysis bags were removed and IDOC Sediments at the upstream site always con-
concentration was measured as described above. tained more organic matter than at the down-
Cores from the top 2 cm of each sub-plot were stream site (t-test, Table 1). Benthic bacterial
pooled, mixed, and sub-sampled to determine biomass was also greater at the upstream site
organic matter content, bacterial biomass, and (t-test, Table 1). No consistent seasonal (t-test)
bacterial production. Sediment organic matter or site (paired t-test) differences in water col-
content and bacterial biomass were estimated umn DOC concentration were observed (Table
as described above. Bacterial production was de- 1). IDOC concentration was greater during the
termined by measuring incorporation of 3H- dormant season (November-April) than during
thymidine into bacterial DNA as adapted for the growing season (May-October) at both sites
stream sediments (Findlay et al. 1984). This (t-test, Table 1). The two sites showed no sig-
method includes a DNA extraction so that only nificant difference in mean IDOC concentration
thymidine incorporated into bacterial DNA is over either season (Table 1). IDOC concentra-
counted. The method also uses an isotope di- tion was greater than water column DOC con-
lution procedure to assess the size of the non- centration at both sites during both seasons
radioactive thymidine pool and hence more (t-test, Table 1). When increases or decreases in
accurately determine specific activity. All di- IDOC concentration occurred between succes-
lutions were done in triplicate and the data sive sampling days, DOC concentration often
combined to yield a single value for production increased or decreased in a similar manner (Fig.
in each plot. 1).
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1987] INTERSTITIALD OC IN A STREAM 163
Lack of treatment replication restricts our 10.0
statements concerning differences between 8.0. .
treatments to statements concerning the plots 6.0 0
only. Comparisons of plot means were made 4.0. U
using ANOVA followed by a PSD (protected 2.5-
least significant difference test, Snedecor and -NJ
Cochran 1980) across POM treatment within ex- oC ] 2.0- 0
periments. Because several sediment cores from E
0
each site were pooled and sub-sampled, within-
0
site variability is unknown for %o rganic matter 0 1.5
content, bacterial biomass, and bacterial 0 00
production. Confidence intervals (95%) are re- 00
ported to indicate the variability of these sub- o 1.0- m0 0
samples, and this variability is used to approx- 000 ) 0 O 0
mimeaatnes w. ithin-plot variability for comparisons of .5 -[ ]0 0. U UU 00 *] .0
The organic matter content of POM treatment
plots for each experiment is given in Table 2. .V I I
Rapid recovery of organic matter on the surface Dormant Growing Dormant Growing
of combusted sediments was due to a dark layer 1982 1984
SEASON
of fine material which settled onto the sedi-
ments. Microscopic examination of this material FIG.1 . Time course of mean IDOC and DOC con-
revealed a predominance of invertebrate and centration at the upstream site beginning in October
copepod fecal material, leaf and wood litter, 1982 and ending in October 1984. The dormant season
was November-April and the growing season was
harpacticoid copepods, fungal hyphae, bacteria,
May-October. Circles represent IDOC and squares
amorphous material, and few mineral grains. represent DOC. Means were calculated from 4 to 5
Bacterial biomass differed significantly among IDOCs amplersa nd triplicatea nalyseso f a single DOC
all POM treatment plots during each experi- grab sample.
ment (Table 3, ANOVA-PSD on log-trans-
formed data). Within two weeks bacterial bio-
mass was generally proportional to organic IDOC concentration of samples taken by di-
matter content of these experimental sedi- alysis did not differ significantly between POM
ments. treatment plots during the third week of the
Bacterial production in each plot was mea- July experiment or during the first week of the
sured after one week in the March experiment March experiment (Table 3, ANOVA on log-
(Table 3). Lowest bacterial production was found transformed data). Five and a half months after
in low POM sediments. Because only single the March treatments, mean IDOC concentra-
measurements were made for each treatment, tion of samples collected by dialysis did not
comparisons of production values are based on differ significantly between POM treatments
the standard error of this method for sediments (Fig. 2, ANOVA). However, the concentration
(30% of value, Findlay et al. 1984). of total IDOC, which includes compounds sam-
TABLE2 . Organic matter content (% AFDM of total sediment dry mass) of the top 2 cm in experimentally
altered stream sediments. Sediments were collected from 6-8 cm depth on 1 October 1984. The July experiment
began on 31 July 1983 and the March experiment began on 19 March 1984. Values are mean + 95% CI. Sample
size equals 5 unless shown otherwise in parentheses.
July Experiment March Experiment
Treatment 31 July 1983 13 August 1983 19 March 1984 26 March 1984 1 October 1984
Low POM 0.24 + 0.17 (4) 1.17 + 0.26 0.14 + 0.02 0.70 + 0.29 (4) 1.10 + 0.08
Medium POM 2.14 ? 0.98 (4) 2.42 + 0.79 3.23 + 1.94 3.02 + 0.57 (4) 2.34 + 0.99
High POM 3.14 + 3.69 (4) 3.10 + 1.29 8.23 + 3.82 11.44 + 9.91 (4) 7.31 + 0.82
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164 M. TAD CROCKERA ND JUDY L. MEYER [Volume 6
TABLE3 . Bacterial biomass, bacterial production, and IDOC concentration following sediment manipula-
tion. In the July experiment bacterial biomass samples were collected two weeks after treatment and IDOC
samples were taken three weeks after treatment. In the March experiment all samples were collected one
week after treatment. Values are mean + 95% CI. Sample size was 4 in the July experiment and 3 in the
March experiment.
Bacterial Bacterial
Biomass Production IDOC
Experiment Treatment (mg C/m2) (mg C m-2 h-1) (mg C/L)
July 1983 Low 96 ? 37 n.d.a 0.69 ? 0.17
Medium 244 ? 92 n.d. 0.67 ? 0.12
High 148 + 23 n.d. 1.00 + 0.47
March 1984 Low 44 + 22 0.4 1.49 + 2.13
Medium 93 + 108 1.1 3.71 + 6.87
High 165 + 87 1.3 1.43 + 1.45
a Not determined.
pled by dialysis and compounds excluded by ident (Fig. 2). These observations show that a
dialysis, was significantly different among all substantial portion of total IDOC is excluded
POM plots (ANOVA-PSD) and increased with by dialysis membranes with a nominal molec-
increasing organic matter content of the 6-8 cm ular weight cut-off of 50,000. Furthermore, these
band on both sampling dates (Fig. 2). high molecular weight compounds accumulate
in proportion to the amount of sediment or-
Discussion ganic matter present (Fig. 2), as would be ex-
pected if leaching rate is proportional to organic
Differences in the nature of IDOC sampled matter content and if the rates of loss of high
by pipet and IDOC sampled by dialysis are ev- molecular weight compounds (via fluvial and
microbial paths) are relatively low and inde-
8- pendent of organic matter content. In contrast,
lower molecular weight IDOC sampled by di-
7-
alysis probably contains a greater proportion of
6- AUGUST2 3 compounds available to microbes. In a related
experiment where leaf leachates were added to
0 sediments, Crocker (1986) observed elevated
E 4- AUGUST 31 IDOC concentrations of samples taken by di-
alysis only from combusted sediments with re-
0 3-
duced bacterial biomass. Also Meyer et al.
2.- (1987a) found greater uptake and bacterial
I.^'.^ ^^~ DIALYSIS growth in the <1000 mol. wt. DOC fraction of
stream water from Coweeta. Thus, sampling by
dialysis selects a component of total IDOC which
0 1 2 3 4 5 6 7 8 9 10 is more available to the microbial community.
%AFDM Water column DOC concentrations were sim-
FIG. 2. IDOCc oncentrationv s. organicm atterc on- ilar to values reported for other Coweeta streams
tent (% AFDM) of experimental sediments 6-8 cm (Table 1; Meyer and Tate 1983, Meyer et al.
deep five and a half months after treatment.P oints 1987b, Tate and Meyer 1983, Wallace et al. 1982).
are means of three samples. Errorb ars show + 1 SD. Sediment organic matter content at all sites was
The lines are plots of least squaresl inear regressions
of the means. The upper two lines are for total IDOC typical of other Coweeta streams (Table 1; %
and the lower line is for IDOC samples taken by organic matter range 1%-20%;J . L. Meyer, un-
dialysis during the period between 23 and 31 August published data). Benthic bacterial biomass (Ta-
1984.W aterc olumn DOCc oncentrationsw ere 5.3 mg ble 1) was similar to that in another stream at
C/L and 0.56 mg C/L on 23 August and 31 August, Coweeta (16-169 mg C/m2, Meyer et al. 1987b).
respectively. The clearest relationship between bacterial bio-
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1987] INTERSTITIAL DOC IN A STREAM 165
mass and sediment organic matter content is was independent of organic matter content in
one of increasing bacterial biomass with in- both undisturbed sediments (Table 1) and in
creasing organic matter content of the sedi- experimental plots (Tables 2, 3, Fig. 2). Microbes
ments (Tables 1-3), although the July 1983 me- influence IDOC released during organic matter
dium POM treatment is an exception to this decomposition by metabolizing labile com-
trend (Table 2). These data do not differ signif- pounds. Refractory IDOC remains.
icantly from the regression of bacterial biomass Higher concentrations of low molecular
on % AFDM reported from sandy sediments of weight IDOC during the dormant season may
a blackwater river (fig. 2 in Findlay et al. 1986), be related to higher IDOC production rates as-
although all of our data points fall below that sociated with increased organic matter supply
best-fit regression line. Benthic bacterial pro- at that time (Crocker 1986) and to the decom-
duction seven days after sediment manipula- position of relatively new organic matter. Even
tion during the March experiment was also though microbial metabolism has been shown
within the range observed in a blackwater river to increase with organic matter supply (Graff et
(table 4 in Findlay et al. 1986). In contrast to al. 1982, Molongoski and Klug 1980), the rate
bacterial biomass, all estimates of bacterial pro- of IDOC consumption may lag behind the rate
duction fall above the best-fit regression line of IDOC production, as has been shown for
for bacterial production on % AFDM from the water column DOC (McDowell and Fisher 1976,
blackwater river (fig. 2 in Findlay et al. 1986). Meyer et al. 1987b), particularly under the low-
Two basic patterns were revealed by studies er temperatures of the dormant season.
at the unmanipulated sites. First, the concen- Seasonal differences in water column DOC
tration of IDOC less than 50,000 molecular concentration were not observed (Table 1).
weight was generally greater than water col- Stream sediments, including those at the ex-
umn total DOC concentration (Table 1, Fig. 1). perimental site, have been shown to remove
Differences between total IDOC and water col- some DOC from the water column rapidly
umn DOC would be even greater. Water sam- (Crocker 1986, Dahm 1981, Kuserk et al. 1984,
ples taken by pipet from the glass tubes at the Lush and Hynes 1978, McDowell and Fisher
manipulated site indicated that the concentra- 1976). Other compounds produced during or-
tion of total IDOC was greater than the dialyzed ganic matter decomposition, such as humic and
samples (Fig. 2). Second, although there was no fulvic substances, are less available to benthic
seasonal difference in water column DOC con- microbial communities and will be released from
centration, IDOC concentration was greater the sediments by fluvial processes. Water in the
during the dormant season (Table 1). Data from headwater stream of Dryman Fork was shallow
the experimental study are useful in interpret- (3-5 cm), flowed slowly (total stream velocity
ing the mechanisms behind these observed pat- ca. 0.2-2 cm/s), and was exposed extensively to
terns. sediments. Seasonal variation in DOC concen-
The observed concentration and molecular tration may not have been observed owing to
weight distribution of IDOC is the net result of the ability of these sediments to remove labile
DOC generation during organic matter decay DOC from the water column, and to the dilu-
and benthic microbial metabolism of the lower tion of refractory compounds by increased
molecular weight compounds. Total IDOC con- stream volume during the dormant season.
centration and bacterial biomass increased with A direct coupling between water column DOC
increasing organic matter content (Tables 1-3, and IDOC was not shown by the data on relative
Fig. 2) indicating both increased DOC genera- concentration and seasonal patterns. This was
tion when sediment POM content is higher and potentially due to differences in sampling
a bacterial response to increased DOC supply. method, since water column samples were in-
In contrast, the relationship between organic stantaneous measures and dialysis IDOC sam-
matter content and low molecular weight IDOC ples were taken over a 6-14 d equilibration pe-
concentration was not simple (Fig. 2). Lower riod. Lack of coupling does not imply that
molecular weight compounds are more suscep- interactions between the water column and in-
tible to removal by the microbial community terstitial water do not occur. For example, sed-
(Kaplan and Bott 1983, Meyer et al. 1987a); hence, iments at the experimental site were in a re-
the concentration of IDOC sampled by dialysis charge area where the water column was a major
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166 M. TAD CROCKERA ND JUDY L. MEYER [Volume 6
source of interstitial water. Differences in total ogy and Coweeta Hydrologic Laboratory, United
IDOC concentration between sampling dates in States Forest Service. S. Findlay and R. Risley
August 1984 (Fig. 2) show a direct effect of water provided technical assistance with the tritiated
column DOC concentration on total IDOC con- thymidine method. L. R. Pomeroy, J. B. Wallace
centration. Both water column DOC and total and two anonymous reviewers provided useful
IDOC concentration were higher on 23 August comments on earlier drafts of the manuscript.
than on 31 August. The patterns of temporal This research was supported by NSF grant BSR
changes in DOC and IDOC concentration at the 85-14328.
upstream site were also similar to each other
(Fig. 1); but, unlike the experimental site, the
upstream site is in a discharge area where water
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