dissolved organic carbon in streams and groundwater

Hydrobiologia 154: 33-48 (1987) 0 Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands 33

Dissolved organic carbon in streams and groundwater

J. E. Rutherford’ & H. B. N. Hynes Department of Biology, University of Waterloo, Waterloo, Ontario, Canada, N2L 3GI 1 Present address: Dept. of Biology, Wilfrid Luurier University, Waterloo, Ont., Canada, N2L 3C5

Received 23 June 1986; in revised form 1 March 1987; accepted 26 March 1987

Key words: groundwater, organic matter (DOC), m ini-piezometers, hyporheic zone

Abstract

M inipiezometers installed at different vertical levels within the streambed (20- 140 cm) were used to study temporal and spatial variation in the dissolved organic carbon (DOC) content of streamwater and groundwater in three southern Ontario streams. Groundwater, as represented by our streambed samples, contained considerable quantities of DOC but variation between replicate samples was high. Die1 fluctuations in DOC content of streamwater were consistent with daytime autochthonous production and night-time uptake by heterotrophs. Water from the streambed neither consistently diluted nor enhanced streamwater levels of DOC. At some stations, DOC variation with depth, including streamwater, seemed to be largely random. At other stations, DOC concentrations from the deepest piezometers were consistently higher than concentrations at intermediate depths, suggesting a loss of DOC from deeper waters to overlying sediments. However, at these stations DOC concentrations were highest at 20 cm and at the surface. Interflow delivery of DOC to the shallow layers of the streambed may be a significant source of carbon for a stream ecosystem, especially in agricultural areas. Late summer die1 fluctuations at one station may be related to changing patterns of intermixing of stream and groundwater in the upper layers of the streambed as governed by velocity heads, convective currents and evapotranspiration.

Introduction

Studies of dissolved organic carbon (DOC) in streams have considered streamwater concentrations only, because measurement of DOC in samples from the surface of springs or seepages indicated the groundwater had low and constant levels of this material, diluting the DOC content of the free- flowing water of the channel (Fisher & Likens; Kaplan et al., 1980; Kaplan & Bott, 1982). However, Wallis et al. (1981) found groundwater extracted from the saturated zone in the vicinity of two small springs contained DOC concentrations of 5.9 mg/l, twice that of the adjacent stream. Their study

showed that DOC concentration in surface spring- water was less than that in water a few centimeters below the seep surface, implying that organic matter was lost to the sediments as water moved up from the groundwater to the stream surface. Lee & Hynes (1977178) found that pore water below a stream contained more organic carbon than the water that seeped up into it. These studies suggest that streambeds are biologically active zones, modifying the chemical content of water that moves through them. Organic carbon entering the stream ecosystem via the groundwater may be an important component of the total carbon budget. Hynes (1983) suggested that the hyporheic zone may act as a sink for

34

organic matter, stripping DOC from inflowing groundwater, from interflow, and from water flushed by storms from bank storage in the unsatu- rated zone.

We set out to evaluate these two competing views of groundwater DOC content, which may be briefly stated as the dilution hypothesis (Fisher & Likens, 1973; Kaplan etaf., 1980) and the stripping hypothesis (Wallis et al., 1981; Hynes, 1983). We believe we are the first to compare the organic carbon content of groundwater at different vertical levels within the streambed to the DOC in water flowing freely in the channel. At this preliminary stage, we did not attempt to characterize the molecular fractions present as DOC, nor did we measure biological activity as such within the steambed. Our purpose was to investigate temporal and spatial variation in DOC concentrations in water from different locations in the stream and streambed.

Methods

Study Areas

Salem Creek and Canagagigue Creek are hardwater tributaries of the Grand River in southern Ontario; they drain fertile agricultural land densely settled by Mennonite farmers over the last 150 years. Farming practices range from techniques already old in the 19th century to those using 20th century chemicals and machinery. As a consequence both streams are subject to enrichment from cultivated fields, pastures, barnyards and septic systems.

Stations consisting of 2 sampling-sites (upper and lower) were established as far as possible from urban influences - on Salem Ck (second order), 1.5 km above its confluence with Canagagigue Ck, and on the Canagagigue (third order) about 1 km above confluence with the Grand R. (Fig. 1). At each sta-

lntematicnal ----

Fig. 1. Map of southern Ontario showing a) the Saugeen River station; b) the Canagagigue and Salem Creek stations. Small triangles enclosed in circles indicate position of sampling-sites at each station.

35

tion, the 2 sampling-sites were about 20-50 m apart, separated by a riffle-pool combination. Our intention was to establish mini-piezometer sets in both discharge and recharge areas, but positive hydraulic heads recorded at all piezometers indicated all were installed in discharge areas - i.e. the net flow was from the groundwater to the stream.

Soils maps of Waterloo County, Ontario, show that, in general, the floodplains of both streams are composed of geologically recent alluvial and sandy loam (derived from dolomite limestones and shales) overlying gravel and cobble, with occasional clay lenses at shallow depths which impede drainage. In the pastures bordering the Salem Ck sampling-sites, 30- 90 cm of highly organic (> 30% organic content), poorly drained soils overlie coarser materials. At the Canagagigue, sand, gravel and loam (clay content < 16%) border the sampling-sites (Presant & Wicklund, 1971).

A third station was established on the Saugeen River, 11 km below Durham, Ont. (population = 5000) and 1 km below the junction of the main river with a major tributary, the Rocky Saugeen (Fig. 1). Both rivers drain semi-forested, sparsely- farmed land; they, too, are hardwater streams. Soil surveys show that most land is well-drained but nu- trient poor, requiring addition of organic matter, phosphorus and potash for succesful agriculture (Gillespie & Richards, 1954). Lands directly bordering the sampling station consist largely of well- drained gravelly outwash. Many farms are no longer worked, resulting in low levels of agricultural enrichment; recently however, there has been increasing recreational use of riverside properties. The streambed is permeable and open, and is composed of large cobbles overlying coarse gravel and sand. A single (lower) sampling-site was installed in an run in August; in October, mini-piezometers were installed about 60 m upstream in the same run (upper sampling-site).

Sample Collection

Mini-piezometers (Lee & Cherry, 1978) were installed at the sampling-sites at each station; each set originally consisted of 5 tubes (polyethylene, inside

diameter 0.31 cm) inserted 20,40,60,80 and 100 cm into the streambed. Later mini-piezometers were installed to depths of 120 and 140 cm at Salem Ck and Canagagigue Ck and to 120 cm at the Saugeen R. At the Saugeen, piezometers were also installed in the banks close to each sampling site (designated as BUl at the upper site, and BLl and BL2 at the lower site). Manometric measurements of hydraulic head (Lee & Cherry, 1978) showed that all mini-piezometers were in discharge zones throughout the study. Handheld vacuum pumps (Nalgene @ ) provided sufficient pressure to extract water from all depths, except when clay clogged the screened tip of a piezometer. At various times, the 80,100 and 120 cm piezometers at Salem Ck and Canagagigue Ck had to be replaced because of clogging. On these occasions, we allowed several days to pass before taking a full set of samples. Triplicate (later, 5 replicate) samples were pumped from each depth; each vial was rinsed twice before filling. Concurrent streamwater samples (i.e. the 0 cm depth) were taken midway in the water column. We used 30 ml polyethylene scintiallation vials, cleaned by soaking overnight in a 2% solution of ExtranO (pH 12) followed by rinsing in de- ionized water. We found this procedure to be as ef- fective at removing contaminant organics as acid washing. Samples were transported on ice to the laboratory, stored overnight at 4 “C to allow particu- lates to settle, decanted then filtered through 0.45 pm membrane filters (Sartorius@ cellulose acetate), acidified and sparged with oxygen to re- move inorganic carbon. Duplicate determinations of dissolved organic carbon (DOC in mg/l) were per- formed on a Dohrmann@ Model DC-80 carbon analyzer.

Data Analysis

To investigate differences in DOC concentration in streamwater between dates or sites, or differences in DOC between streamwater and intersitial water from various depths within the streambed, we em- ployed hierarchial analyses of variance, in which the replicate samples were nested within the next higher factor under consideration, allowing for random variability of DOC measurements from each water

36

source. The nested variable always accounted for much of the variation in the data (F’s were always highly significant), and was used as the appropriate mean square to test the effect of the other factors, date, site, depth or time of day (period). We considered the depth to which a piezometer was installed in the streambed to be a random factor, for although the depths were regularly spaced at 20 cm intervals, these intervals were chosen for convenience (Zar, 1974). We wished to test the hypothesis that in general, there is no difference in mean DOC concentration between water from different locations in the water column extending from the free-flowing stream into the streambed; appropriate F-tests for mixed model Anova’s were constructed according to standard statistical rules (Zar, 1974).

To investigate whether there was a repeated pattern in the relative magnitude of DOC concentration from depth to depth at each station and sampling- site, we standardized the data (z-score transforma- tion) to eliminate the differences in absolute concentrations between sampling dates. For each date, the overall mean concentration (all depths) were set to 0, with unit standard deviation; the resultant z- scores were plotted against sampling depth, combin- ing data from all dates on one plot. We organized the Saugeen R. samples into four separate data sets: late summer daytime, late summer night-time; fall daytime and fall night-time. We attempted to find significant statistical fits of descriptive equations to the resultant patterns of DOC variation with depth using polynomial regression. We do not propose to attempt to interpret these higher order equations in terms of biological phenomena; rather our intent was to use the equations to detect repeating patterns of variation, whatever the controlling mechanisms might be. Die1 differences in DOC concentration at the Saugeen River were tested by a Wilcoxon paired- sample test: for example, for the August 17 data, daytime sample #l of the stream water was paired with the night-time sample #l, and so on. Die1 changes in late summer and fall were plotted by subtracting mean night-time concentrations from daytime concentrations.

Results

Salem Creek

The mean ( f standard deviation) daytime dissolved organic carbon concentration of the free-flowing streamwater of Salem Creek was 12.42 (f 2.42) mg/l over the study period (June-August). DOC concentration differed significantly between sampling dates and on July 12, between the upper and lower sampling-sites (Table l), with a threefold difference between the lowest and greatest DOC concentration recorded.

Comparisons of DOC concentration in streamwater and in streambed water samples for each sampling-site on each date tested the hypothesis that there was no difference in DOC concentration between sampling depths (0 to 140 cm); this null hypothesis was rejected in all 8 tests (Table 2). In all tests, the variation of the replicate samples drawn from each depth was high, yielding a highly significant F-statistic for the sample-nested-within-depth variable. For the upper sampling-site, examination of the means for each depth (SNK multiple range tests) showed that water from 20 cm and free- flowing streamwater (i.e. 0 cm depth) generally had significantly greater concentrations of DOC than water from greater depths (Table 2). Water samples from 60 and 80 cm consistently ranked lowest in

Table 1. Mean daytime DOC concentrations ( ?Z standard deviation)ls2 in streamwater (0 cm), Salem Creek (June-August, 1984).

Date Site Mean DOC (k 1 S.D.) mg/l

18 June

12 July

7 August

15 August

upper lower upper lower upper lower upper lower

17.91sb (+2.038) 19.393a (+ 0.729)

9.943d (+- 3.024) 15.713b (k2.119) 5 .42ge (kO.261) 5.600e (kO.199)

12.837c (rtO.739) 12.508c (k0.226)

1 Nested Anova statistics: Date, F= 59.522, P<O.OOOl; Site, F=6.468, P=O.O230; Date x Site, F=566.640, P<O.OOOl; Sample (Date x Site), F= 143.640, P<O.OOOl.

2 Superscripts denote significant differences between means (SNK test, P<O.O5).

37

Table 2. Salem Creek comparisons of mean DOC concentration (mg/l) between streamwater and streambedwater.

Date Site Summary of nested Anova’s: Depths of piezometers (and mean DOC) in descending order: SNK tests’ (PcO.05)

variable DF F P

June 18

July 12

August 7

August 15

June 18

July 12

August I

August 15

upper

upper

upper

upper

lower

lower

lower

lower

depth 5,12 sample (depth) 12,18

depth 5,12 sample (depth) 12,18

3.71 0.0292 1768.05 <O.OOOl

5.50 0.0074 776.02 <O.OOOl

depth 7,16 103.87 sample (depth) 16,24 116.18


depth 5,12 15.52 sample (depth) 12,lS 226.21

depth 4,lO 4.11 sample (depth) 10,15 995.05



<O.OOOl <O.OOOl

<O.OOOl < 0.0001

<O.OOOl <O.OOOl

0.0319 <O.OOOl

<O.OOOl <0.0001

<O.OOOl <O.OOOl

0 20 100 80 40 60 (17.9) (16.0) (10.2) (9.8) (8.0) (7.5)

20 0 100 40 80 60 (12.6) (9.9) (7.6) (6.4) (3.9) 2.9)

20 0 40 100 120 60 140 80 (8.4) (5.4) (4.7) (3.8) (3.0) (2.9) (2.4) (2.1) ---~ 20 0 40 120 140 100 60 80

(13.0) (12.8) (6.7) (6.3) (5.1) (4.8) (4.0) (3.5)

0 20 100 40 80 60 (19.4) (8.7) (7.7) (4.9) (4.8) (4.7)

0 80 60 20 40 (15.7) (10.7) (7.5) (7.4) (5.6)

100 0 140 120 20 80 60 40 (5.9) (5.6) (4.4) (3.2) (2.7) (2.2) (2.1) (2.1)

100 0 140 120 80 20 60 40 (18.1) (12.5) (5.6) (4.0) (3.6) (3.6) (3.5) (2.6) --~

r Underscores join depths with similar means (P~0.05).

DOC concentration. For the lower sampling-site, streamwater generally had significantly greater DOC concentrations than most samples drawn from the streambed; however, in two comparisons, DOC concentrations in water from 100 cm were greatest of all samples, and in one of these comparisons, the mean at 100 cm was significantly greater than mean streamwater DOC (Table 2).

DOC concentrations varied considerably between sampling dates (Table 2), however, for each separate set of piezometers, the plots of z-scores revealed a repeatingpattern of variation in DOC concentration with depth (Fig. 28~3). At the upper sampling-site, DOC concentrations in streamwater and water from 20 cm were greater than the mean, while water from intermediate depths (60 and 80 cm) were lower than 0; other z-scores clustered near the mean (Fig.2a). The pattern was described by a 5th order polynomial equation (F = 125.02, P < 0.0001) which accounted for 76% of the variation (Fig. 2). At the lower sampling-site, the DOC concentration of streamwater was consistently much greater than the mean concentration on any sampling date; DOC concen-

Depth within Streambed (cm)

- I

0 1

Depth wlthin Streambed (m)

Fig. 2. Changes in DOC concentration (daytime, June-August, 1984) at the upper sampling-site, Salem Creek: a) mean (f 1 standard error) z-scores, by depth (cm); b) best fit to z-scores plotted by depth (m) is a 5th order equation: y = 1.048 + 16.277x - 100.595~~ + 176.832x3 - 124.873x4 + 31.074~~.

38

A.

7 1.5..

g 1

r-4 05.. f 6

4

f O

I I t

-0 5..

d b

-7, , 0 20 40 60 80 100

Depth within Streambed (cm)

120 140

2 F?= 0.75

z b 1

x N

z 0

-1

Fig. 3. Changes in DOC concentration (daytime, June-August, 1984) at the lower sampling-site, Salem Creek: a) and b), details as in Fig. 2. Best fit equation is 7th order: y = 1.751 + 6.650x - 241.118x2 + 1202.655x3 - 2651.076x4 + 2972.637~~ - 1648.602~6 f 357.813~‘.

trations in samples from 100 and 140 cm were also higher than the mean. A 7th order polynomial equation described the pattern in z-score distribution (F = 62.186, P < O.OOOl), accounting for 75% of the variation (Fig. 3).

Canagagigue Creek

Daytime DOC concentration in the Canagagigue was highly variable over the study period (July- October), averaging 9.33 (-+ 5.65) mg/l; variation in samples within each sampling date at each site (i.e. samples nested within date and site, F = 5926.06, P c 0.0001) masked the differences between dates, although the greatest mean DOC concentration recorded was 2 times greater than the smallest (Ta- ble 3). On October 5, the mean daytime DOC con-

Table 3. Mean daytime DOC concentrations ( f standard deviation)tr2 in streamwater (0 cm), Canagagigue Creek (July- October, 1984).

Date site Mean DOC (rt 1 S.D.) mg/l

12 July

7 August

17 August

5 October

upper 10.815 (* 0.457) lower 10.946 (of: 1.208) upper 7.347 (kO.419) lower 6.945 (+0.323) 4-w-r 8.550 (+ 1.461) lower 7.268 (+0.132) upper 8.172 (kO.383) lower 14.621 (+ 8.286)

1 Nested Anova statistics: Date, F= 1.991, P =0.148; Site, F= 1.701, P=O.207; Date x Site, F=9834.540, P<O.OOOl; Sample (Date x Site), F=5926.06, P<O.OOOl.

2 No significant differences between means (SNK test, P<O.O5).

centration was greater than the night concentration (F = 5.921, P = 0.027); the daytime DOC at the lower site was significantly greater than all other mean DOC values (Table 4).

For the upper sampling-site, the hypothesis that there was no difference in DOC concentration in water from the stream or the streambed was accepted in 2 of 4 comparisons (Table 5). Significant depth differences were found for samples collected on 2 dates: in August 7, water from 140 cm was lowest in DOC, but on October 5, water from this depth had a significantly greater mean concentration than samples from all other depths, including the streamwater. Variation of DOC concentration with depth

Table 4. Mean DOC concentrations (+ standard deviation)ts2 in streamwater (0 cm), daytime vs night-time, Canagagigue Creek (October 5, 1984).

Period site Mean DOC (+ 1 S.D.) mg/l

Day

Night

upper 8.172b (kO.383) lower 14.621= (k8.286) upper 6.431” (+0.358) lower 5.802b (kO.014)

t Nested Anova statistics: Date, F= 5.921, P =0.027; Site, F=4.421, P=O.O56; Period x Site, F=8575.770, P<O.OOOl; Sampe (Period x Site), F=7415.460, P<O.OOOl.

2 Superscripts denote significant differences between means (SNK test, PcO.05).

Tabl

e 5.

Can

agag

igue

Cre

ek c

ompa

rison

s of

mea

n DO

C co

ncen

tratio

ns

(mg/

l) be

twee

n st

ream

wate

r an

d st

ream

bed

wate

r. Fo

r O

ct.

5, r

anke

d da

ytim

e de

pths

(an

d m

eans

) pr

eced

e ni

ght-t

ime

data

.

Date

Si

te

Sum

mar

y of

nes

ted

Anov

a’s:

De

pths

of

pie

zom

eter

s (a

nd m

ean

DOC)

in

des

cend

ing

orde

r: SN

K te

sts1

(P<O

.O5)

Varia

ble

DF

F P

July

12

up

per

Augu

st

7

Augu

st

17

Oct

ober

5

__

uppe

r

uppe

r

July

12

lo

wer

Augu

st

7 lo

wer

Augu

st

17

lowe

r

Oct

ober

5

lowe

r

dept

h sa

mpl

e (d

epth

)

dept

h sa

mpl

e (d

epth

)

5,lO

lo

,16

6,14

14

,21

1.93

0.

1754

49

2.94

<O

.OO

Ol

24.7

1 <O

.OO

Ol

61.4

6 <O

.OO

Ol

dept

h 7,

lS

0.74

sa

mpl

e (d

epth

) 16

,24

116.

18

perio

d 1,

56

2.88

de

pth

7,56

4.

37

perio

d x

dept

h 7,

66

6747

.38

sam

pe (

p x

d)

56,6

6 10

37.2

1

dept

h 5,

12

19.0

1 sa

mpl

e (d

epth

) 12

,18

355.

26

dept

h 7,

16

sam

ple

(dep

th)

16,2

4 11

.11

<O.O

OO

l 16

7.96

<O

.OO

Ol

dept

h 7,

16

sam

ple

(dep

th)

16,2

4

perio

d 1,

56

dept

h 7,

56

2.81

0.

0412

10

2.43

<O

.OO

Ol

8.76

0.

0045

3.

87

10.0

017

perio

d x

dept

h sa

mpl

e (p

x d

) 7,

66

4125

.51

<O.O

OO

l 56

,66

1690

.90

< 0.

0001

<0.6

412

<O.O

OO

l

0.09

53

<O.O

Oll

<O.O

OO

l <O

.OO

Ol

<O.O

OO

l <0

.000

1

80

20

100

40

0 60

(1

5.8)

(1

5.0)

(1

3.9)

(1

2.1)

(1

0.8)

(1

0.1)

60

20

80

0 10

0 40

14

0 (8

.2)

(7.7

) (7

.4)

(7.3

) (6

.7)

(6.0

) (5

.2)

120

20

0 60

40

80

14

0 10

0 (1

0.3)

(9

.5)

(8.6

) (8

.4)

(8.3

) (7

.9)

(7.6

) (7

.5)

140

20

0 60

10

0 40

80

(D

: 8.

76*

3.80

6)

(15.

8)

(8.3

) (8

.2)

(7.4

) (7

.1)

(6.9

) (6

.9)

80

20

60

40

140

0 10

0 (N

: 7.

69 +

2.4

40)

(10.

0)

(9.3

) (7

.5)

(7.4

) (6

.8)

(6.4

) (6

.3)

20

0 40

10

0 60

80

(1

7.0)

(1

0.9)

(7

.3)

(7.0

) (5

.0)

(3.6

)

0 12

0 20

80

40

10

0 60

14

0 (6

.9)

(6.8

) (5

.6)

(5.2

) (4

.6)

(4.1

) (3

.2)

(2.6

)

20

80

100

120

40

0 60

14

0 (9

.7)

(8.9

) (8

.5)

(8.2

) (7

.7)

(7.3

) (5

.6)

(5.3

) 0

20

80

40

100

60

120

140

(D:

7.43

k4.

410)

(1

4.6)

(1

0.1)

(7

.5)

(6.7

) (6

.1)

(5.6

) (5

.3)

(3.5

)

20

120

0 14

0 40

80

10

0 60

(N

: 5.

54k2

.152

) (7

.2)

(6.4

) (5

.8)

(5.7

) (5

.4)

(4.4

) (4

.2)

(4.1

)

t Und

ersc

ores

joi

n de

pths

with

si

mila

r m

eans

(Pr

O.0

5);

for

Oct

. 5,

day

(D)

and

nig

ht

(N)

over

all

mea

ns (

+_S.

D.)

follo

w ra

nked

de

pths

.

40

40 60 80 100

Depth within Streambed (cm) 40 60 80 100

Depth wthln Streambed (cm)

1

Depth wthin Streambed (m)

2

-\ 1

Depth within Streambed (m)

2

Fig. 4. Changes in DOC concentration (daytime, July-October, Fig. 5. Changes in Dot concentration (daytime, July-October, 1984) at the upper sampling-site, Canagagigue Creek: a) and b), 1984) at the lower sampling-site, Canagagigue Creek: a) and b), details as in Fig. 2. Best fit equation is 6th order: y = - 0.081 + details as in Fig. 2. Best fit equation is 7th order: y = 0.920 - 26.401x - 219.880x2 + 654.840~3 - 896.458x“ + 573.579x5 - 19.738x + 265.899x2 - 1288.161x3 + 2817.503x4 - 3089.490x5 138.594x6. + 1662.814x6 - 349.932x7.

showed little consistent pattern of deviation from the mean DOC concentration (i.e. z = 0); although a 6th order polynomial equation provided a significant fit to the standardized data (F = 17.668, P < 0.0001) it described only 9% of the variation (Fig. 4).

In 3 of the 4 comparisons (daytime data) of DOC concentration with depth for the lower sampling-site the hypothesis that there were no differences was rejected. Streamwater and water from 20 cm generally had the greatest mean DOC concentration, water from 140 and 60 cm the lowest; there was considerable overlap between groups of significantly different means. No depth difference in mean DOC concentration was found for the night samples of October 5 (Table 5). The pattern revealed by plotting z-scores for each date showed that DOC concentrations in streamwater and at 20 cm within the streambed were consistently greater than the mean, concentrations

d= 0.45

Table 6. Mean DOC concentrations (+ standard deviation)r,2 in streamwater (0 cm), daytime vs night-time, Saugeen River (August-December, 1984).

Date Site Mean DOC (k 1 SD.) mg/l

Day Night

17 August lower lo.nP (k2.211) 4.343b,C (kO.180) 25 August lower 5.599b~c (kO.604) 5.280bsC (+0.335)

3 September lower 16.603a (f 8.753) 6.296b*c (kO.496) 26 September lower 6.559bJ ( f 0.469) 9.744bJ (f 3.064) 25 October upper 6.193bsC (+ 1.395) 5.332b,C (20.485)

lower 7.187b.C (kO.260) 3.816c (kO.940) 14 December upper 6.824bJ (+0.497)

lower 5.023b~c (f 0.333)

1 Nested Anova statistics, taking each Date-Site-Period combination as a separate group: Group, F=6.235, P<O.OOOl; Sample (Group), F = 1180.80, P <O.OOOl.

2 Superscripts denote significant differences between means (SNK test, PcO.05).

Tabl

e 7.

Sau

geen

Riv

er c

ompa

rison

s of

mea

n DO

C co

ncen

tratio

n (m

g/l)

betw

een

stre

amwa

ter

and

stre

ambe

d wa

ter.

Date

Si

te

Sum

mar

y of

nes

ted

Anov

a’s:

De

pths

of

piez

omet

ers

(and

mea

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Saugeen River

The average DOC concentration in both daytime and night-time samples of streamwater was 7.10 (+ 4.09) mg/l over the study period (August- December); when each date-site-period combination was treated as a data group, significant variation of samples within each group (F = 1180.80, P < 0.0001) tended to mask significant differences between group means despite a range of values from 3.8 to 16.6mg/l. Mean daytime DOC concentrations tended to be greater than night-time concentrations although these differences were not statistically significant except for that found on September 3 (Ta- ble 6).

Comparisons of DOC concentration with period and depth (6 tests) showed that there was significant sample variation at each depth (nested variable), and that the pattern of DOC concentration with depth varied considerably between day and night (highly significant period-depth interactions in all tests, Ta- ble 7). In the 14 possible daytime and night-time comparisons of means, there were detectable depth differences in mean DOC concentration in only 7, with much overlap between group means. Stream- water DOC was significantly higher than DOC concentrations at all other depths in only 1 test; DOC at 20 cm was greater than all others in another test (SNK tests, Table 7).

The pattern of DOC variation with depth was considered in 4 parts: late-summer days, late- summer nights, fall days and fall nights. The trend on summer days indicated greater than mean DOC concentrations at 0 cm (i.e. in streamwater), at 80 cm, and to a lesser extent at 100 and 120 cm; lower than mean concentrations occurred at 20,40 and 60 cm. Although this pattern is significantly described by a 3rd order polynomial equation (F = 20.730, P < O.OOOl), only 10% of the variation in the standardized data was explained by depth

A.

-2 / 0 05 10 15

Depth wlthm Streambed (ml

Fig. 6. Changes in DOC concentration (summer daytime, Au- gust 17, 25, and September 3, 1984) at the lower sampling-site, Saugeen River: a) and b), details as in Fig, 2. Best fit equation is 3rd order: y = 0.411 - 6.101x + 12.381x2 - 6.328~~.

(Fig. 6). On summer nights, DOC concentrations in streamwater and at 20 cm tended to be lower than the mean; concentrations at 40 and 120 cm tended to be higher than the mean. However, the 3rd order polynomial equation describing this pattern (F = 29.906, P < 0.0001) accounted for only 14% of the variation (Fig. 7). On fall days, there was a tendency for DOC concentration in streamwater and at 40 cm to exceed the mean concentration, and for DOC concentrations at 20 and 80 cm to be lower than the mean. The 5th order polynomial equation describing this pattern was significant (F = 23.320, P < 0.0001) but explained only 10% of the variation (Fig. 8). During fall nights, DOC concentrations at 20, 40 and 80 cm were lower than the mean, while concentrations in streamwater and at 60 cm were higher. A 6th order polynomial equation was a significant fit to this pattern (F = 16,943, P < 0.0001) but accounted for only 11% of the variation in the standardized data (Fig. 9).

In summer the mean DOC concentration of streamwater was greater during the day than at

43

Fig. 7. Changes in DOC concentration (summer night-time, Au- gust 17, 25, and September 3, 1984) at the lower sampling-site, Saugeen River: a) and b), details as in Fig. 2. Best fit equation is 3rd order: y = - 0.810 + 5.695x - 10.013~2 + 5.159x3.

8.

20 40 60 80 100 120 Depth within Streambed (cm)

2 E 8

F?=OlO

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Depth wlthfn Streambed (m)

15

Fig. 8. Changes in DOC concentration (fall daytime, September 16, October 25, December 14, 1984) at the lower sampling-site, Saugeen River: a) and b), details as in Fig. 2. Best fit equation is 5th order: y = 0.300 - 18.286x + 122.136x2 - 276.281 x3 + 253.897x4 - 81.792x5.

Fig. 9. Changes in DOC concentration (fall night-time, Septem- ber 16, October 25, December 14,1984) at the lower sampling-site, Saugeen River: a) and b), details as in Fig. 2. Best fit equation is 6th order: y = 0.188 + 27.184x - 326.868x2 + 1244.677x3 - 2071.219x4 + 1570.588x’ - 443.390x6.

night, but DOC concentrations within the streambed tended to be greater at night than during the day (Fig. 10a). In the fall (October), the mean die1 difference in both streamwater DOC concentration and DOC within the streambed indicated a consistent trend for higher daytime concentrations (Fig. lob). The magnitude of these differences was quite variable, from less than 0.5 mg/l to almost 6 mg/l. Non-parametric comparisons of DOC concentrations in samples from the streambed revealed these differences were significant: in summer, night- time DOC concentrations in the streambed were higher than daytime concentrations, and in October, the reverse was true (Table 8).

Discussion

Mean daytime concentrations of DOC in Salem Creek, Canagagigue Creek and the Saugeen River were generally greater than those reported for streams of the Hubbard Brook Experimental Forest

44

20 40 60 80 100 120

Depth of Samples wlthln Streambed (cm)

Fig. 20. Mean (f 1 standard deviation) die1 change (i.e. daytime concentrations minus night-time concentrations) in DOC (mg/l), by depth: a) summer (August 17,25 and September 3); b) fall (Sep- tember 26, October 25 upper and lower site samples).

T&e 8. Comparison of daytime DOC concentrations to night- time concentrations from piezometer tubes only (i.e. streamwater excluded), Saugeen R.; Wilcoxon 2-tailed tests.

Date and site Wilcoxon test P statistics

Conclusion

17 Aug., lower Ts=205, n=36 <0.05 night DOC higher

25 Aug., lower t=2.142, n=60 0.018 night DOC higher

3 Sept., lower t=22.99, n=58 *0.0001 night DOC higher

26 Sept., lower t=1.775, n=78

25 Oct., upper t=4.159, n=57

25 Oct., lower t = 4.914, n = 57

0.080 no difference

<O.OOOl day DOC higher

<O.OOOl day DOC higher

(1- 3 mg/l, Fisher & Likens, 1973; Johnson et al., 1981; McDowell, 1985), for streams of the Pennsyl- vania Piedmont (means usually < 5 mg/l, Kaplan et al., 1980; Kaplan & Bott, 1982; Bott et al., 1984; Ku- serk et al., 1984) or for streams of the Marmot Basin, Alberta (2-2.5 mg/l, Wallis et al., 1981), but were comparable to DOC concentrations reported for other hardwater streams in Michigan (Augusta Ck, annual mean 4.4 (k 45%) mg/l, Wetzel & Manny, 1977) and southern Ontario (2.3 - 15.5 mg/l, Lock et al., 1977). Our Waterloo County Streams (Salem and Canagagigue Ck) were probably the most affected by agriculture of all these streams and their high daytime mean DOC concentrations may be attributable in part to enrichment by livestock manure and chemical fertilizers.

All but the southern Ontario and Michigan studies were conducted in forested headwater regions of mountainous drainage basins, where the soil development and underlying geology was quite different from the deep glacial deposits characteris- tic of our study areas. Indeed, the streams at Hub- bard Brook lack a true groundwater component due to an impermeable fragipan within 1 m of the soil surface (Cole et al., 1984); streamwater DOC concentrations there varied little despite seasonal changes in discharge and organic matter deposition (Fisher & Likens, 1973) or drainage basin deforesta- tion (Hobbie & Likens, 1973). There, streamflow must be derived from water percolating through the thin soil and flowing along the impermeable surface to the stream. Adsorption of DOC by soils (podzolization processes) and by streambed sediments were felt to control DOC concentrations in streamwater in one of these streams, Bear Brook (McDowell & Wood, 1984; McDowell, 1985).

In the Pennsylvania streams (Kaplan et al., 1980; Kaplan & Bott, 1982) and in the Marmot Basin (Wal- lis et al., 1981), seepage areas at the stream sources and along their banks provided direct evidence of groundwater contribution to streamflow. Isotopic separation of storm hydrographs showed groundwater contributed over 65% of total streamflow and must have supplied almost all streamwater during periods of low flow in the Marmot Basin (Wallis et al., 1981). Groundwater was felt to be a significant contributor to streamflow in Augusta Creek (Manny

45

& Wetzel, 1973; Wetzel & Manny, 1977). Measurement of DOC concentrations in springs

and seeps led the Pennsylvanian workers to conclude that groundwater had low and constant concentrations of DOC, serving to dilute streamwater. Die1 fluctuations in streamwater DOC, with predawn minima and post-noon maxima were attributed to release of organic exudates during daytime algal primary production and uptake by heterotrophic processors at night (Kaplan et al., 1980; Kaplan & Bott, 1982). We observed die1 fluctuation in streamwater DOC concentrations that were consistent with daytime release of photosynthetic products and night-time uptake by heterotrophs. Cladophoru was abundant at both Salem and Canagagigue Ck; mosses and algae associated with travertine deposits were abundant at the Saugeen R. However, at none of these streams could we conclude that groundwater, as represented by our interstitial water samples, contained low or constant concentrations of DOC. These concentrations varied with both depth and time of day; patterns of variation were site-and- time specific (Fig. 2- 9). Yet none of these data une- quivocally supported the stripping hypothesis of Wallis et al. (1981).

Despite the statiscally significant fit of descriptive equations relating relative DOC concentration to depth, depth generally accounted for little of the variation observed. The exceptions were the data sets (i.e. daytime concentrations) collected at Salem Ck and the lower site on the Canagagigue; depth accounted for 45 -76% of the variation in relative DOC concentration. At these stations, water from 20 cm equalled or exceeded streamwater DOC concentrations (at 3 of 4 sites), water from mid-depths was depleted in DOC and there was a tendency for increased concentrations at the greatest depths, although these were lower than those at the surface and near-surface.

The shape of the curves generated by the polynomial regressions for these stations offer limited support to the idea that DOC in the water from greater depths is lost to the sediments of the intermediate depths as it moves upward; the data do not show that the DOC concentration of water from the greatest depths exceed that of the streamwater. If one examines the distribution of z-scores for depths

140 cm, it becomes apparent that almost none are more than 1 standard deviation away from the overall mean DOC concentration (i.e. z = 0). The scatter of points may simply represent random variation in interstitial water DOC concentration. Apparent dis- continuities - as between 80 and 100 cm at Salem Ck, upper and lower sites, or between 60 and 80 cm and 120 and 140 cm at Canagagigue Ck, lower site - may have resulted from clay lenses at these points adsorbing DOC or deflecting the movement of water. While we know the net movement of the interstitial water was upward, we do not know the direc- tion of the flow paths within the streambed. When we installed the piezometers, we often encountered greater resistance at about 80 cm, and also at 120 cm, which we attributed to hitting a pocket of clay. On these occasions, the lower portions of the steel insertion pipe was covered with clay when with- drawn from the streambed. Permeability of clay is much lower than that of sand or gravel (Freeze & Cherry, 1979) and thus would retard the rate of upward flow of groundwater. Amorphous, non- crystalline clays containing trivalent cations (e.g. Al, Fe, Ca and Mg - all present in hardwater) can ad- sorb significant fractions (26-33%) of total DOC (Dahm, 1981).

Similarities in DOC concentration of streamwater and streambed water at 20 cm - and their apparent dissimilarity to deeper water samples - occurred at two sites: Salem Ck, upper site and Canagagigue Ck, lower site. But at the lower site at Salem Ck, streamwater and water at 20 cm were clearly unlike one another, with 20 cm concentrations almost always significantly lower than streamwater but indistinguishable from water from 40, 60 or 80 cm (Table 2). It appears as though intermixing patterns of streamwater and streambed water may be highly localized. In a sandy-bedded Michigan stream (Ma- ple R), temperature profiles revealed that Chara hummocks and other impediments to streamflow caused local upwelling of hyporheic flow (Hendricks & White, 1986). At the lower site on Salem Ck, the 20 cm piezometer may have been in a region of strong upwelling. At the other sites, the high DOC concentrations at 20 cm may have owed more to input from interflow, or at least flow from the uppermost layer of the saturated zone. At all stations, the

20 cm piezometer was quite close to the bank and was probably more influenced by shallow inputs. If the neighbouring soils were relatively poorly drained, as suggested by the soil surveys (Presant & Wicklund, 1971), lateral movement of drainage waters through the upper soil horizons may have contributed to elevated DOC concentrations at 20 cm (Dawson et al., 1981; Frape & Patterson, 1981; Hynes, 1983).

The bed material of the Saugeen R. was much more open and permeable than either Salem or Canagagigue Ck, potentially allowing much faster movement of water through the streambed. Mini- piezometers, because of their small size and tendency to clog, generally do not yield accurate groundwater discharge rates (Woessner & Sullivan, 1984). We did not attempt to quantify discharge rates; however, we always found it much easier to extract water from Saugeen R. piezometers than from any of the Salem or Canagagigue Ck piezometers.

At the Saugeen R., depth had little influence on the magnitude of DOC concentration; best-fit equations accounted for 14% of the variation at most. Even when day and night samples were treated separately (note large time period-depth interactions, Table 7), variation of samples within each depth was usually much larger than variation between depths (Fig. 6 - 9, part b). Mean z-scores were never more than 1 standard deviation away from the mean (Fig. 6-9, part a). We conclude that, on the whole, water from the streambed neither diluted nor augmented DOC concentrations of the free-flowing streamwater in either summer or fall, day or night.

In late summer, although the DOC content of the deepest waters (2 80 cm) remained fairly constant from day to night, we noted an elevation of streamwater DOC concentration, and a smaller, but consistent, relative depression of DOC content in the shallow (20- 60 cm) depths during the day (Tables 5 & 8, Fig. 10a). Daytime elevation of streamwater DOC due to autochthonous production has been dis- cussed; what remains puzzling are the die1 fluctuations in DOC content of the upper layers of the streambed. It appears as though water, and conse- quently DOC concentration, at 60,40, and to a lesser extent, 20 cm, was derived primarily from deeper sources at night (Fig. 7) than by day (Fig. 6). A mid-

afternoon temperature profile (June 4, 1985) from this set of piezometers showed that the 20 and 40 cm depths were more like streamwater (stream, 16.6 “C; 20 cm, 14.3 “C; 40 cm, 16.1 “C) than the deeper piezometers (range 11.2- 14.3 “C). Streamwater is driven into the streambed by a high velocity head at the tail of pools, depressing groundwater input in the following riffle; the effect becomes less pronounced towards the tail of riffles (Bencala et al., 1984; White & Elzinga, 1986). In the Maple R., living algal cells were retrieved from depths of 25 cm and more, carried there passively by flow from the channel into the streambed (Elzinga & White, 1986). Hydrologic studies of cation and anion transport revealed that conservative tracers released into streamwater later appeared in sediments in the streambed and in pits dug several metres away from the wetted channel during periods of low flow, presumably carried there by subsurface (i.e. hyporheic) flow patterns (Bencala et al., 1984). Whitman & Clark (1982) suggested that convective currents may be established at dawn in midsummer, when surface water temperatures are cooler than streambed waters; these denser surface waters would sink into the streambed, displacing water already present in the interstices. Such a temperature differential would occur in late summer at the Saugeen R. - daytime temperatures reach the high 20’s, but night-time temperatures often drop below IO “C. Coincidentally, in summer, DOC levels in streamwater are lowest at dawn. Movement of streamwater into the streambed later in the day would no longer be driven by convection, but by velocity head as well as depression of groundwater flow by increased evapotranspiration by nearby plants. Lee 8z Hynes (1977/78) found die1 changes in seepage rates (i.e. groundwater seepage), with mid- afternoon minima, at Hillman Ck, Ontario; they attributed the unsteady groundwater flow to maxi- mum evapotranspiration by phreatophytes near the stream.

At the Saugeen R. in late summer, we propose that a combination of these factors could cause daytime entry of streamwater into the upper streambed, yielding a mid-day temperature profile like that described (June 4, 1985); this water would be stripped of DOC at the streambed surface, resulting in relatively depleted daytime DOC in the uppermost

47

20-40 cm of streambed. Lock & Hynes (1976) and Lush & Hynes (1978) demonstrated rapid uptake of organic material by stream substrates, particularly those from hardwater. Mechanisms of uptake at the streambed could include both rapid bacterial metabolism (Dahm, 1981; Kaplan& Bott, 1983) and abiotic adsorption by the calcareous deposits (Wetzel, 1969; McDowell, 1985). At night, evapotranspiration declines and groundwater discharge to the stream is enhanced, making groundwater relatively more important in the shallow depths at night than during the day. The DOC concentrations at these depths would resemble those of the deeper piezometers (as in Fig. 7); this sequence of events would produce the die1 fluctuation of DOC shown in Fig. 10a.

On Sept. 26, there was no difference between daytime and night-time DOC concentrations at any depth (Tables 7&8), but in October we noted a small, but statistically significant, die1 fluctuation in DOC content of streambed water (Table 8). Streambed waters generally had slightly higher DOC concentrations by day, although the mean differences rarely exceeded 1 mg/l, with considerable variation (Fig. lob). Indeed, there was no clear cut pattern in DOC variation with depth either by day or night (Table 7, Fig. 8&9). In the fall, we would not expect the daytime depression of groundwater upwelling as we postulated for late summer. The Oc- tober samples followed leaf-fall; streamside herba- ceous plants had been killed by frost. Reduced vegetation, cooler daytime temperatures, and reduced metabolic activity by both deciduous and evergreen trees would result in lower rates of evapotranspiration, reducing this mechanism of depression of groundwater upwelling. In addition, in October, the temperature differential between stream and groundwater would be minimized, reducing convective mixing of streamwater into the bed. Overall DOC concentrations were somewhat lower in October (4.26-6.08 mg/l) than earlier in the year (6.36-7.62 mg/l, but precipitation and stream discharge were higher (field notes). The fall die1 fluctuation may reflect hydrologic events rather than temporal ones.

DOC content of the waters of streambeds appears to be much more heterogeneous than either the dilution or stripping hypotheses would suggest. Flow

patterns within the hyporheic zone are probably very complex, and much influenced by bed materials (e.g. clay deposits) as well as surface forms such as large rocks or hummocks of algae (Hendricks & White, 1986). Biotic and abiotic uptake of organic matter must also influence DOC concentrations in interstitial waters as has been demonstrated for channel water. We are currently investigating the microbial activity within the streambeds (to depths of 150 cm) of several southern Ontario streams. Tracer transport studies in California showed that temporary storage and sorption of solutes take place on long time scales (Bencala et al., 1984), indicating that DOC entering the streambed from either the channel or groundwater would be available for much longer periods than streamflow transport models predict. Thus, even rather low microbial uptake rates may contribute significantly to the overall carbon budget.

Acknowledgements

This work was supported by a Natural Sciences and Engineering Research Council of Canada operating grant (to H. B. N. Hynes), for which we are grateful. We thank Stacey Hunter for her hard work in the field and laboratory, Dr. R. Barker of the Dept. of Earth Sciences, U. of W., for the use of the carbon analyzer, and Mr. Ralph Dickhout for technical help. Various landowners generously gave us unres- tricted access to their properties, and we thank them.

References

Bencala, K. E., V. C. Kennedy, G. W. Zellweger, A. P. Jackman & R. J. Avanzino, 1984. Interactions of solutes and streambed sediment. 1. An experimental analysis of cation and anion transport in a mountain stream. Wat. Res. Res. 20: 1797 - 1803.

Bott, T. L., L. A. Kaplan&F. T. Kuserk, 1984. Benthic bacterial biomass supported by streamwater dissolved organic matter. Microb. Ecology 10: 335-344.

Cole, J. J., W. H. McDowell & G. E. Likens, 1984. Sources and molecular weight of “dissolved” organic carbon in an oligo- trophic lake. Oikos 42: 1-9.

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