Habitat effects on invertebrate drift in a small trout stream: implications for prey availability to drift-feeding fish
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Hydrobiologia (2009) 623:113–125
DOI 10.1007/s10750-008-9652-1
PRIMARY RESEARCH PAPER
Habitat effects on invertebrate drift in a small trout stream:
implications for prey availability to drift-feeding fish
Elaine S. Leung Æ Jordan S. Rosenfeld Æ
Joanna R. Bernhardt
Received: 5 June 2008 / Revised: 3 October 2008 / Accepted: 8 November 2008 / Published online: 2 December 2008
Ó Springer Science+Business Media B.V. 2008
Abstract In this study, we focused on the drivers of abundance at the mesohabitat scale, although drift
micro- and mesohabitat variation of drift in a small concentration was highest in riffle habitats. Similarly,
trout stream with the goal of understanding the factors there was no differentiation of drifting invertebrate
that influence the abundance of prey for drift-feeding community structure among summer samples col-
fish. We hypothesized that there would be a positive lected from pools, glides, runs, and riffles. Drift
relationship between velocity and drift abundance concentration was significantly higher in winter than
(biomass concentration, mg/m3) across multiple spa- in summer, and variation in drift within individual
tial scales, and compared seasonal variation in mesohabitat types (e.g., pools or riffles) was lower
abundance of drifting terrestrial and aquatic inverte- during winter high flows. As expected, summer
brates in habitats that represent the fundamental surface samples also had a significantly higher
constituents of stream channels (pools, glides, runs, proportion of terrestrial invertebrates and higher
and riffles). We also examined how drift abundance overall biomass than samples collected from within
varied spatially within the water column. We found no the water column. Our results suggest that turbulence
relationship between drift concentration and velocity and the short length of different habitat types in small
at the microhabitat scale within individual pools or streams tend to homogenize drift concentration, and
riffles, suggesting that turbulence and short distances that spatial variation in drift concentrations may be
between high- and low-velocity microhabitats mini- affected as much by fish predation as by entrainment
mize changes in drift concentration through settlement rates from the benthos.
in slower velocity microhabitats. There were also
minimal differences in summer low-flow drift Keywords Invertebrate drift Drift-feeding
Spatial variation Trout Salmonids Streams
Handling editor: Robert Bailey
E. S. Leung (&) J. R. Bernhardt
Introduction
British Columbia Conservation Foundation, #206,
17564 56A Avenue, Surrey, BC, Canada V3S 1G3
e-mail: elaineswleung@gmail.com Drift, the downstream transport of invertebrates
suspended in the water column (Shearer et al.,
J. S. Rosenfeld
2003), is a defining attribute of stream ecosystems.
British Columbia Ministry of the Environment,
2202 Main Mall, Vancouver, BC, Canada V6T 1Z4 Drift affects the recolonization of denuded habitats
e-mail: Jordan.Rosenfeld@gov.bc.ca following disturbance (Townsend & Hildrew, 1976;
123114 Hydrobiologia (2009) 623:113–125 Sagar, 1983) and strongly influences invertebrate has been observed to increase with velocity (Brittain community structure and composition (Hansen & & Eikeland, 1988) since invertebrates are dislodged Closs, 2007). One of the most important trophic as the scouring effect of flow increases with stream consequences of variation in drift is to directly affect discharge (Ciborowski, 1983; Mackay, 1992), and the availability of food for drift-feeding fishes once entrained in the water column resettlement rates (Elliott, 1967; Sagar & Glova, 1988; Dedual & of drifting invertebrates are lower in swift water Collier, 1995; Hayes et al., 2000), thereby influenc- (Bond et al., 2000). Since velocity and habitat type ing both habitat selection and individual growth may influence drift abundance (Reisen & Prins, 1972) (Hayes et al., 2007). Drift dynamics should also have and thus prey availability for fish, understanding how a profound effect on fish production and the effi- these factors drive differences in drift concentration ciency of transfer of primary consumer biomass to the between habitats is vital to understanding the ecology fish trophic level. It is therefore important to under- and production of species that feed on drift. stand (1) how drift varies seasonally, since this will In order to better understand the relationship affect seasonal variation in prey availability for fish, between habitat structure and variation in drift abun- and (2) how drift varies at micro- and mesohabitat dance in small streams, we compared the abundance scales within a stream channel, since this will of aquatic and terrestrial invertebrate drift across determine habitat choice and the energetic benefits different habitat types in the summer and winter in that fishes experience in different habitats, as well as Husdon Creek, a small cutthroat trout stream in local fish abundance and production. coastal British Columbia, Canada. Since the funda- Average fish biomass and production as well as the mental constituent habitat types in small streams— spatial distribution of fish have been found to be pools, glides, runs, and riffles—differ in depth and positively correlated with drift abundance (e.g., Elli- velocity (Peterson & Rabeni, 2001), we anticipated ott, 1973; Wilzbach et al., 1986; Ensign et al., 1990; that drift abundance might also differ between habitat Shannon et al., 1996). Predictability of drift distribu- types. Based on previous research and the assumption tion within a stream may therefore allow fish to that higher velocities lead to higher entrainment and identify the most profitable feeding locations (Hansen lower settling rates of drifting invertebrates, we & Closs, 2007). Since increased prey availability predicted (1) that there would be a positive correlation enhances fish growth (e.g., Rosenfeld et al., 2005) and between drift concentration and water velocity at survival increases with body size (Holtby et al., 1990), multiple spatial scales, (2) that the highest drift food availability is an important index of habitat abundances would therefore occur in riffle habitats, quality for fish (Nislow et al., 1998). Terrestrial and (3) that drift abundance in the winter would be invertebrates also represent a significant component higher than in the summer due to increased winter of drift, particularly at the stream surface (Sagar & discharge and water velocity. Given the hierarchical Glova, 1992) and are an important energy source for nature of stream habitats, we also expected that fish (Elliott, 1973; Allan et al. 2003; Romaniszyn variation in drift within a single habitat type (e.g., a et al., 2007). Managing the production of drift-feeding pool) would be less than variation between habitats fishes (e.g., stream salmonids) should therefore be (e.g., pools vs. riffles). In addition to these predicted informed by an understanding of the habitat factors relationships between absolute drift abundance and that influence the abundance of both terrestrial and habitat factors, we further hypothesized that the aquatic prey (Romaniszyn et al., 2007). community structure of drifting invertebrates would Research on invertebrate drift has largely focused respond to habitat (e.g., Cellot, 1996), and (1) that on temporal variation in drift, the effects of photo- terrestrial drift, and therefore total drift abundance, period, water temperature, predators, life-cycle stage would be higher at the water surface than in the water (reviewed in Brittain & Eikeland, 1988) and the roles column, and (2) that potential differences in benthic of disturbance and resource competition in drift invertebrate community structure between habitats initiation (Statzner et al., 1987; Ramirez & Pringle, (e.g., pools vs. riffles) would result in differences in 1998; Siler et al., 2001). However, fewer studies have the taxonomic composition of drifting invertebrates investigated microhabitat variation in drift within a associated with different habitat types. Finally, to stream and the factors that drive this variation. Drift understand the importance of fish predation relative to 123
Hydrobiologia (2009) 623:113–125 115
abiotic drivers of drift abundance, we applied simple by protruding substrata, and shallow) as described in
bioenergetic modeling to assess the potential for drift- Johnston & Slaney (1996) and Moore et al. (1997).
feeding fishes to reduce drifting invertebrate abun- Drift nets consisted of a 60-cm long bag of 250-lm
dance in Husdon Creek. Nitex screen attached to a 17.5 cm by 17.5-cm square
collar molded from a PVC pipe. A slightly smaller but
shallower drift net (17 cm by 5 cm) made of the same
Methods materials was fitted into the larger drift net, and could
be adjusted vertically within the larger net to sample
Study site the surface 1–2 cm of the water column, while the
larger net simultaneously sampled subsurface flow.
Drift samples and habitat measurements were col- Water depth and velocity were measured using a
lected from a 1 km reach of Husdon Creek, a small Marsh–McBirney model 2000 flow meter in both the
coastal stream on the Sunshine Coast of British larger and smaller drift nets when the nets were set and
Columbia, 50 km north of the city of Vancouver, lifted, in order to calculate the total volume of water
Canada. Husdon Creek has an average bankfull width filtered by a drift net over the duration of each set.
of 3.4 m, a summer baseflow of approximately 20– In order to ensure that drift samples collected on
30 l/s, and drains a 3.4 km2 watershed of second the same day were set and lifted within a relatively
growth forest. The reach of stream used for exper- narrow time frame, we sampled summer drift over
iments had approximately 75% canopy cover, a 1% 2 days during summer low-flow discharge (approxi-
gradient, substrate dominated by gravel and sand, and mately 30 l/s). Ten drift nets were set in separate
abundant large woody debris (LWD; 0.37 pieces of channels units between 10:20 and 11:00 on July 21,
LWD per linear meter) in a forced pool-riffle channel 2001 and lifted between 15:15 and 16:15, and 10 drift
(Montgomery et al., 1995). Husdon Creek typifies the nets were set on July 22 from 11:30 to 12:20 and
small stream habitat where juvenile anadromous lifted from 16:30 to 17:40.
coastal cutthroat trout are most abundant (Rosenfeld The durations of sets ranged from 4.9 to 5.3 h on
et al., 2000), and the fish community consists of July 21 and from 4.9 to 5.5 h on July 22. Set and lift
cutthroat trout in three age classes (young-of-the-year times were chosen to represent the daylight hours
YOY, 1?, and 2? fish) and YOY coho salmon. when juvenile cutthroat trout and coho salmon were
Juvenile cutthroat trout and coho densities are observed to forage in Husdon Creek, and to avoid pre-
relatively high, averaging 0.92 and 0.2 fish/m2, dawn (5:07) and post-dusk (21:19) peaks in drifting
respectively (J. Rosenfeld, unpublished data). invertebrates that may not be available to drift-feeding
fishes at low light levels (juvenile trout and salmon
Drift sampling were not observed to forage at night; J. Rosenfeld pers.
obs.). Since the number of invertebrates collected in
Variation between habitat types drift nets is extremely sensitive to disturbance that can
suspend benthic organisms, drift nets were set from
As part of a broader study to understand the physical upstream to downstream, and extreme care was taken
habitat attributes of small coastal streams (Rosenfeld to minimize disturbance and avoid stepping in the
et al., 2000), and the fitness consequences of different stream when setting drift nets. In addition, we placed
habitats to juvenile trout (Rosenfeld & Boss, 2001), 250-lm Nitex screen covers over the mouths of drift
we surveyed different habitat types in Husdon Creek nets immediately after they were set, so that we could
and selected five representative pool, glide, run, and initiate sampling by removing covers more or less
riffle habitats for drift sampling (total of 20 channel simultaneously and avoiding temporal artefacts. Final
units). Habitat types were defined as pools contents of drift nets were rinsed into a bucket and
(0% gradient, low current velocity, and deep), glides then transferred into a sampling jar using a 250-lm
(0–1% gradient, slow current velocity, and minimal sieve and preserved in 5% formalin.
water surface turbulence), runs (1–2% gradient, high We sampled winter drift on December 20, 2001,
current velocity, and turbulent flow), or riffles (1–3% when discharge was near bankfull (approximately
gradient, high current velocity, water surface broken 440 l/s). We sampled three pools, two glides, two
123116 Hydrobiologia (2009) 623:113–125
runs, and two riffles using a single drift net that published length–weight regressions for aquatic
included surface and subsurface drift (i.e., we did not (Meyer, 1989; Benke et al., 1999; Sabo et al., 2002)
collect surface and subsurface drift separately). Drift and terrestrial (Edwards, 1966; Gowing & Recher,
was sampled between 9:00 and 15:30 and the sets 1984; Sample et al., 1993) invertebrates.
ranged from 8 to 30 min. Winter drift sets were of
shorter duration than summer samples because of the Data analysis
higher concentration of suspended organic matter at
high discharge that clogged the drift nets over longer Drift abundance was calculated as both drift biomass
sets. Although the winter drift sets were short, high (mg/m3; total biomass of invertebrates collected in a
water velocities resulted in nets filtering a volume of net divided by water volume filtered) and drift density
water similar to the longer duration summer sets, so (number of invertebrates/m3). Rather than expressing
that short set times should not contribute excessively drift in terms of concentration, many researchers (e.g.,
to variation in drift abundance (e.g., Culp et al., 1994). Bacon et al., 2005; Romero et al., 2005) have
expressed prey availability in the drift as a total flux
Variation within habitat types (or drift rate) of invertebrates (the product of concen-
tration and discharge, either through the mouth of a
In order to determine the variation in invertebrate drift net at the microhabitat scale, or through an entire
drift abundance within a single channel unit, we stream cross section). This unfortunately confounds
collected 14 drift samples from each of the single the independent effects of drift concentration and
pool and riffle channel units. Four equally spaced discharge on total prey availability. Total flux and
transects were established across each channel unit concentration are both important metrics of prey
and drift nets were set evenly spaced along each availability; total invertebrate flux through a drift net
transect. Transects were sampled on four different or stream cross-section will influence capacity (the
days (August 29, 30, 31, and September 1, 2001) to total number of fish that can be supported), while
prevent upstream drift nets from depleting drift concentration will influence the energy intake and
collected in downstream transects; day of sampling growth rate that individual fish experience when
was later used as a covariate in analysis to test for a exposed to a fixed volume of water. Although we
day effect on drift abundance (no significant day briefly consider the influence of total flux on capacity,
effect was detected). Drift nets were set between 9:30 we present most data in terms of biomass concentra-
and 11:30, and lifted between 15:30 and 16:30 from a tion (mg/m3), since biomass can readily be converted
portable wooden bridge set across the stream channel to flux if discharge is known, and the influence of flux
to prevent disturbance associated with walking in the on individual consumption is well described else-
stream. Invertebrate drift samples were preserved as where (e.g., Hughes & Dill, 1990; Hayes et al., 2007).
described above. (Note that drift biomass, a measure of standing crop,
and drift rate, biomass of drift passing through a
Estimation of invertebrate biomass vertical cross-section per unit time, are both distinct
from drift production rate, which is the biomass of
Invertebrates in drift samples were sorted from drift entering the water column per m2 of stream bed
detritus in the lab at 169 magnification and preserved per unit time. Drift production is considerably more
in 70% ethanol. Most aquatic invertebrates were difficult to measure in the field than drift biomass
identified to genus using Merritt & Cummins (1984), (e.g., see Romaniszyn et al. 2007).)
with the exception of chironomids, which were Although our sampling design was inherently
identified to subfamily, and terrestrial invertebrates, hierarchical—we sampled variation in drift within
which were identified to family. Invertebrate length habitats (pool vs. riffle), between habitats (pool, glide,
was measured to the nearest 0.05 mm using a run, and riffle), and between seasons (summer low flow
binocular microscope equipped with a drawing tube vs. winter high flow)—we do not formally analyze it as
that projected images of invertebrates onto a digitiz- a nested analysis, partly because the design is unbal-
ing pad (Roff & Hopcroft, 1986), and invertebrate anced. Logistic constraints limited us in measuring the
biomass (mg dry weight) was then estimated using drift variation within a single pool and riffle, and
123Hydrobiologia (2009) 623:113–125 117
sampling was conducted on a single day during winter drift abundance, we used simple bioenergetic model-
high flows. Instead, we use a combination of analyses ing and average size and density of fish in pool habitats
at different scales to evaluate our predictions. in Husdon Creek (from Rosenfeld et al., 2000) to
We used linear regressions to test for a relationship estimate bioenergetic demand within a typical pool
between drift abundance and velocity in the summer relative to the flux of invertebrate drift into it. We
at different spatial scales, i.e., between different assumed a 12-h (daylight) foraging period, a wetted
habitat types (n = 20), and within habitat types, by pool area of 8.3 m2 (3.6 m long by 2.3 m wide, the
analyzing replicate samples collected along four average for Husdon Creek), and a combined density of
transects nested within an individual pool (n = 14) YOY cutthroat trout and coho salmon in pool habitat
and riffle (n = 14). We used one-way analyses of of 1.01 fish/m2, and densities of yearling and older
variance (ANOVA) to test for differences in drift (year 1? to 2?) cutthroat trout in 80–100 and 110–
abundance between mesohabitats in the summer 190 mm fork length size classes of 0.15 and 0.13 fish/
(n = 5 each for pools, glides, runs, and riffles) and m2, respectively (Rosenfeld et al., 2000). This trans-
winter (n = 2 per habitat type except n = 3 for lated into eight YOY and 1.3 and 1.1 each of the older
pools). Using a t-test, we also compared drift size classes in a representative 8.3 m2 pool. We used
abundance in riffles to all other habitat types com- average measured weights in each size class of 1.45,
bined in the summer to increase power by reducing 8.2, and 32.4 g, respectively, to calculate bioenergetic
variation in drift abundance. Similarly, we compared demand in each size class at 10°C assuming either
drift in pools to all other habitat types combined in 25% or 50% of maximum daily consumption (MDC;
the winter. Differences in drift abundance between calculated using equations from Elliot, 1976) to
surface and subsurface samples as well as the bracket a realistic range of consumption rates by
proportion of terrestrial versus aquatic biomass were juvenile salmonids. We then estimated YOY trout and
tested using a Wilcoxon two-sample test and t-test, salmon bioenergetic demand as a percent of total
respectively. We used a t-test to assess seasonal energy flux (drift) into the pool, as well as for all age
differences in drift abundance and a Wilcoxon two- classes of fish combined. We assumed energy content
sample test to examine seasonal differences in the of drifting invertebrates of 21,790 J/g dry weight
coefficient of variation for drift biomass. We tested (Cummins & Wuycheck, 1971).
for greater variance in drift abundance between We estimated the flux of drifting invertebrates into
different habitat types than within habitat types using the pool using an average measured summer discharge
Levene’s test for homogeneity of variance. of 0.022 m3/s over a drift foraging period of 12 h, and
We performed correspondence analyses on sum- assumed a drift concentration of 0.103 mg/m3 dry
mer drift samples to (1) assess differences in drifting weight (the average observed in pool habitats in
invertebrate community structure between habitats Husdon Creek during this study). Since the drift
(n = 20 for subsurface samples) and (2) to assess concentrations we measured in pools in Husdon Creek
differences in community structure between surface may be partially depleted through fish predation, we
and subsurface drift samples (n = 40). We ordinated also used the higher average observed drift concen-
untransformed abundance data (numbers/m3) so as to tration in riffles (0.37 mg/m3) to provide an upper
reflect differences in both taxonomic composition and bound on maximum potential energy influx into pools.
absolute abundance of different prey (Jackson, 1993).
All data were analyzed using SAS 9.1 (SAS
Institute Inc., 2002). Data were log10 transformed Results
where necessary to meet assumptions of normality
and homogeneity of variance. Relationship between velocity and drift
Estimation of drift consumption by juvenile Average velocities increased from pools, to glides, to
salmon relative to drift supply runs, and riffles along a gradient of decreasing habitat
depth; average velocities of drift net sets showed a
In order to determine whether consumption by drift- similar pattern (Table 1). Velocities were also higher
feeding fish has the potential to significantly impact across all mesohabitat types during the winter high
123118 Hydrobiologia (2009) 623:113–125
discharge sampling (Table 1). Although we predicted
a positive relationship between drift concentration
and velocity for drift samples collected within
individual pool and riffle habitats, there was no
correlation between drift abundance and velocity at
the microhabitat scale (Fig. 1) within either the pool
(r2 = 0.004, P = 0.84, n = 14) or riffle habitat
(r2 = 0.007, P = 0.78, n = 14). There was a signif-
icant positive relationship between drift and velocity
at the mesohabitat scale (i.e., between multiple
channel units; Fig. 2; r2 = 0.31, P = 0.01, n = 19),
but the correlation was relatively weak and became
non-significant (P = 0.09) with the removal of the Fig. 1 Relationship between drift biomass (mg/m3) and
velocity (m/s) at the within-habitat scale. Data points represent
highest velocity data point (upper right point in drift samples collected across multiple transects in a single
Fig. 2). pool or riffle habitat at summer low flow
Habitat and seasonal effects on drift abundance
Despite our expectation of higher drift abundance in
high velocity habitats, there were no significant
differences in drift biomass among pools, glides, runs,
and riffles, in the summer (Table 2; F3,16 = 1.61,
P = 0.23) or winter (F3,5 = 5.09, P = 0.056). How-
ever, variability was high, and summer drift biomass in
riffles was significantly higher than all other habitat
types combined (Fig. 3; t = -2.10, P = 0.0498).
Similarly, pools had significantly higher biomass than
all other habitat types in the winter (t = -4.46,
P = 0.0029). Fig. 2 Relationship between drift biomass (mg/m3) and
velocity (m/s) at the mesohabitat scale. Data points represent
As expected, drift abundance varied spatially
drift sampled from a single pool, glide, run or riffle during
within the water column (Fig. 4). Surface samples summer low flow
had significantly higher biomass (z = 2.58, P = 0.01)
and a higher proportion of terrestrial invertebrates (t = -3.60, P = 0.001) during the winter sampling
(t = -2.54, P = 0.015) than subsurface samples. (Fig. 5). Terrestrial and aquatic invertebrate drift
Total drift abundance also varied seasonally, with biomasses were (mean ± SD) 0.05 ± 0.06 and
both higher drift biomass (t = -3.79, P = 0.001) 0.13 ± 0.15, respectively, in the summer and
and a greater proportion of terrestrial invertebrates 0.36 ± 0.25 and 0.24 ± 0.17, respectively, during
Table 1 Average velocity (m/s) ± standard deviation at drift net set locations and in different habitat types in the summer and
winter
Habitat type Pool Glide Run Riffle Average
Average drift net velocities
Summer 0.11 ± 0.10 0.21 ± 0.09 0.20 ± 0.05 0.35 ± 0.12 0.22 ± 0.10
Winter 0.38 ± 0.19 0.30 ± 0.20 0.46 ± 0.21 0.53 ± 0.03 0.42 ± 0.10
Average habitat velocities
Summer 0.05 ± 0.01 0.12 ± 0.03 0.16 ± 0.03 0.22 ± 0.03 0.14 ± 0.07
Winter 0.25 ± 0.12 0.27 ± 0.11 0.31 ± 0.18 0.45 ± 0.11 0.32 ± 0.09
123Hydrobiologia (2009) 623:113–125 119
Table 2 Mean drift biomass (mg/m3) ± standard deviation and mean density (number/m3) ± standard deviation in different habitat
types in the summer and winter
Habitat type Summer Winter
3 3
Biomass (mg/m ) Density (per m ) Biomass (mg/m3) Density (per m3)
Pool 0.10 ± 0.08 1.51 ± 0.57 1.06 ± 0.46 24.58 ± 7.16
Glide 0.15 ± 0.15 2.82 ± 1.35 0.32 ± 0.08 8.14 ± 2.87
Run 0.10 ± 0.09 1.73 ± 0.39 0.40 ± 0.18 16.59 ± 7.47
Riffle 0.37 ± 0.35 3.20 ± 1.58 0.39 ± 0.07 12.66 ± 0.30
Fig. 3 Mean drift biomass (mg/m3) in different habitat types
in the summer. Error bars represent one standard deviation
Fig. 5 Average contribution of terrestrial and aquatic drift to
total biomass (mg/m3) in the summer and winter. Error bars
represent one standard deviation for total drift biomass
all habitats (t = 5.68, P = 0.01, for n = 4 habitat
types). The CV between habitat types was also lower
in the winter (CV = 0.70, n = 9) than in the summer
(CV = 1.18, n = 20) indicating greater homogeneity
of drift concentration when stream discharge is high.
Variance in drift abundance within and between
habitat types
We predicted that variance in summer drift within a
single habitat type would be lower than the variance
Fig. 4 Average contribution of terrestrial and aquatic drift to between different habitat types. However, the variance
total biomass (mg/m3) in surface versus subsurface drift
samples at summer low flow. Error bars represent one standard in biomass between different mesohabitats (CV =
deviation for total drift biomass 1.18, n = 20, representing variation between single
drift samples collected from each of the five pools, five
winter sampling. The coefficient of variation for drift glides, five runs, and five riffles) was not significantly
biomass within habitat types was significantly lower different from variance within a single pool (CV =
in the winter (mean CV ± SD = 0.33 ± 0.13) than 0.97, n = 14) or riffle (CV = 0.61, n = 14; F2,45 =
in the summer (mean CV ± SD = 0.91 ± 0.08) in 1.97, P = 0.15 for a comparison of all variances).
123120 Hydrobiologia (2009) 623:113–125
These results collectively indicate that variation in
drift concentration within individual channel units
(e.g., individual pools or riffles) is similar to the
variation between different habitat types.
Habitat effects on drift community structure
Given well-documented differences in benthic (e.g.,
Huryn & Wallace, 1987) and drift (e.g., Cellot, 1996)
community between different habitats (e.g., pools vs.
riffles), we anticipated some differentiation of drift-
ing invertebrate community structure between habitat
types. However, correspondence analysis demon-
strated broad overlap of drift community structure
for subsurface samples collected in different habitat
types (Fig. 6), and habitats could not be differentiated
by the presence or abundance of particular taxa.
These taxonomic results, along with the observation Fig. 7 Ordination of surface (open diamonds) and subsurface
summer drift samples (filled squares) in taxonomic space.
of similar variation in drift abundance at multiple Dimension 1 is negatively correlated with invertebrates of
spatial scales, supports the inference that turbulence terrestrial origin (Collembolans (Collem), spiders (Arach), and
and downstream transport processes tend to spatially adult flies (Dipt-A)) and positively correlated with common
homogenize drift in small streams. aquatic taxa (chironomids (Chiro), caddisfly larvae (Trichop),
and Tanypodinae chironomids (Tanypod))
In contrast, correspondence analysis demonstrated
distinct differences in community structure between
surface and subsurface drift samples (Fig. 7), with surface drift differentiated by the presence of spiders,
collembolans, and dipteran adults, while subsurface
drift was characterized by a greater abundance of
orthoclad and tanytarsini chironomids.
Bioenergetic demands of juvenile fish relative
to drift flux
The estimated combined bioenergetic demand of
YOY cutthroat trout and YOY coho salmon in a
typical pool in Husdon Creek (assuming eight 1.45 g
YOY fish per pool) ranged from 45% to 90% of total
energy flux into the pool (Table 3), assuming prey
consumption at 25%–50% of a fish’s physiological
maximum at 10°C, and an invertebrate drift concen-
tration of 0.1 mg/m3 (the average measured in pool
habitat). If drift concentrations are assumed to be the
average measured in riffle habitat (i.e., assuming that
measured concentrations in pools already reflect
Fig. 6 Ordination of summer drift subsurface samples from
pool, glides, runs, and riffles in taxonomic space (taxa labels depletion by fish consumption), the percent con-
represent the location of taxa with the largest scores on sumption by YOY drops to 13–25% of total drift flux
Dimension 1 and Dimension 2; mayfly larvae (Ephem), spiders over a 12-h period. However, if bioenergetic demand
(Arach), caddisfly larvae (Trichop), Leptophlebiid mayflies
of older fish (year 1? and 2? cutthroat trout at
(Lepto), and Nemourid stoneflies (Nemour)). There is no
apparent taxonomic differentiation of samples from different realistic densities of 0.28 fish/m2) is included, then
habitat types bioenergetic demand ranges from 36% to 71% of
123Hydrobiologia (2009) 623:113–125 121
Table 3 Estimated flux of invertebrate drift into a typical pool assuming consumption rates at 25% or 50% of Maximum Daily
(g dry weight per 12 h day) in Husdon Creek, and the estimated Consumption (MDC), and average drift concentrations from
bioenergetic demand (g dry weight of invertebrates per 12 h) pools (0.10 mg/m3) or riffles (0.37 mg/m3)
by YOY salmonids and all juvenile salmonids combined,
Biomass flux into YOY bioenergetic All sizes bioenergetic YOY consumption All sizes consumption
pool (g dry wt.) demand (g dry wt.) demand (g dry wt.) as % of flux as % of flux
Pool drift
25% MDC 0.098 0.044 0.125 45% 128%
50% MDC 0.098 0.088 0.25 90% 255%
Riffle drift
25% MDC 0.351 0.044 0.125 13% 36%
50% MDC 0.351 0.088 0.25 25% 71%
total drift flux assuming high (riffle) drift concentra- invertebrates/m3, or the total mass flux of suspended
tions (Table 3). invertebrates carried in the water column. Support for
our expectation of a positive relationship between
velocity and drift abundance (concentration) was
Discussion limited, despite previous observations of higher
invertebrate drift in higher velocity habitats (Brittain
Despite half a century of research on the ecology of & Eikeland, 1988). There was only a weak positive
invertebrate drift, our understanding of how habitat relationship between drift abundance and velocity at
affects drifting invertebrates lags far behind our the mesohabitat scale (Fig. 2), although riffle habitats
knowledge of habitat effects on the fish that consume had higher drift abundance than lower velocity
them (e.g., Elliot, 1994; Rosenfeld, 2003). A variety habitat types. However, there was no relationship at
of complex bioenergetic foraging models have been all between drift abundance and velocity at the
developed to predict habitat selection and growth of within-habitat scale (Fig. 1).
drift-feeding fishes (e.g., Hughes & Dill, 1990; Van Lack of a positive relationship between velocity
Winkle et al., 1998; Hayes et al., 2007), but reliably and drift at the microhabitat scale suggests that
parameterizing the abundance and dynamics of drift- turbulence and short distances between high- and
ing invertebrate prey remains one of the primary low-velocity microhabitats within a channel unit (e.g.,
sources of model uncertainty. Relatively few studies a single pool) minimize drift settlement in slower
have investigated how drift varies between discrete microhabitats. With the exception of higher drift
habitat types within a stream, at scales that are concentrations in summer riffle habitats, there was
relevant to habitat selection and use by individual fish surprisingly little difference in drift abundance
(see Nielsen, 1992; Keeley & Grant, 1997; Hansen & between habitat types (Fig. 2). This indicates, in
Closs, 2007 for notable exceptions). Given the conjunction with the lack of taxonomic differentiation
importance of invertebrate drift as an energy flux in of drift from different habitat types, that turbulence
stream ecosystems and a food source for drift-feeding and downstream transport processes tend to homog-
fishes (e.g., salmonids; Elliott, 1967; Sagar & Glova, enize drift between sequential channel units as well as
1988; Dedual & Collier, 1995; Hayes et al., 2000), within them. This effect may be most pronounced in
understanding how habitat, flow, and riparian condi- small streams like Husdon Creek, where the average
tions affect drift are vital for the effective channel unit length (4.0 ± 2.0 m for 44 channel units
management of stream habitats and the fish that surveyed over a 176 m reach) is short relative to
depend on them (Elliott, 1973; Wilzbach et al., 1986). typical invertebrate drift distances (typically 3–10 m
Discharge and water velocity are key hydrological in small streams; e.g., Elliot, 1971). However, spatial
factors associated with increased invertebrate drift variation in drift may be well differentiated in larger
(O’Hop & Wallace, 1983; Brittain & Eikeland, streams and rivers where the greater length of
1988), both in terms of biomass or numbers of different habitats facilitates differentiation. For
123122 Hydrobiologia (2009) 623:113–125 instance, Stark et al. (2002) found consistent habitat be tempered by the source and delivery mechanisms and depth differentiation of drift abundance in a larger of invertebrate prey. Our observation of elevated river, and Cellot (1996) also found differences in drift terrestrial inputs during high winter discharge are community structure and abundance between main- likely the result of rising water levels flooding gravel stem and side channel habitat. bars and stream banks that are colonized by terrestrial Our conclusions with respect to the homogeniza- invertebrates at low flow (Angermeier & Karr, 1983), tion of stream drift need to be tempered by several rather than terrestrial drop of invertebrates from the considerations. First, we did not sample microhabitats riparian canopy (O’Hop & Wallace, 1983). Terres- within a channel unit repeatedly over time (c.f. trial and aquatic drift abundance will also depend on Hansen & Closs, 2007). Therefore, we cannot deter- the timing of drift sampling relative to peak discharge mine whether the microspatial variation in drift and the duration of low flows before and high flows concentration we observed within channel units is after a storm event. In our study, we sampled winter consistent over time, or the outcome of unpredictable drift on only one occasion at high flow during early stochastic variation (e.g., turbulence). It remains winter. In general, drift concentrations are usually possible that invertebrate drift is locally concentrated lowest during periods of prolonged high discharge by hydraulics or surface and subsurface eddy lines in (Shaw & Minshall, 1980; Sagar & Glova, 1992; streams, rather than being homogenized by turbulence Hansen & Closs, 2007), and winter drift concentra- as we suggest above. Coarse metrics such as water tions may very well decrease in coastal streams as column velocity may not adequately represent prolonged high flows flush out invertebrates and hydraulics that are relevant to drift transport but are benthic organic matter accumulated during low flows easily detected by fish (e.g., Crowder & Diplas, 2002). (e.g., O’Hop & Wallace, 1983). The increased drift we observed during higher We based our expectation of a positive relation- velocity winter flow was consistent with our predic- ship between drift and velocity on the assumption tion and past studies (e.g., O’Hop & Wallace, 1983). that entrainment and suspension of benthic inverte- The expectation of higher invertebrate drift concen- brates are higher in faster water and once entrained tration with increasing velocity may be valid when drift should remain in suspension for longer at higher associated with rising discharge, which initiates the velocities (Bond et al., 2000). To some extent this movement of bed load and suspension of organic assumption was supported by our observation of matter and invertebrates (O’Hop & Wallace, 1983; higher drift concentrations in riffle habitats, and the Gibbins et al., 2007). In contrast, spatial differences independent observation by Hansen & Closs (2007) in microhabitat velocity during stable summer low that drift abundance is positively correlated with riffle flows may have limited effects on drift concentration length. However, drift concentration in the water because benthic disturbance is minimal. However, column will respond to factors that influence both the differences in microhabitat velocity will have large exit as well as the entry of invertebrates into the water effects on the total flux of prey drifting through column. While velocity may influence the entrain- different microhabitats and should therefore strongly ment of invertebrates into the drift, predation by fish influence habitat selection by fish, as discussed later. can have a much larger effect on the removal of drift The contribution of terrestrial invertebrates to drift than settling of drift in low-velocity microhabitats. was significant in both summer and winter, as The energetic benefit that a drift-feeding fish observed by many others (e.g., Romero et al., 2005; experiences in any given microhabitat will be jointly Romaniszyn et al., 2007), and total drifting inverte- determined by prey concentration and the volume of brate abundance was consequently higher at the water water passing through a fish’s reactive field (Hughes & surface (see Sagar & Glova, 1992 for similar results). Dill, 1990; Hill & Grossman, 1993). Our results The proportion of terrestrial invertebrate biomass was suggest that microhabitat effects on prey concentra- considerably higher in the winter than summer tion vary in a much less predictable way than either (Fig. 5), which was surprising since arboreal inver- physical constraints on the volume of water that a fish tebrate abundance and activity is generally higher can scan (imposed by water depth, velocity, and during the summer (Romero et al., 2005). However, channel configuration; Hughes & Dill, 1990; Keeley & consideration of seasonal variation in drift needs to Grant, 1997; Grossman et al., 2002), or depletion of 123
Hydrobiologia (2009) 623:113–125 123
drift by upstream conspecifics. Our simple bioener- were also likely influenced through consumption by
getic estimates of consumption indicate that energetic juvenile salmonids present at moderate densities
demand by juvenile salmon and trout in Husdon Creek ([1 fish/m2), which may tend to reduce measured
pools is sufficient to consume a substantial proportion drift abundance. Conservative bioenergetic modeling
of the invertebrate flux into pools, even at higher drift indicates that juvenile trout at these densities locally
concentrations (Table 3). This suggests that predation consume a minimum of 13–36% of invertebrate drift
by fish in pools can act as an effective filter to remove at summer low flow.
drifting invertebrates in smaller streams, as suggested Drift biomass or numbers/m3 remain measures of
by Waters (1962) and studies demonstrating con- standing crop that are affected by both the rate of drift
sumption of a significant proportion of total production (recruitment into the water column) and
invertebrate production by trout (e.g., Huryn, 1996; the rate of drift removal (by settling or fish preda-
Kawaguchi & Nakano, 2001). Accounting for the tion). At present there are few accurate field estimates
effects of consumption by fish (e.g., Hughes, 1992; of drift production rates or how they differ between
Hayes et al., 2007) should therefore greatly improve habitats, although some field experiments to directly
our understanding of spatial variation in drift abun- measure areal recruitment rates from the benthos are
dance, particularly in small streams where velocities in progress (K. Shearer, pers. comm.) or completed
are relatively low and a significant proportion of the (e.g., Romaniszyn et al. 2007). In the short term,
water column can be searched by drift-feeding fishes. stream ecologists can use drift concentrations mea-
However, downstream increases in velocity (Rosen- sured in different habitats (e.g., pools vs. riffles) as a
feld et al., 2007) and decreases in densities of juvenile surrogate of production rate and the availability of
salmonids along the river continuum (e.g., Rosenfeld prey for drift-feeding fish. In the long term, reliable
et al., 2000) should result in decreasing efficiency of estimates of drift production and models that incor-
transfer of energy from drifting invertebrates to the porate transport and consumption (c.f. Hayes et al.,
fish trophic level. Our drift sampling may also have 2007) are needed to understand the true dynamics of
underestimated daily drift flux into pools because we invertebrate drift abundance.
did not include the dawn and dusk spikes in drift
abundance and consumption observed by some Acknowledgments We would like to thank Leonardo Huato
for assistance in the field and Diane Srivastava for providing
researchers (e.g., Sagar & Glova, 1988; but not others, laboratory space. This research was partly funded by the
e.g., Allan, 1981; Dedual & Collier, 1995). British Columbia Conservation Corps and Forest Renewal, BC.
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