Subsurface hydrology and degree of burial affectmass loss and invertebrate colonisation of leavesin a woodland stream

DANIELLE C. TILLMAN, ASHLEY H. MOERKE, CARRIE L. ZIEHL and GARY A. LAMBERTI

Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556-0369, U.S.A.

SUMMARY

1. In deciduous forest streams, fallen leaves form a large component of the total organic

matter budget, and many leaves become buried within stream sediments. We examined

the processing of buried leaves as compared with those at the surface, and the influence of

subsurface hydrology on processing rates.

2. Leaf packs were secured on the streambed surface or buried 10 cm deep in upwelling

and downwelling reaches of a second-order stream in Michigan, U.S.A. Mass loss and

invertebrate colonisation were measured from October to February.

3. Leaves buried in upwelling reaches lost mass more slowly (exponential decay

coefficient, k ¼ )0.0097) than did leaves from the other treatments (buried downwelling:

)0.017; surface upwelling: )0.022; surface downwelling: )0.021).

4. Initially, more invertebrates colonised surface leaf packs than buried packs. During the

remainder of the study, however, hydrology had a greater effect on invertebrate

abundance than did burial, as more invertebrates were found in packs in downwelling

reaches than in upwelling reaches.

5. Local subsurface hydrology and degree of burial, factors rarely considered in studies of

detritus processing, can significantly influence mass loss and invertebrate colonisation of

fallen leaves in streams. Furthermore, because of slower processing, subsurface zones may

function as organic matter reservoirs that gradually ‘spiral’ carbon to downstream

subsurface and surface habitats.

Keywords: carbon cycling, detritus processing, hyporheic zone, meiofauna, Michigan stream

Introduction

Local vertical hydraulic gradient (VHG) can influence

many stream patterns and processes, including nutri-

ent concentrations (Triska, Duff & Avanzino, 1993;

Valett, 1993; Findlay, 1995), invertebrate distributions

(Williams, 1993; Plenet & Gibert, 1994), metabolism

(Jones, Fisher & Grimm, 1995) and carbon dynamics

(Vervier & Naiman, 1992; Wagner, Schmidt &

Marxsen, 1993; Marmonier et al., 1995). Most studies

of hyporheic carbon dynamics have focused on

dissolved organic carbon (DOC). However, each

autumn in temperate regions, huge quantities of

particulate organic carbon (POC) enter woodland

streams as deciduous leaves (Fisher, 1973), and much

of that carbon may become buried within the sedi-

ments (Smock, Metzler & Gladden, 1989; Metzler &

Smock, 1990). Knowledge of the fate of buried POC is

important for a complete understanding of stream

carbon cycling.

Processing of POC is influenced by a number of

environmental factors (see below and review by

Webster & Benfield, 1986). Conditions can differ

between locations on or within the sediments and

can also vary based on local VHG. For example, faster

leaf processing has been associated with increased

Correspondence: Gary A. Lamberti, Department of Biological

Sciences, University of Notre Dame, Notre Dame, IN 46556-0369,

U.S.A. E-mail: lamberti.1@nd.edu

Freshwater Biology (2003) 48, 98–107

98 � 2003 Blackwell Publishing Ltd

oxygen availability (Reed, 1979). Surface water gen-

erally contains more dissolved oxygen (DO) than does

pore water (Hendricks & White, 1991), and DO

concentration is usually higher in downwelling than

in upwelling reaches (Valett, 1993). Faster leaf break-

down has also been associated with higher tempera-

ture (e.g. Rowe et al., 1996). Pore water temperature

generally fluctuates less than surface water tempera-

ture (Silliman & Booth, 1993), and pore water tem-

perature in upwelling reaches tends to be higher than

that in downwelling regions as the air temperature

drops. Thus, in the late autumn and winter, after

deciduous leaf-abscission, the temperature in upwel-

ling reaches can be higher than that in downwelling

reaches. Water nutrient concentrations can also

influence leaf breakdown (Fairchild, Boyle & Robin-

son-Wilson, 1983; Suberkropp & Chauvet, 1995), and

these concentrations often differ between pore and

surface water (Hendricks & White, 1991; Valett, 1993;

Hendricks & White, 1995).

Physical and chemical variability in aquatic envir-

onments also leads to variability in the abundance and

distribution of biota (Dole-Olivier et al., 1994; Boulton

& Stanley, 1995). Because invertebrates are important

processors of leaves in many lotic systems (Anderson

& Sedell, 1979; Kirby, Webster & Benfield, 1983;

Cuffney, Wallace & Lugthart, 1990), spatial hetero-

geneity in invertebrate communities may lead to

heterogeneity in the pattern of leaf processing. Inver-

tebrate abundance and diversity can differ between

assemblages on and within the sediments (Godbout &

Hynes, 1982; Strommer & Smock, 1989; McElravy &

Resh, 1991; Schmid-Araya, 1995; Palmer et al., 2000),

and more individuals and taxa have been found in

downwelling than in upwelling reaches (Creuze des

Chatelliers, 1991; Dole-Olivier, Marmonier & Beffy,

1997).

We conducted a 120-day experiment in a Michigan

stream to investigate how VHG (upwelling or down-

welling) and location within the streambed (buried or

on the surface) affects the breakdown of leaf detritus.

We hypothesised that leaves would be processed (i.e.

lose mass) more rapidly on the surface than when

buried and more rapidly in downwelling than in

upwelling reaches. In addition, we expected inverte-

brate colonisation to differ, based on degree of burial

and local subsurface hydrology, with greater inverte-

brate abundance and diversity in surface than in buried

packs and in downwelling than in upwelling zones.

Methods

Study site

Bertrand Creek (Berrien Co., MI, USA; 41�46¢N,

86�17¢W) is a second-order woodland stream of

about 3 km in length and with a catchment of

5.6 km2. Maple (Acer spp.) and oak (Quercus spp.) are

the most common riparian trees, and the channel has

a dense canopy. The stream drains an area of glacial

till, and sediment grain size is heterogeneous. Bert-

rand Creek cuts through several geological strata

and, as a result, the direction of VHG within the

sediments changes along the stream course. Our

study sites were positioned within four predomin-

antly sandy reaches: two downwelling (positive

VHG) and two upwelling (negative VHG) reaches.

Reaches were approximately 250 m apart and the

substratum was comprised of small gravel and

coarse sand (27%), medium sand (60%), and fine

sand (13%) according to a modified Wentworth scale

(Cummins, 1962). The VHG, determined by ran-

domly placing multiple minipiezometers throughout

the study reaches (Lee & Cherry, 1978), averaged

)0.01 cm cm)1 in downwelling reaches and

0.03 cm cm)1 in upwelling reaches.

Ambient sampling

Surface water, pore water and invertebrate samples

were collected to provide background information for

this study. An Orion 835 DO meter (Orion Research

Incorporated, Boston, MA, USA) was used to measure

temperature and DO concentration in the field. Surface

water and pore water samples (n ¼ 6 per reach) were

collected with a hand vacuum-pump attached to a 0.6-

cm diameter piezometer. Surface water was collected

at mid-depth of the thalweg and sediment pore water

was collected at 12–13 cm deep at randomly selected

locations within the downwelling and upwelling

reaches. Water samples were kept on ice in the dark

until returned to the laboratory for processing. Water

subsamples were analysed for NH4-N, NO3-N, PO4-P,

and DOC. Water subsamples for NH4-N analyses were

frozen until analysed with the manual method of

Solorzano (1969) and Harwood & Kuhn (1970).

Subsamples for NO3-N and PO4-P analyses were

filtered through Gelman 0.45 lm pore size membrane

filters and frozen. NO3-N concentration was measured

using the hydrazine method (Kamphake, Hannah &

Stream subsurface hydrology and leaf breakdown 99

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 98–107

Cohen, 1967), and PO4-P concentration with the

manual ascorbic acid method (Murphy & Riley,

1962). For DOC analyses, water was filtered through

preashed Whatman GF ⁄F glass fibre filters (0.7 lm

pore size). Sodium azide (5 lg mL)1 of 0.54 MM) was

added to each sample, and samples were refrigerated

until DOC concentration was determined with a

Shimadzu 5000 Total Organic Carbon Analyser (Shi-

madzu Scientific Instruments, Columbia, MO, USA).

Invertebrates were collected using sediment cores

(7.5 cm diameter) from upwelling and downwelling

reaches (n ¼ 6 per reach). From each core, the sedi-

ments from 0 to 5 cm and 10–15 cm deep were

separated. Magnesium chloride solution was added

to each bag to promote release of fauna from the

sediments (Palmer & Strayer, 1996). Rose Bengal was

also added to stain the invertebrates and ease detec-

tion. Invertebrates were identified at 100· under a

dissecting microscope.

Leaf pack construction, deployment, and analysis

During September–October 1996, falling sugar maple

(Acer saccarhum Marsh) leaves were collected on

fibreglass nets suspended between riparian trees

along Bertrand Creek. These leaves were pooled, air-

dried for about 7 days, randomly divided into

approximately 5-g (dry mass) packs, and their petioles

were secured together with bands of 2-cm width

labelling tape. In order to eliminate any potential

enclosure effects, as reported by Rosemond, Pringle &

Ramırez (1998), the leaf packs were not placed within

a mesh bag or cage. After construction, each pack was

weighed to the nearest 0.01 g and taken to the field in

a separate plastic bag.

The leaf packs were placed in Bertrand Creek at the

end of October, within few days of peak litterfall. In

the field, we carefully removed each pack from its bag

and soaked it in a bucket of stream water for several

minutes. This procedure softened the leaves and

reduced immediate mechanical damage when the

leaves were exposed to the stream current. After the

packs were removed from the plastic bags, the latter

were returned to the laboratory where any leaf pieces

that had broken off were weighed and subtracted from

the initial mass of each pack. Ten packs were tied onto

4-m lengths of monofilament, spaced approximately

30 cm apart. Half of the packs were secured at random

to the main line with 5-cm long tethers, while the other

half of the packs were secured with 15-cm long tethers.

In each of the four reaches, four lines of bags were

secured across the stream on transects approximately

30 cm apart. The lines were placed as close to the

sediment–water interface as possible and secured to

stakes on the bank. The packs with 15-cm long tethers

were buried about 10 cm below the sediment–water

interface, whereas the packs with the 5-cm long tethers

were allowed to rest on the sediment surface.

A subset of the leaf packs was collected at random on

days 7, 14, 25, 38 and 120 after initial placement of the

packs in the stream. Because more variability in

response variables was expected as the study pro-

gressed (Boulton & Boon, 1991), more packs were

collected later in the study. From each site, three packs

per treatment (buried or surface) were collected on

days 7 and 15, four packs were collected on days 25 and

38, and five packs were collected on day 120. The total

number of packs also varied because of occasional leaf

pack losses. When the packs were collected, a plastic

bag was held open just downstream from each pack, the

tether was cut, and the pack was carefully and quickly

slid into the bag. Before buried packs were collected,

they were carefully exposed by gradually removing

overlying sediment; we saw little evidence that leaf

fragments or invertebrates escaped during this process,

but some loss was possible.

In the laboratory, each leaf pack was gently rinsed

with tap water over a 45-lm mesh sieve, and the

contents of the sieve were then transferred to a plastic

bag and preserved with 70% ethanol. Rose bengal

was also added to each bag. After at least 48 h, the

contents of the bag were transferred to a 45-lm mesh

sieve, rinsed thoroughly to remove excess dye, and

examined under a dissecting microscope at 100·.

Invertebrates collected on days 7, 38 and 120 were

identified to the lowest practical taxonomic level.

The rinsed leaf packs were placed in paper bags,

hung in the laboratory, air-dried to constant mass and

weighed to the nearest 0.01 g. Percent mass remaining

was calculated as

[Final dry mass/(initial dry mass

� pieces broken off in plastic bag)] � 100

Data analyses

Leaf decomposition rates (k) for each of the four

hydrology–depth treatment combinations (down-

100 D.C. Tillman et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 98–107

welling buried, downwelling surface, upwelling bur-

ied, upwelling surface) were calculated using an

exponential decay model, Mt ¼ Moe–kt, where Mo is

the initial mass, Mt is the mass remaining at time t,

and t is the time in days (Benfield, 1996). Data for days

7, 14, 25, 38 and 120 were included in the model (day 0

was set at 100% but not sampled with true replicates).

The fit of the lines to the exponential model was

assessed using linear regression, and the slopes of the

lines were statistically compared with ANCOVAANCOVA (Zar,

1996). To examine patterns in leaf mass loss and leaf

pack invertebrate communities at specific periods

during the study (beginning, middle and end), leaf

mass loss data and invertebrate data (expressed both

on a per leaf pack and a per unit mass basis) were

analysed separately by date (days 7, 38 and 120) using

a split-plot ANOVAANOVA (Johnson & Leone, 1977; Steel &

Torrie, 1980). The VHG direction (upwelling or

downwelling) was used as the whole plot factor,

and depth (surface or buried) served as the sub plot

factor. The split-plot ANOVAANOVA is appropriate because

the treatments required experimental units of unequal

sizes (Johnson & Leone, 1977; Steel & Torrie, 1980). In

our case, hydrology treatments required a larger

spatial scale than did depth treatments. Data were

log-transformed when necessary to meet the assump-

tions of ANOVAANOVA, and post hoc comparisons were

performed with Tukey’s multiple comparison tests

(Zar, 1996).

Results

Ambient sampling

Water temperature and concentrations of NH4-N,

PO4-P or DOC did not significantly differ between

surface and pore water or between water from

downwelling and upwelling reaches (Table 1).

However, NO3-N concentration and DO concentra-

tion were significantly lower in upwelling reach

pore water than in downwelling reach pore water

or in any surface water (NO3-N: P < 0.05; DO:

P < 0.001). Invertebrate abundance and richness

were significantly higher in sediments 0–5 cm deep

than in sediments 10–15 cm deep (P < 0.05; Ta-

ble 2). Abundance and richness also were signifi-

cantly higher in downwelling than in upwelling

reaches (P < 0.05).

Leaf breakdown

An exponential decay model regressing each hydrol-

ogy-depth combination against time significantly fit the

leaf breakdown data from each treatment (Table 3).

Decay coefficients ranged from )0.0097 for upwelling

buried packs (slowest processing) to )0.022 for

upwelling surface packs (fastest processing), and the

slopes of the regression lines were not parallel

(F3,127 ¼ 7.43, P ¼ 0.0001; Fig. 1). Mass loss over the

120 days was significantly slower for leaves buried in

Table 1 Mean (1 SE; n ¼ 12) physical and chemical parameters measured in Bertrand Creek for surface water and pore water

(collected from 12 to 13 cm deep in sediments) in downwelling and upwelling reaches

Parameter

Surface water Pore water

Downwelling Upwelling Downwelling Upwelling

Temperature (�C) 10.2 (0.2) 10.2 (0.3) 9.9 (0.3) 10.5 (0.2)

Dissolved oxygen (mg L)1) 11.3 (0.4) 11.6 (0.7) 8.9 (1.0) 2.6 (0.6)

NO3-N (lg L)1) 664 (59) 664 (37) 737 (68) 305 (157)

NH4-N (lg L)1) 40.6 (6.6) 35.4 (3.7) 34.3 (6.7) 36.4 (4.9)

PO4-P (lg L)1) 21.1 (2.5) 21.0 (2.0) 25.3 (1.9) 20.0 (1.4)

DOC (lg L)1) 2.75 (0.85) 3.98 (1.34) 4.10 (1.13) 6.09 (2.48)

Table 2 Mean (1 SE; n ¼ 12) invertebrate density and taxon richness in sediment cores collected from Bertrand Creek

Metric

Surface (0–5 cm) Buried (10–15 cm)

Downwelling Upwelling Downwelling Upwelling

Density (invertebrates dm)3) 87.9 (25.7) 35.2 (7.2) 21.8 (7.0) 8.8 (2.5)

Taxon richness (taxa dm)3) 6.50 (0.86) 4.75 (0.81) 3.50 (0.71) 3.17 (0.68)

Stream subsurface hydrology and leaf breakdown 101

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 98–107

upwelling reaches than for buried (P ¼ 0.0087) and

surface leaves (P < 0.0001) in downwelling reaches.

Split-plot ANOVAANOVA was used to analyse treatment

effects on days 7, 38 and 120, which represented early,

mid- and late processing (Fig. 1). Surface leaves had

significantly more mass remaining after 7 days than

did buried leaves (F1,16 ¼ 31.6, P ¼ 0.0004), but there

was no effect of hydrology on leaf mass at this time

(F1,16 ¼ 0.473, P ¼ 0.50). In contrast, after 38 days

buried leaves had significantly more mass remaining

than did surface leaves (F1,24 ¼ 5.83, P ¼ 0.024), and

leaves in upwelling reaches had more mass remaining

than did leaves in downwelling reaches (F1,24 ¼ 6.90,

P ¼ 0.015). A depth effect was also observed on day

120, with significantly more mass remaining for

buried than for surface leaves (F1,22 ¼ 6.68,

P ¼ 0.017), but hydrology had no significant effect

on leaf mass remaining on day 120 (F1,22 ¼ 2.42,

P ¼ 0.14).

Invertebrate colonisation

For most treatment combinations and dates, Chiro-

nomidae, Nematoda or Rotifera were the most com-

mon invertebrate taxa per unit of leaf mass (Fig. 2).

Taxon richness increased over time (Fig. 3). Inverte-

brates initially colonised surface leaf packs more

rapidly than buried packs. At day 7, surface packs

contained more individuals per unit mass

(F1,15 ¼ 16.3, P ¼ 0.0011; Fig. 2) and more taxa per

unit mass (F1,15 ¼ 31.1, P < 0.0001; Fig. 3) than did

buried packs. At 38 days, the effects of burial on

invertebrate communities were not evident, whereas

the effects of local subsurface hydrology became more

important. Packs in downwelling reaches contained

more individuals per unit mass (F1,20 ¼ 17.2,

P ¼ 0.0005) and more taxa per unit mass

(F1,20 ¼ 6.43, P ¼ 0.020) than did packs in upwelling

reaches. At day 120, packs in downwelling reaches

still contained more individuals per unit mass

(F1,21 ¼ 7.46, P ¼ 0.012), but degree of burial did not

significantly affect invertebrate density (F1,21 ¼ 1.70,

P ¼ 0.21). Neither degree of burial nor hydrology

significantly influenced taxa richness at day 120

(F1,21 ¼ 0.300, P ¼ 0.59; F1,21 ¼ 1.54, P ¼ 0.23). Patterns

Fig. 1 Mean (±1 SE) percent leaf dry mass remaining as a

function of time from initial leaf pack placement in Bertrand

Creek. P-values given for days 7, 38 and 120 are for a split-plot

A N O V AA N O V A, with hydrology as the whole plot and depth as the sub

plot.

Fig. 2 Abundance of most common taxa found in leaf packs in

Bertrand Creek (n ¼ 2–5). P-values are for a split-plot A N O V AA N O V A,

with hydrology as the whole plot and depth as the sub plot.

Error bars represent +1 SE of total abundance. DB, downwelling

buried; DS, downwelling surface; UB, upwelling buried; US,

upwelling surface.

Table 3 Decay coefficients (k), R2 values and regression

P-values of leaf breakdown for the four treatment combinations

over time

Treatment k R2 P

Downwelling buried ) 0.017 0.88 < 0.001

Downwelling surface ) 0.021 0.89 < 0.001

Upwelling buried ) 0.0097 0.62 < 0.001

Upwelling surface ) 0.022 0.93 < 0.001

102 D.C. Tillman et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 98–107

were similar when invertebrate density or taxa rich-

ness were analysed on a per pack rather than a per

unit mass basis, with one exception: more taxa per

pack were present in surface leaves than in buried

leaves on day 38 (F1,20 ¼ 4.62, P ¼ 0.044).

Discussion

Leaf pack breakdown

Leaves that fall into streams can become buried

during bed scour and redeposition events (Metzler

& Smock, 1990) or by infiltration of leaf fragments

through coarse sediments (Brunke & Gonser, 1997). In

Bertrand Creek, most of the mass of buried leaf packs

was gone after 4 months, suggesting that organic

matter cycling is rapid despite burial. After 1 week in

the stream, buried leaves had less mass remaining

than did surface leaves, perhaps because of increased

mechanical breakage while leaves were being buried

and recovered. However, leaf breakage would prob-

ably also occur during natural burial. Surface leaves

had less mass after 38 and 120 days, indicating that

biological processing may be slower for buried than

for surface leaves. Slower breakdown of buried,

compared with surface leaves has been reported by

several researchers (Herbst, 1980; Rounick & Winter-

bourn, 1983; Smith & Lake, 1993).

Lower temperature has been associated with slower

decomposition (e.g. Rowe et al., 1996). Temperature,

however, did not differ among treatments in our

study and therefore is unlikely to account for patterns

of leaf breakdown. Rounick & Winterbourn (1983)

postulated that reduced water flow over buried leaf

surfaces may have led to a decline in microbial

oxygen consumption, resulting in a slower break-

down of buried packs relative to surface packs in their

study. Hyporheic pore water in Bertrand Creek is

generally oxic at the depth (10 cm) at which leaves

were buried in this study, although oxygen may have

been depleted locally within the leaf packs. Low DO

concentrations (<3 mg L)1) in upwelling reaches also

may have contributed to slower leaf breakdown

relative to downwelling reaches. The combination of

burial and upwelling water produced the slowest

decay rates in our study.

After 120 days, leaves on the streambed surface in

the upwelling reach had lost a percentage of initial

mass similar to both surface and buried packs in

downwelling reaches. This pattern may be because of

the similarity in many physical and chemical attrib-

utes between pore water in downwelling reaches and

surface water in both downwelling and upwelling

reaches. In contrast, buried leaves in upwelling

reaches were exposed to the least amount of surface

water and broke down more slowly than surface and

buried leaves in downwelling reaches. Boulton (1993)

suggested that leaf packs in upwelling zones may

have faster leaf breakdown because of higher nutrient

supply or possibly because they support a different

microbial community that is more palatable to stream

detritivores. However, our study suggests that differ-

ences in water chemistry, such as lower DO and

inorganic nitrogen, may have contributed to slower

leaf processing rates in upwelling zones of Bertrand

Creek.

Invertebrate colonisation

Invertebrates were probably important processors of

leaf material during this study, as also found by

other researchers (Anderson & Sedell, 1979; Kirby

et al., 1983; Webster & Benfield, 1986; Cuffney et al.,

1990). Invertebrates tended to be more abundant and

diverse in leaf packs that had the fastest mass loss,

particularly those leaves on the streambed surface.

Invertebrate colonisation was slowest in upwelling

and buried leaves. Ease of colonisation, as well as

natural spatial variability in the invertebrate benthos,

Fig. 3 Invertebrate diversity in leaf packs in Bertrand Creek

(n ¼ 2–5). P-values are for a split-plot A N O V AA N O V A, with hydrology

as the whole plot and depth as the sub plot. Error bars represent

+1 SE. DB, downwelling buried; DS, downwelling surface; UB,

upwelling buried; US, upwelling surface.

Stream subsurface hydrology and leaf breakdown 103

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 98–107

may have contributed to these differences. Inverteb-

rate drift would have led to more rapid colonisation

of surface than buried packs. Initially, we found

fewer individuals and taxa in buried packs than in

surface packs, similar to the findings of Rounick &

Winterbourn (1983) in a New Zealand stream. From

core samples, we also found that surface inverte-

brates in Bertrand Creek were generally more

abundant and diverse than invertebrates within the

sediments. Gibert et al. (1990) suggested that hypor-

heic communities may be less diverse than surface

communities for reasons that include reduced pore

space and decreased availability of oxygen and

organic matter.

In Bertrand Creek sediments, hyporheic inverte-

brates were more abundant and diverse in downwel-

ling than in upwelling reaches, consistent with the

patterns that we observed in leaf packs. Possible

explanations for the greater abundance and diversity

of invertebrates in downwelling reaches include

higher DO (from surface water) and greater food

availability (e.g. periphyton products and leaf frag-

ments drawn into the pore spaces). Previous studies

have shown a positive relationship between the

diversity of interstitial invertebrate and DO concen-

tration (Strommer & Smock, 1989; Plenet & Gibert,

1994). Lenting, Williams & Fraser (1997) found that a

higher quality of hyporheic organic matter (as

measured by the carbon-to-nitrogen ratio) was asso-

ciated with increased invertebrate abundance and

taxon richness. Although leaves are a food resource,

Boulton & Foster (1998) did not find significant

changes in local invertebrate abundance and taxon

richness in streams with buried leaf packs. In Bertrand

Creek, a combination of better physical conditions and

more food resources may have provided invertebrates

with better habitat in downwelling reaches.

Invertebrate taxonomic composition was quite

variable both within and amongst sites. Although

abundant in this study, rotifers and tardigrades have

rarely been reported as common leaf pack inverte-

brates. Other studies have generally used a larger

mesh size than the 45-lm mesh sieve used in this

study to retain invertebrates (Smith & Lake, 1993;

Meegan, Perry & Perry, 1996; Rowe et al., 1996) and

consequently may have missed these small inverte-

brates. Amphipods (Gammarus sp.) were the only

macroinvertebrates commonly found in the leaf

packs, although other large shredders such as cranefly

larvae (Tipula sp.) were present in the stream and

probably contributed to leaf breakdown.

Implications for stream processes

Several factors, including degree of burial, VHG, local

chemistry, water temperature, hydraulics and inver-

tebrate colonisation, probably operate in concert to

determine the breakdown of riparian leaves in

streams. Our study showed that variation in subsur-

face hydrology can affect leaf processing rates, with

faster decomposition occurring in downwelling rea-

ches than in upwelling reaches. Furthermore, burial of

organic matter may reduce the rate of leaf processing

when compared with the surface environment. Thus,

the hyporheic zone may serve as a temporary reser-

voir for organic matter in streams and affect the

overall pattern of stream metabolism.

Because over 20% of autumn leaf input can become

buried in stream sediments (Metzler & Smock, 1990),

processing of buried leaves should be considered in

future studies of stream carbon cycling. Newbold

et al. (1982) noted that ‘the carbon turnover length is

the ratio of the downstream transport flux of carbon

(per unit width of stream) to the respiratory utilisation

of carbon (per unit area of stream bottom)’. Although

our study did not quantify leaf transport, this flux was

probably low because Bertrand Creek is a highly

retentive headwater stream with many debris dams.

Furthermore, Webster et al. (1999) concluded that, in

headwater streams, leaves are generally broken down

near their point of entry into the stream. Our study

indicates that leaf litter carbon may be processed more

rapidly in downwelling reaches than in upwelling

reaches because of faster subsurface processing. As a

consequence, the carbon turnover length may be

shorter and reach efficiency (sensu Newbold et al.,

1982) greater in downwelling than in upwelling

reaches. Although local subsurface hydrology may

affect carbon processing rates, the eventual fate of

the unrespired carbon depends on whether it leaves

the hyporheic zone via scour (i.e. resuspension) or

percolation. If the carbon is not scoured from the

streambed during high flow, it may percolate through

the sediment interstices as fine particulates or follow

hyporheic flow paths in dissolved form (Mathieu,

Essafi & Doledec, 1991; Fiebig, 1995; Marmonier et al.,

1995; Claret et al., 1997). In that case, the faster

decomposition of organic matter in downwelling

104 D.C. Tillman et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 98–107

reaches may result in carbon leaving the hyporheic

zone more rapidly in downwelling reaches than in

upwelling reaches. As a result, leaf packs in down-

welling reaches may act as a source of particulate and

dissolved organic carbon to groundwater systems,

which may then be transported to downstream

upwelling areas. In this way, hyporheic zones may

be important sites for a subsurface carbon ‘spiral’,

which can have significant implications for carbon

metabolism in stream ecosystems.

Acknowledgments

We are grateful to Katherine Mosca for her help in the

field and laboratory and to Eric Strauss for statistical

assistance. We also thank the Brazo and White

families for access to the study sites. This research

was supported by an NSF Predoctoral Fellowship to

DCT, and by U.S. Geological Survey Grant no. 1434-

HQ-96-GR-02669 through Purdue University.

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