patterns of dissolved organic carbon in transport

Patterns of Dissolved Organic Carbon in TransportAuthor(s): Louis A. Kaplan, Richard A. Larson and Thomas L. BottSource: Limnology and Oceanography, Vol. 25, No. 6 (Nov., 1980), pp. 1034-1043Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2835781 .

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Limnol. Oceanogr., 25(6), 1980, 1034-1043 ? 1980, by the American Society of Limnology and Oceanography, Inc.

Patterns of dissolved organic carbon in transport'

Louis A. Kaplan Stroud Water Research Center of the Academy of Natural Sciences of Philadelphia, R.D. 1 Box 512, Avondale, Pennsylvania 19311, and Department of Biology, University of Pennsylvania, Philadelphia 19104

Richard A. Larson2 Stroud Water Research Center of the Academy of Natural Sciences of Philadelphia

Thomas L. Bott Stroud Water Research Center of the Academy of Natural Sciences of Philadelphia, and Department of Biology, University of Pennsylvania

Abstract

Two distinct patterns of dissolved organic carbon (DOC) in transport were observed in a southeastern Pennsylvania piedmont drainage basin under low flow conditions. In relatively undisturbed woodland spring seeps, DOC concentrations increased with distance from the groundwater sources as did the apparent percentage of high molecular weight DOC. Changes in pH, color, and phenolic-C paralleled those for total DOC in a seep, while the concentration of carbohydrate-C remained relatively constant. In headwater areas perturbed by humans, cattle, or waterfowl, elevated DOC concentrations decreased rapidly from point source inputs.

Dissolved organic matter (DOM), fre- quently measured as dissolved organic carbon (DOC), is an important compo- nent of the organic energy budget of temperate stream ecosystems (Fisher and Li- kens 1973; Fisher 1977) and often is the dominant form of carbon in transport (Wetzel and Manny 1977). In one undisturbed watershed DOC came predomi- nantly from surface water and groundwater (Fisher and Likens 1973), although seasonally important contributions from leaf leachates (McDowell and Fisher 1976) and algal exudates (Kaplan and Bott in prep.) have been measured else- where. Storms are a primary mechanism of DOC export from watersheds because they produce increases in both DOC concentration and discharge (e.g. Manny and Wetzel 1973). Transport during storms

I Predoctoral fellowships from the National Sci- ence Foundation and the Shell Companies Foun- dation supported L.A.K. The Boyer Research En- dowment Fund supported T.L.B. and R.A.L. Additional support for R.A.L. provided by NSF grant BMS-77-23389. National Science Foundation grant DES 77-12902 and the Stroud Foundation provided funds for the DOC analyzer.

2 Current address: Institute for Environmental Studies, University of Illinois, Urbana 61801.

may dominate DOC budgets, but low- flow conditions prevail about 75% of the year (Leopold et al. 1964) and represent periods of the greatest processing of DOC.

DOC can enter streams from diffuse or point sources. Decreases in DOC concentration downstream from point sources have been found in streams (Klotz and Matson 1978; Hynes et al. 1974), and a spring seep (Lush and Hynes 1978b). In contrast, increases in DOC concentration from diffuse sources (Lush and Hynes 1978a; Mulholland and Kuenzler 1979) or increases in the percentage of refractory DOC (Wetzel and Otsuki 1974) with distance downstream have also been described. Although biotic and abiotic processing of DOC from diffuse sources can create distinct longitudinal gradients in concentration or composition in undisturbed streams, several such systems lack clear patterns (e.g. Lewis and Tyburczy 1974; Comiskey 1978; Moeller et al. 1979). Here we describe longitudinal patterns of DOC concentration and composition during transport that are distinct and consistent, in undisturbed and dis- turbed headwater reaches of a temperate woodland stream ecosystem.

We thank A. L. Rockwell for help with 1034



DOC in transport 1035

*..- Ledyards Springk

/15 Ponid (-0

/ 11 Walton s

v Sprin?o

Pasture s

Marshj

w M(ill Srein a -woods

p watershed 6 boundary

Fig. 1. White Clay Creek drainage basin and sampling stations.

compound classification analyses and C. A. Staub for help with the data analyses.

Study sites The study sites (Fig. 1) are in a 720-ha

SE Pennsylvania piedmont watershed in the headwaters of the east branch of White Clay Creek (39053'N,75047'W). Saw Mill Spring (sta. 1-3), the primary study site, is a permanently flowing woodland seep which suffaces in a broad wetted area of 175 m2 and narrows into a spring brook, flowing 44 m from source to outlet. The spring surfaces in Worsham silt loam, a poorly drained soil on weath- ered quartz, gneiss, and schist substrate. The surface water, 1-4 cm deep, flows slowly over a loose flocculent detrital lay- er covering a sand and silt substrate. The flow varies spatially and the average residence time of water moving through the seep and spring brook is about 1 h (Table 1). Discharge increases with distance from the head of the seep as does the range of annual water temperatures.

Walton's Spring (sta. 12-14) and Led- yard's Spring (sta. 9, 10), seeps from dif-

Table 1. Physical characteristics of Saw Mill Spring stations. Discharge and residence time values based on two determinations with Rhodamine WT.

Residence time (h)

Dis- Annual Trailing charge temp edge (liters, range Leading (5.0%o of

Sta. s-1) (OC) edge Peak peak)

1 0.08 8.0-12.0 2 0.33 3.5-14.9 0.13 0.30 1.55 3 1.19 -0.2-17.2 0.47 0.85 >2.00

ferent watershed subdrainages, but also arising in Worsham soils, were used as corroborative study sites.

Point sources of DOC studied included a pasture (sta. 7), a farm pond outflow (sta. 15), and sewage outflow through a marsh (sta. 8). The pasture source is a permanently flowing spring with DOC levels similar to those of other springs, except that when cattle were present concentrations were much higher. The pond is fed by springs having DOC concentrations typical of the DOC-depleted groundwater in the drainage; downstream from it the stream traverses a grassy marsh and enters a wooded area. The marsh below station 8 drains through a shallow, nar- row channel, also into a wooded area.

Samples from White Clay Creek where the stream is third order and leaves the watershed (sta. 6) were collected from a meadow reach with riparian growth of deciduous trees, downstream from a wooded reach.

Methods and materials DOC analysis-Samples were collect-

ed in 125-ml precombusted (500?C for 6 h) glass serum bottles and stored at 50C. Two or three replicate samples were filtered through precombusted Gelman AE glass-fiber filters and the filtrates analyzed for DOC with a Dohrmann DC 54 organic carbon analyzer (precision, +2%).

Molecular weight determinations- Stirred cells with membrane ultrafilters (PTGC, 47-mm-diam Millipore) were used to separate DOC <10,000 nominal molecular weight (nmw) from the total



1036 Kaplan et al.

1500-_ Saw Mill Spring (52)

1000 -

(45)

500 -(52

stal I sta 2 sta3 3 ? 10 20 30

1500 - Ledyard's Spring

_7 (5) Q 1000 -

500 5

0

to sta 9 1 statO I 0 10 20 30

1500 - Walton's Spring

1000 (2)

500 ()(2)

stal2 stal3 1 I stal4 1 0 50 100 150

DISTANCE DOWNSTREAM (m)

Fig. 2. Increases in DOC with distance from groundwater sources. Values are annual means based on ( ) daily means (+2 SE).

DOC. To remove organic contamination from these filters, we amended the methods of Ogura (1974) and Wheeler (1976) by leaching with a 1.0% NaCl solution before the organic-free water leaching. Samples (80 ml) were placed in the stirred cell; the first 5 ml of filtrate were discarded and the next 25 ml of filtrate analyzed in the DC 54 system.

Compound classification analyses- Samples were taken in rinsed 2-liter polyethylene bottles, filtered through precombusted Gelman AE glass-fiber filters, and the color (OD300) and pH re- corded. The sample was acidified to pH 2-3 with 50% HC1, freeze-dried, the res- idue diluted with 40 ml of deionized water, and the concentrate filtered through Whatman 42 filter paper. Total carbohydrates were assayed by the phen- ol-surfuric acid method (DuBois et al.

sta 3 20W0 _h ('3)

1000 _ ii_)02 1500 4) 4

(12) (ri) ~~~~~~~~~~~~~sta 2

(12) (12)~~~~~~~~~~~~~~~~~~~~(4

A I I I I winter spring sumrmer autum

SEASON

Fig. 3. Seasonal changes in DOC at Saw Mill Spring. Values are seasonal means based on ( ) daily means (+2 SE).

1956), total dissolved phenolics by the tannin-lignin procedure (Am. Public Health Assoc. 1975). Spectrophotometric assays were conducted with a Beckman model DB-GT grating spectrophotome- ter, and a Corning model 10 pH meter and probe were used for pH determinations.

Statistical analyses-Data were analyzed for statistically significant differences with ANOVA and the Scheffe mul- tiple range test at the 0.050 level. All references to significance are at the 95% confidence level. Seasons were delimit- ed by the solstices and equinoxes.

Results At baseflow, the concentration of DOC

increased with distance from groundwater sources in all three spring seeps at all times of year (Figs. 2, 3), more than doubling in 100 m of travel. At Saw Mill Spring, the pattern of increase down-




uo Saw Mill Spring

50 _

(1) (5)

0stoi2l sto31

E

Ledyard's Spring

A 0

50

-(1)

sta 9 1 sta l l 0 10 20 30

DISTANCE DOWNSTREAM (m) Fig. 4. Increase in percentage of high molecular

weight DOC with distance from groundwater sources. Values are based on (-) daily means (+2 SE).

stream held on each of 52 days that samples were taken. Groundwater coming to the surface at the Saw Mill Spring source (sta. 1) had a concentration of ca. 600 ,mg liter-' which did not change seasonally, but the downstream sites showed significant increases in summer and autumn (Fig. 3). At stations 2 and 3 the summer and autumn values were significantly greater than the winter and spring values; at station 3 the autumn value was significantly higher than the summer value.

The molecular weight characteristics of the DOC shifted during transport, with a higher percentage of large molecules with increasing distance from the source (Fig. 4). We call molecules of <10,000 nmw "small,"2 those >10,000 nmw, "large." Any colloidial organic carbon would be included in the large size frac-

tion. Molecular weight differences between stations were not statistically significant for Ledyard's Spring, but in Saw Mill Spring, stations 2 and 3 were significantly different from station 1. In Saw Mill Spring, three additional molecular weight determinations were made on a single day, using both 1,000- and 10,000- nmw membranes; an average of 67% (range 51-78) of the DOC <10,000 nmw was also <1,000 nmw.

The concentrations of phenolic compounds and carbohydrates, as well as pH and color, based on samples taken from Saw Mill Spring and White Clay Creek (sta. 4-6), are listed in Table 2. At stations 1 through 3, the ratio of carbohydrate-C to phenolic-C decreased from 7.1 to 2.5. The carbohydrate-C and phenolic-C data separately showed no statistically significant trends, but when carbohydrate-C was expressed as a percentage of the total DOC concentration there was a distinct decrease. Color increased through the seep as did color per unit of DOC, but the only significant difference was between station 1 and 3. The pH increased significantly with distance through the seep.

The DOC at station 3 after it has tra- versed the short distance of Saw Mill Spring shares several characteristics with that in White Clay Creek. The phenolic- C levels approximate concentrations in the stream and the percentage of func- tional group carbon in the DOC pool is no longer significantly different from that at sites 4, 5, and 6 (Table 2). The DOC molecular weight pattern also ap- proaches that of the main channel, with only 65% of the molecules being <10,000 nmw.

Below point sources such as the marsh and the farm pond, concentrations of DOC at baseflow decreased sharply with distance from the source throughout the year (Fig. 5). Processing of DOC within 0.45 km of the marsh was such that no further significant differences were found in the next 0.20 km, between stations 4 and 5. At both these sites DOC concentrations increased in the summer and autumn to levels significantly higher



1038 Kaplan et al.

0) 00 kO 0= Ot ~ Cn 0. X

10 1 00 0 6 6 6 ~o +1 +1 +1 +1 +1 +1 +1 +1 co C 1>-. 00 0 o Co ,It 9 1 0

00~~~ Cd

Ut

rA ~~~~~~~00 =

~~ o - 6 O o

0 ~~~~ +1 +1 +1 +1 +1 +1 +1 +1 C. 00 I 0 t 10

Rt C6 00 c-o c qi

Ct~ ~~ 1 -{ 0O -- 0{

"d" '1 i +1 +1 +1 +1 +1 +1 +1 +1 0 0 ~ 00 m It 00 0 co) Co C'1

co 0 00 I C'1

ci 0~~~~~c

-.o co +1 +1 +1 +1 +1 +1 +1 + 0010 U I- C'1 10 0 10 "- 00 irS cq1 m

Ct ?~~~~~~~-

C1 10 o 0

oo

Cc co O

6f ?? K )

6 6 6 Cq +1 +1 +1 +1 +1 +1 +1 +1 ct~~ 1O~Ci10 10 0~

0 ~1

E t t N s 10 t Co 10

00

Cd X

N 0o

Cd Cd ~ ~ ~ C C Cd ~ 00 P C1 04

I- + + +1 +1 +1 +1 +1 +1 0C 0 m It 00 ~ C

cd0 Cd~~~~~

14 --QQ

Cd~~~~~ c'1

14 1 Q~~~~o~~~~o ~~~~~ ~ ~ c

cd~ ~ c CQ Q Cd 1 -

than at station 6. The pH increased with distance downstream (sta. 4-6) in the Saw Mill branch while color per unit of DOC, and carbohydrate-C decreased; however only the pH differences were statistically significant (Table 2). The phenolic-C was variable; the ratio of carbohydrate-C to phenolic-C ranged from 2.0 to 3.3 and the percentage of function- al group carbon in the DOC pools remained quite constant, though markedly lower than the values for the seep. Mo- lecular weight fractionation showed that three samples from a single day from the marsh outflow had a high percentage of DOC <10,000 (72 + 1%: x + SD), while water downstream from point sources had considerably less DOC of that size: 44 samples over 10 days at station 6 had 54 + 8% of DOC <10,000 nmw.

Discussion Metabolism, adsorption, precipitation,

photochemical destruction, hydrolysis, and dilution by groundwater depleted of DOC will all lead to a decrease in DOC downstream. On the other hand, if the time required to remove a class of molecules from the DOC pool by whatever mechanisms exceeds the residence time of the molecules in the reach, concentrations of that class of molecules should increase downstream with additional DOC inputs. Our data reveal two distinct patterns which result from the interaction of these factors.

In one pattern, contributions from humans, cattle, or waterfowl elevate DOC concentrations at headwater sites in the watershed, which decrease sharply in a short distance downstream. Tributary dilution effects were not applicable at station 16 below the pond and for 300 m below the marsh. Dilution by groundwater cannot be discounted (we did not measure discharge), but we suspect that metabolism or abiotic processes are the prime removal mechanisms below stations 8 and 16. However, after the rapid decrease in DOC from these point sources, we found no further changes downstream (sta. 4, 5). Possibly DOC inputs balanced outputs (steady state), or




Be low Pond Below Marsh

(8)

0 53000-

(34) 81)

nO 1 2 D1 o 4A 4-

1 20 12 DISTANCE DOWNSTREAM (km)

Fig. 5. Decreases in DOC downstream from point source inputs. Values are annual means based on ()daily means (?2 SE).

our analytical techniques were not sen- sitive enough to detect changes in the DOC pool. The higher DOC concentrations at stations 4 and 5 during summer and autumn (Fig. 6), when biological processing should be maximal, may result from additional biotic inputs at stations 7 or 8 (as at Saw Mill Spring: Fig. 3). The rapid decreases in DOC from point sources result from a self-purification phe- nomenon, which is largely biological and well documented for sugars and amino acids (Wiihrmann et al. 1975), sewage (Wiihrmann 1974; Klotz and Matson 1978), and fresh, unprocessed leaf leachates (e.g. Cummins et al. 1972; Bott et al. 1977).

The second pattern was observed in the relatively undisturbed spring seeps in forested areas of the watershed and is characterized by low concentrations of DOC which increase downstream. Spring seeps are highly productive aquatic environments with immediate and obvious terrestrial and benthic connections. De- ciduous trees growing around the seeps in the poorly drained Worsham soil are extremely susceptible to wind throw. Openings in the forest canopy thus cre- ated permit extensive growth of macro- phytes throughout the seeps during late spring and summer. In early spring and autumn, in the absence of macrophyte shading, algal blooms occur. In addition,



1040 Kaplan et al.

3000

> 2(7 t (< i~~~~~~~~~~~~~~~~~~~~~~~~~7

(1) sta( -t41

- 1000 .I1

(8) (9 (15 )

0~~~~~~~~~~~~~~~0

0

(i ) (16)

winter spring summer autumn

SEASON

Fig. 6. Seasonal changes in DOC in Saw Mill branch and third-order station. Values are seasonal means of ( ) daily means (?2 SE).

allochthonous litter inputs to seeps in the White Clay Creek drainage exceed 450 g C.m-2 yr-' (R. L. Vannote pers. comm.). Thus the total inputs of plant biomass exceed those reported for most aquatic environments, except possibly for swamps, marshes, and estuaries (Whittaker and Likens 1973). If inputs were expressed on a volumetric basis, to emphasize the large surface-to-volume ratio of spring seeps, this would amplify their relative importance. The effect of the surface-to- volume ratio is illustrated by the contrast between the concentration changes in Walton's Spring and those in Ledyard's or Saw Mill Spring (Fig. 2). Walton's Spring has a greater discharge than the others and therefore required a greater distance for the concentration of DOC to show an increase, assuming equal inputs. Groundwater temperatures, which would affect microbial metabolic rates, were similar at all three sites, although the greater discharge from Walton's Spring slightly diminishes seasonal temperature fluctuations downstream. The DOC dra-

matically changed over short distances in seeps, but the pattern may be a special case not duplicated over similar or extended distances downstream.

The low concentrations of DOC at station 1 are typical of most groundwaters sampled in the United States (Leenheer et al. 1974), but are considerably lower than those for groundwater entering Lawrence Lake (Wetzel et al. 1972) or Bear Brook (Fisher and Likens 1973). Concentrations of DOC at station 1 re- main constant through the year; groundwater temperatures fluctuate at most 40C, apparently buffering microbial processing from seasonal changes. The relatively high carbohydrate: phenolic ratio at station 1 may reflect a lesser interaction of the largely neutral carbohydrates with the soils through which the groundwater percolates than that of the phenolic compounds, which are probably largely charged or polar species such as phenolic acids or degraded tannins which interact with soil particles. Larson and Rockwell (1980) found that fluorescence spectra of DOC from stations 1 and 2 are similar to those of degraded lignins and oxygenated cinnamic acid derivatives, while those from the other stations are similar to the more oxidized coumarins, suggesting transformation with distance downstream.

We attribute the seasonal effects observed in the downstream increases in DOC in Saw Mill Spring to variations in algal and macrophyte primary productivity and excretion, as well as to leaching and decomposition of detritus. Standing crops of living plants and plant detritus are maximal during summer and autumn. Despite the higher temperatures and the accelerated activity of decomposers in the summer and autumn, concentrations of DOC at station 3 were higher than at station 1 by 250-300%. Discharge also increases with distance downstream from station 1, presumably because of additional groundwater coming in. This dilution tends to counteract increases in DOC. The contribution of DOC from plant exudates and plant detritus leaching must be many times higher in sum-




mer and autumn than in winter and spring, with autotrophs controlling the levels of DOC in summer. The summer and autumn values at station 3 are com- parable to levels found in a small Cana- dian seep (Lush and Hynes 1978b) but considerably lower than those for springs in Denmark (Iversen and Madsen 1977).

In addition to the quantitative changes in DOC during transport through Saw Mill Spring, there were qualitative changes probably resulting from both microbial activity and abiotic reactions. A selective uptake of low molecular weight organic compounds by aquatic microorganisms was found in both studies with labeled compounds (Wright and Hobbie 1965; Allen 1969; Hoppe 1977) and experiments on molecular size changes in complex natural mixtures (Ogura 1975; Kaplan and Bott in prep.). Bacteria can also excrete large organic molecules after taking up smaller ones such as glucose (Allen 1976) or a heter- ogenous algal exudate (Nalwejko and Lean 1972). Ogura (1975) found that the surface waters in Tokyo Bay have a higher concentration of low molecular weight DOC than the deep water, which contains less total DOC but a greater percentage of high molecular weight DOC; he interpreted this pattern to result from vertical processing. This is possibly anal- ogous to the longitudinal processing characteristic of streams. The molecular weight patterns described here and by Ogura (1975) may be good indicators of biological lability. In addition to prefer- ential microbial selection for small molecules, enzymatic attack on large molecules may create reactive sites on a polymer backbone which could then by abiotic reactions give still larger molecules. For example, a principal step in the degradation of lignin by fungi is the cleavage of methoxyl groups to phenols (Kirk et al. 1978). The catechols released by this step are easily oxidized by air in the presence of clays (Wang et al. 1978), metal oxides, and transition metal cations (Larson and Hufnal 1980) to materials ca- pable of polymerization. From 70 to 99.4% of the DOC inputs to undisturbed

streams are considered refractory to rapid microbial degradation (Wetzel and Man- ny 1977; Fisher 1977); this material would be exported downstream if abiotic removal mechanisms were not important.

The use of ultrafiltration for molecular weight determination is relatively new; the only other report for total DOC in freshwaters is for a small soft-water Ver- mont lake where 62% of the DOC was >10,000 nmw (Allen 1976). Previous fractionation of White Clay Creek waters (Larson 1978) showed an absence of large polymers with molecular weight >5,000, but these determinations were made with strongly acidic concentrates on a Sephadex column and were subject to possible errors from isolation and concentration (Visser 1974), hydrolysis or disaggregation of DOC subunits (Wer- shaw and Pickney 1971; Stabel and Steinberg 1976), or polar interactions of DOC with gel media (Gjessing 1973; Woof and Pierce 1967).

Reports of increases in concentration of DOC downstream are limited to headwaters of stream systems. Moeller et al. (1979) reported only minor changes in concentrations of DOC with stream size in two systems incorporating large (sev- enth-order) rivers. Malcolm and Durum (1976) also found that concentrations of DOC did not increase continually from headwaters to estuaries. Concentrations might be expected to increase along a river system as biologically refractory molecules, which supposedly constitute the bulk of DOC inputs, are transported downstream. However, as the surface-to- volume ratio decreases downstream, the impact of terrestrial and benthic inputs is dampened and the distance over which DOC changes is extended. In addition, the bacterial flora of various stream areas should be able to utilize the organic molecules found there. Downstream populations are probably adapted to use more of the refractory molecules; such a distri- bution of physiological groups in different brackish water biotopes has been documented by Hoppe (1977). Finally, abiotic reactions of DOC at environmen- tally realistic levels have not been ex-



1042 Kaplan et al.

amined. Upstream perturbations in our watershed obscured our determination of those DOC changes due to natural inputs, and the study of longitudinal changes in DOC through most large river systems will be similarly complicated.

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Submitted: 26 December 1979 Accepted: 6 May 1980



patterns of dissolved organic carbon in transport

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