PALMER, MARGARET A. Incorporating lotic meiofauna into our

Oceanogr., 37(2), 1992,329-34 1
0 1992, by the American Society of Limnology and Oceanography, Inc.
Limnol.
Incorporating lotic meiofauna
faunal transport processes
into our understanding
of
Margaret A. Palmer
Department of Zoology, University of Maryland, College Park 20742-44 15
Abstract
This work represents the first detailed study using field data and flume experiments to evaluate
the magnitude and pattern of meiobenthic drift in a stream, the evidence that both active and
passive processes operate during entry and transport, and how meiofaunal transport in streams
compares with what is currently known about the transport of lotic macrofauna and of the meiofauna and larval macrofauna of marine systems. Abundances of drifting meiofauna in Goose Creek,
Virginia, were extremely high (up to 250,000 per 100 m3) and represented a significant amount of
biomass when compared to literature biomass estimates for macrobenthic drift in other warmwater streams. Drift of the dominant taxa in this study (rotifers, early instar chironomids, oligochaetes, copepods) was flow-dependent. Flume experiments examining live and dead entry processes provided evidence that a critical threshold velocity existed (-9-l 2 cm s-l), above which
the number of animals entering the water increased dramatically. This threshold was less than the
critical threshold velocity of the streambed substrate (- 14-17 cm s-l), suggesting that whenever
the streambed is scoured, potentially large numbers of meiofauna may enter the water. Despite
the evidence that animals may enter the water passively due to entrainment by flow, active control
over drift entry also operated since all four of the dominant taxa exhibited significant increases in
drift at night. These diel drift patterns were reflected in reduced streambed abundances at night
for chironomids and oligochaetes but not for copepods and rotifers. Additionally, in flume experiments which examined the return of live vs. dead drifting fauna to the bottom, copepods were
able to influence their return to the bottom once in transport, a finding consistent with field
observations.
The downstream transport or drift of benthic stream invertebrates has been studied
extensively (reviewed by Brittain and Eikeland 1988). Most of the studies have focused on macroinvertebrates, despite the fact
that total stream drift may be underesti-
mated dramatically by ignoring the smaller
invertebrates (Sandlund 1982). In marine
environments, meiofauna are known to be
regularly transported in the water and their
drift has been shown to have important ecological consequences including controlling
their availability as prey to macrofauna and
fish and influencing benthic recruitment
Acknowledgments
(Walters and Bell 1986; Palmer and Gust
This research was supported by grants from the Na198 5). Because meiofauna are extremely
tional Science Foundation (RI1 86-20422, BSR 90abundant
in streams (e.g. Pennak and Ward
06002) and the Maryland Agricultural Experiment Sta1986), occur in the drift, and may be food
tion.
Thanks go to Peter Arensburger, Alexa Bely, Kathsources for macroinvertebrates and fish
leen Berg, Dan Cristini, Chris Hakenkamp, Lies1 King,
(O’Doherty 1988; Vadas 1990), their drift
and Linda Mitchell for field and laboratory assistance. dynamics should influence their availability
The University of Maryland stream group made suggestions that improved the manuscript as did Dave as prey and their recruitment, just as is true
for marine systems.
Allan, Art Benke, Susan Bell, Cheryl Ann Butman,
LeRoy Poff, David Strayer, and an anonymous reAlthough stream meiofauna have reviewer. Special thanks go to Dave Allan who played a ceived little study, considerable progress has
pivotal role in helping with the transition from the
been made in the study of the transport of
study of marine systems to inland streams and with
whom I had many hours of discussions. I thank Paul marine meiofauna. These studies have foTurner, Janet Reid, and Kathy Coates who offered their
cused on the same sorts of questions which
expertise in the identification of rotifers, copepods, and have been asked concerning the drift of
oligochaetes. Fred J. Easton of the Virginia State Water stream macrofauna, e.g. why do the animals
Control Board made discharge data easily available. I
enter the water column and what are the
thank the sisters of the Notre Dame Academy and Mr.
and Mrs. Ridgley White for freely permitting access to consequences of this entry? (e.g. Allan et al.
the study site.
1986; Walters and Bell 1986). Because
329
330
Palmer
freshwater and marine meiofauna inhabiting sandy bottoms are similar in size and
may live in similar flows, it is tempting to
assume that stream meiofauna exhibit
transport patterns similar to their marine
counterparts. However, oceanic and stream
habitats are quite different physically and
evolutionarily, so selective pressures that
influence drift patterns may also be very
different. Indeed, stream meiofauna may
exhibit drift patterns (e.g. high nocturnal
drift) more similar to their macrobenthic
counterparts. Support for this comes from
Schram et al. (1990) who showed that “zooplankton” in Arkansas streams drift in high
numbers, particularly at night. Although this
study did not involve benthic sampling, the
specieslists they provided suggestthat many
of the drifting animals they collected were
probably members of the meiobenthos.
I designed a study to examine meiobenthic drift in lotic systems to answer the following. specific questions with respect to the
magnitude of drift. Is the drift of stream
meiofauna significant and, if so, what percentage of the meiobenthos occurs in the
drift? .Are drift rates comparable to those
found for stream macrobenthos or marine
meioflwna? I also asked specific questions
regarding the evidencefor active and passive
components to drift. Do drift rates increase
with flow (suggestive of passive drift)? Is
drift a nonlinear function of benthic density
(evidence animals actively influence drift)?
Do the meiofauna show nocturnal peaks in
drift as do their macrobenthic counterparts
(evidence animals actively influence drift)?
Do live meiofauna enter the drift at higher
threshold current velocities and exhibit
shorter drift distances than dead fauna (evidence: animals actively influence drift)?
Pasc;ive drift as used herein means animals enter and exit the water as if they are
inert particles; active (influence over) drift
means that through their behaviors (e.g.
swimrning into the water, clinging to substrates on the bottom, etc.), animals reduce
or enhance drift entry or exit.
38”57’N). Goose Creek is tributary to the
Potomac River and the creek is situated in
the Marshall Formation geologic region; the
underlying bedrock is biotite granitic gneiss.
The drainage area is -750 km2; the stream
has year-round flow and a width of lo-20
tn. Water depth at the study site varies from
- 15 to 150 cm during the year. The stream
channel has not been altered by humans;
however, portions of the catchment area are
subject to human activity for recreation and
farming. The riparian zone is dominated by
deciduous trees common to the area especially oaks, sycamore, and box elder. L,ong
stretches (- 100-300 m) of sandy bottom
grade into occasional, short riffles (- 10 m)
with cobble and pebbles. Sampling was con‘ducted in the sandy channels where the median grain size is 1 mm, the hyporheic zone
extends from the sand surface to a depth of
40-60 cm, and the meiofauna are extremely
abundant (Palmer 1990a,b).
The site is just downstream from a USGS
gauging station, thus long-term records of
daily discharge and water quality were
available. Mean annual discharge is - 8.5
m3 s-’ and peak discharge during floods is
- 100 m3 s-l. Each tirne we sampled, vertical velocity profiles were measured with a
Marsh-McBirney model 20 1 electromagnetic meter.
Field sampling- On each sampling date,
streambed meiofauna ‘weresampled with 35 replicate cores (1.8-cm diam) taken in 1Ocm intervals between the sand surface and
bedrock. Separate cores were collected at
each depth (O-10 cm, 10-20 cm, etc.) to
ensure that data from various depth levels
were independent. Replicate samples of
meiofauna in the drift were collected at midday with drift nets (44+m mesh) positioned
5 cm above the sediment-water interface for
15-30 min. Nets were anchored above and
not on the bottom to minimize flow disturbances which might artificially enhance
erosion. The exact volume filtered was determined midway through each sampling by
measuring mean velocity through the known
cross-sectional area of the net. Nets were 2
Materials and methods
m long with a 0.1 m2 opening and a widened
Study site-The study site was Goose area 0.5 m below the mouth to minimize
Creek, a low-gradient, fourth-order stream turbulence at the net entrance. Preliminary
sampling showed that drift nets made of
in Eoudoun County, Virginia (77’45’W,
Lotic meiofauna drift
mesh finer than 44 pm became quickly
clogged and did not result in significantly
greater meiofauna catches (20-pm nets vs.
44-ym nets: abundance of meiofauna not
significantly different, P > 0.05).
Samples were collected monthly from
January 1988 to January 1989 and more
frequently when discharge changed markedly; however, I was unable to collect drift
samples on three dates (September, October, November) when flows were too low.
Diel patterns were investigated with replicate streambed cores and drift samples collected at midday and 2 h after sunset on
four dates (22 April, 27 April, 13 May, 23
August).
After collection, samples were immediately placed in 1% MgC12 for - 3 min to
promote meiofaunal relaxation, then rinsed
and those fauna retained on a 44-pm sieve
preserved in 10% Rose Bengal-Formalin.
Fauna were identified to major taxonomic
category; five taxa made up 90% of the community by abundance: rotifers, oligochaetes, copepods, early instar chironomids, and nematodes. For three of the
sampling dates, rotifers, oligochaetes, and
copepods were identified to species.
Flume experiments-To
investigate the
importance of active vs. passive drift, I
measured drift distances and critical threshold drift velocities of live vs. dead animals
in a Plexiglas flume (5.5 m long, 3 5 cm wide,
50 cm deep) with freshly collected water at
field temperature.
Measurements of drift distance served as
a tool for assessing whether the animals act
like passive particles in transport. Specifically, if live animals released into the water
returned to the bottom just as quickly as
dead animals then both would drift the same
distances at the same flow. A low flow setting (2.5 cm s-l) was used in the flume for
these measurements because it allowed animals that were released into the water to
settle to the channel bed before they were
transported out of the channel itself. The
channel was ~6 m long and the gravitational sinking rates of the animals were slow
(-0.02-0.39 cm s-l; Palmer 1990a), so using higher velocities would have required
an unrealistically long flume to allow settlement (e.g. at flows of 30 cm s-l, rotifers
331
released 3 cm from the bed would drift -45
m). It was not my goal to make drift distance
measurements for extrapolation to the field
although the measurements made here
probably are reasonable for the long periods
of low flow that generally occur in Goose
Creek for -5 months of the year (see Fig.
I and Palmer et al. 1992).
The procedure used to measure drift distance was as follows. The flume channel was
filled with a layer of azoic sand and a drift
net positioned at the end of the channel.
Water depth in the flume was kept at 6 cm,
flow was set at 2.5 cm s-* , and animals were
released into the flow at 3 cm above the
bottom. For release, animals were extracted
from freshly collected stream sand by swirling it in streamwater and decanting the water and suspended animals into a dish. The
contents of the dish were poured into a sample splitter and divided into four aliquots
(each with - 1,000 animals in 500 ml of
water). Before use, each aliquot was examined to be certain that animals were active. Aliquots were poured into a Plexiglas
chamber (11 x 30 cm) which was lowered
into the flowing water of the flume channel,
and the two ends of the chamber normal to
the flow were immediately raised from the
chamber base so that the animals entered
the flow. Only the chamber base (0.96 cm
thick and beveled to 1 mm thickness at its
edges to minimize flow disturbance) intercepted the flow (designed by Woods Hole
Oceanographic flume group; C. Butman
pers. comm.).
One aliquot of animals was released at a
distance of 0.5 m upstream of the catchment
net and allowed to drift-settle for 5 min.
The drift net was then retrieved and all captured animals (i.e. not settled in the flume)
were preserved in Formalin with Rose Bengal stain. This procedure was repeated at
additional release points upstream: the second, third, and fourth aliquots of animals
were released at 1, 2, and 3 m upstream of
the drift net. Before each release, the sand
downstream of the last release point was
removed and replaced with new azoic sand.
The entire procedure (aliquot release at four
distances) was repeated three times with live
animals and three times with animals that
had been killed with ethanol. Nematode drift
332
Palmer
from the box cores. Box cores (20-cm diam
X 10 cm deep) consisted of sand and associated fauna collected from the field ‘by
inserting the core into the streambed and
then sliding a bottom plate below the core.
All
cores were collected 24 h before their
J,
use in the flume and held in a living stream.
Previous experiments have shown that the
vertical distribution of IGoose Creek meiofauna in box cores collected and allowed to
equilibrate before use in the flume was not
significantly different frlom the vertical distribution in the field (P.almer et al. 1992).
Box cores fit into the flume channel 4.5
>
20000
CHIRONOMIDS
m downstream of the entrance, and the surI1
bz 15000
face of the substrate in the core was flush
with
the bottom of the: flume. Azoic sand
8
10000
was
placed
upstream (evenly all along the
tbottom of the channel) of and surrounding
5000
&
the core to ensure no changes in bottom
D
0 I-~,,,,,,
roughness as flow approached the core. The
maximum velocity used in these experiCOPEPODS
ments (20 cm s-l) was chosen to exceed critical erosion velocity for the substrate. The
drift vs. flow measurements were made for
three replicate box cores with live animals
and three “dead” box cores that had been
injected with ethanol 2 h before use. This
7
120
technique
did not alter the vertical distrim
bution of animals in the box cores (live vs.
20
DISCHARGE
L
II
I I
dead, percentage of animals in top 5 mm
not different, P > 0.05).
Data analysis -Abundances of fauna in
the drift are expressed as drift densities with
those units most commonly reported-No.
of fauna per 100 m3. For comparison of drift
Fig. :I. Temporal patterns of drift for 1988-1989.
Mean values for the four dominant taxa with standard between dates and with physical variables,
error bars; data points absent if flow was too low for drift was also expressed as drift rate (drift
drift sampling. Mean daily discharge from USGS gaug- density x discharge >: time), which is the
ing station near sample site.
total number of animals drifting in a cross
section of the stream per unit time. Drift
rate is less dependent on the dilution and
distances were not measured because there concentration effects that changing flow rewere not enough live animals.
gimes have on drift d’ensity.
Throughout, parametric statistical tests
If drift entry is passive, a critical entrainment velocity (hereafter, threshold) should were used after the data were examined to
exist analogous to critical erosion velocities determine that appropriate assumptions
for se,diments. To investigate, I exposed box were valid. When necessary, abundances
cores of fresh stream sand to increasing flows were log& + 1) or arcsin-transformed be(O-20 cm s-l in 2 cm s-l intervals) for 2 min fore tests. ANOVA and linear regressions
at ealch flow. Drift nets positioned at the were performed with general linear model
end of the flume and changed before each techniques (SAS 19(35) and comparisons
made with the Tukey-Kramer method when
flow jincrease caught all animals originating
I
333
Lotic meiofauna drift
Table 1. Percentage of bottom meiofauna in drift.
Percentages are calculated for each taxon by dividing
numerical abundances found in the drift (no. vol.-‘)
by numerical abundances found in an equal volume of
sediment from the top 10 cm of the hyporheic zone.
Estimates are mean values (N = 34) for entire year; SE
in parentheses.
Rotifers
Chironomids
Oligochaetes
Copepods
Nematodes
.
Drifting benthos
w
Max drifting
w
0.019 (0.015)
0.004 (0.003)
0.00 1 (0.00 1)
0.054 (0.03)
0.005 (0.008)
0.05
0.02
0.02
0.14
0.03
significant main effects were present from
an ANOVA.
Results
The magnitude of drift -Large numbers
of meiofauna were found in all drift samples
collected at Goose Creek. Over the entire
year mean abundance of total meiofauna
drifting was N 89,000 per 100 m3, but in
April drift densities exceeded 250,000 per
100 m3. The number of drifting meiofauna
varied significantly by date (ANOVA, P <
0.001) with highest drift abundances generally in late spring (Fig. 1). The percentage
of benthos drifting was estimated from field
data on drift and benthic densities, which
ranged from 0.00 1 to 0.14% depending on
taxon and date (Table 1). The predominant
taxonomic group in the water was rotifers,
which constituted > 86% of the drift by numerical abundance (Table 2).
Evidence for active and passive components to drift-The temporal pattern in drift
over the year (Fig. 1) typically showed greatest drift densities in April, May, and June.
Statistically, drift rates and drift densities
increased with current velocity for all groups
(P < 0.01; e.g. Fig. 2, drift rate for rotifers).
Drift was not density-dependent, i.e. there
was not a significant increase in percent
drifting when benthic density increased (P >
0.05) for any of the taxa.
When samples were collected to determine whether there was diel variation in
drift, flow was not significantly different between night and day on any of the sampling
dates (P > 0.05). However, discharge magnitude was significantlv~ lower
- -~ during
- - ----” the
_--_
Table 2. Percentage representation of common
meiobenthic taxa in drift and bottom samples. Percentages are calculated from numerical abundances for
each taxon divided by the total faunal abundance (all
taxa) found in the sediment (bottom) or drift. N = 34.
SE in parentheses.
Rotifers
Chironomids
Oligochaetes
Copepods
Nematodes
Bottom
w
Drift
v-4
47.7 (5.2)
12.9 (2.3)
19.1 (2.8)
4.2 (0.9)
3.1 (0.5)
86.2 (1.5)
6.9 (1.1)
2.9 (0.4)
1.5 (0.2)
2.4 (0.4)
August sampling (1.4 m3 s-l) than during
the April and May samplings (X = 8.1 m3
s-l). Diel variation in drift was pronounced
for all taxonomic groups except nematodes.
Significantly more chironomids, oligochaetes, and copepods were drifting at night
than during the day for all four dates (Fig.
3) on which day-night samples were collected (ANOVA, P < 0.01 for each taxon).
Abundances of chironomids and oligochaetes in the streambed (Fig. 3) also exhibited a diel pattern, with lower numbers
in the hyporheic zone at night (ANOVA, P
< 0.00 1). Diel patterns in abundance of copepods in the streambed were quite variable
between dates and overall there was no statistically significant diel effect (P > 0.05).
Rotifers exhibited higher night than day drift
rates, but this diel pattern was highly dependent on date (ANOVA, date x time-ofday interaction significant at P < 0.001)
with no diel variation in August (Fig. 3).
Q 6000
a
t 4000.
B
L 2000
.
.
.
.
:a
z 0L L%.,..L..,
.. ...._.....,.__.....
T
0
20
VELOCITY
40
60
(‘cm
60
s- ’ 1
Fig. 2. Drift rate of rotifers (total drift past a point
in the stream over 24 h) vs. mean current velocity for
same time period. Drift samples were not collected
when- flows
were too low (September, October, No.
vember).
334
Palmer
DRIFT
DAY
0
NICXT
m
Chronomlds
STREAMBED
DAY
0
NIGHT
a
Fig. 3. Diel patterns of meiofaunal abundance in the drift and the streambed on 22 and 27 April, 13 May,
and 23 .4ugust 1988. Values are means per unit volume sampled based on benthos samples taken from the
entire hyporheic zone (O-50 cm) and drift samples taken with nets positioned 5 cm above the bottom. Standard
error bars shown.
Diel patterns were not detectable in streambed abundances of rotifers (P > 0.05; Fig.
3).
For the rotifers and copepods, there was
not co:mplete concordance between species
found in the drift and those found in the
benthic samples, indicating that some drifters originated from benthic habitats not
sampled such as debris dams or streambanks. Over 80 species of rotifers have been
identified from Goose Creek benthic and
drift samples to date (Turner and Palmer in
prep). Most of the species found in the drift
were also found in benthic samples (N 70%);
however, several of the dominant drift species were among those species never found
in the sandy benthos [e.g. Lecane (Monostyla) cornuta, Macrochaetus subquadratus]. For the copepods, species diversity was
higher in the drift samples than in benthic
samples. One species, Diacyclops albus, was
the overwhelming dominant in the benthic
samples yet this species was no more common in the drift than Eucyclops agilis and
Macrocyclops albidus. Among the oligochaetes, most of the species found in the
sediment (e.-g. Chaetogaster diaphanus,
Chaetogaster diastrophus, Slavina appendiculata, Nais variabilis, .4ulodrilus pluriseta) were also found in the drift.
In the drift distance experiments, the
number of animals caught in drift nets gen-
erally decreased exponentially as the distance from the point of release to the drift
net increased (Fig. 4). In order to determine
the mean drift distance for each group, I
regressed log catch (number of animals
caught in the drift) on :releasedistance. The
inverse of the slope of that regression line
gives mean drift distance (Elliott 197 1). All
regressions were signilircant with r2 values
of 0.87-0.99. Because three live and three
dead replicate release e:xperiments were run
for each group, there were six regression lines
per taxonomic group. The overall mean drift
distance (n = three replicates) for dead animals was similar to live for all taxa except
copepods (Fig. 5) which drifted significantly
farther before returning to the bottom than
did live copepods (P < 0.05). Differences
in mean drift distance between taxa were
apparent (Fig. 5). Oligochaetes drifted the
shortest distances; relative to oligochaetes,
chironomids drifted CU1.6 x farther, copepods -2.5~ farther, and rotifers -5.5~
farther. Similar differences in drift distance
among taxa were obtained from calculations based on gravitational sinking rates
made in still water (Palmer 1990a): relative
to oligochaetes (with the shortest drift distance), chironomids would drift 1.4 X farther, copepods 2.7 x farther, and rotifers 7 X
farther.
The experiments to determine the critical
335
Lotic meiofauna drift
Oligochaetes
1
2
1
3
Drift
Distance
2-
3
(m)
Fig. 4. Relationship between total catch of meiofauna and horizontal distance between drift net and the
release point in a laboratory flume. Mean water velocity was 2.5 cm s-l and release height was 3 cm above the
bottom. Three replicates for live animals (dashed lines) and three for dead (solid lines).
drift thresholds of the animals as a function
of velocity yielded variable results between
taxa and, for some taxa, between live and
dead animals (Fig. 6). If we assume passive
entry into the water via erosion, few or no
animals should be present in the water at
low flows and, then, above a “threshold”
velocity, the numbers drifting should increase dramatically. The threshold points
were visually approximated from the graphs
(arrowheads on Fig. 6) as is commonly done
to estimate critical erosion velocity of sediments. All of the animals had entry thresholds between m9 and 12 cm s-l, which was
less than the critical erosion threshold for
the sediment (range, 14-17 cm s-l; mean,
16.4 cm s-l).
There was little difference between the
curves for live vs. dead for the oligochaetes
and chironomids, suggesting they did not
exert active control over entry in these experiments. For the copepods, the threshold
flow at which the number of animals entering the water increased noticeably was
slightly higher for live animals than for dead
animals, which also may have been true for
rotifers; however, there was large variability
between the three threshold curves for live
rotifers (Fig. 6).
Discussion
The magnitude of drift-Drift
densities of
total meiofauna in Goose Creek were ex-
tremely high: 250,000 per 100 m3 during
spring spates and an average of -89,000
per 100 m3 for the remainder of the year.
These densities vastly exceed drift densities
of macrobenthic invertebrates in streams,
which usually fall between 100 and 1,000
per 100 m3 (Allan 1987); however, the Goose
Creek values are comparable to the highest
meiobenthic drift found in marine systems
(Hagerman and Rieger 198 1; Palmer and
Gust 1985). Additionally, Schram et al.
(1990) reported extremely high drift densities for “zooplankton” in a third-order
stream in Arkansas. Although they did not
sample the benthos, their species list indi-
Oligochaetes
Chmnomids
Rotifers
Taxa
Fig. 5. Mean drift distance for live and dead meiofauna calculated from slopes of regression lines in Fig.
6; seetextforjiirther details. Standard error bars shown.
336
Palmer
Oligochaetes
24
d
Live
6
-2
:I,,p:
24
/
/
/’
/’
Dead
A’
/’
/’
/’
,/--’
,/’
,y”-r----*
---
___C--
6
/’
LLII-2
12
-5%
c---
16
20
24
20
24
Copepods
0
24 c
----I
12
Velocity
km
s-’
16
)
Fig. 6. Number of meiofauna drifting (per 2 min) from flume box-core sediments as a function of current
velocity. Three replicate box cores with live animals and three with dead animals. For each line, drift threshold
was approximated visually (H) as the velocity at which drift began to increase most dramatically. Arrowheads
indicate mean threshold velocity for the three lines.
cates that many of the rotifers collected in
their drift nets (80qm mesh) were probably
meiobenthic in origin, not planktonic.
Clearly, the present study and that of Schram
et al. indicate that meiofaunal drift in
streams can be substantial and that use of
large-mesh drift nets has caused most stream
investigators to underestimate drift (seealso
Benke et al. 1986).
The magnitude of meiofaunal drift in
Goose Creek was estimated in terms of biomass using literature values for similarly
sized freshwater meiofauna. I used the yearly average drift densities for each taxon (Fig.
1) and the range in individual biomass reported in the literature (Table 3) and found
that meiofaunal biomass in the drift in the
creek averaged 8-9 1 mg dry mass per 100
m3. When I used peak drift densities (generally found in spring) for these calculations,
meiofaunal biomass in the drift was 24-308
mg per 100 m3. Ranges are given to emphasize that these are merely estimates because species-specific: biomass and lengthweight data are not yet available for Goose
Creek. The estimates indicate that, in terms
of biomass, meiofaunal drift is not insignificant as has often been assumed. The bio-
Lotic meiofauna drift
337
Table 3. Literature estimates of mean individual dry mass @g ind.-‘) for benthic taxa of meiofaunal size
that are important in Goose Creek. Values shown cover entire range of values reported.
Biomass range
(pg ind.-‘)
Rotifers
Chironomids
Oligochaetes
Copepods
Nematodes
0.07-0.44
0.50-4.00
0.03-l 3.7
0.20-6.96
0.04490
References
Oden 1979; Quiglcy and Nalepa 1983; Strayer 1985
Benke et al. 1984*
Giere 1975; Strayer 1985
Goodman 1980; Quigley and Nalepa 1983; O’Doherty 1988; Strayer 1985
Quigley and Nalepa 1983; Strayer 1985
* Data from two smallest size classes used.
mass of macrofauna drifting in warm-water
streams such as Goose Creek has been reported to be 20-50 mg per 100 m3 for the
Satilla River (Benke et al. 1986) and 240246 for the Ogeechee River (Benke et al.
199 1). A comparison of the range in biomass estimates for meiofauna in Goose
Creek with the range in biomass reported
by Benke and coworkers indicates that
meiofaunal drift may be 1O-l 00% of macrofaunal drift!
The percentage of bottom meiofauna
drifting at any point in time was within the
range generally reported for most stream
macrobenthos (usually ~0. 10%) (Williams
1980). The percentages presented here (Table 1) can be considered conservative for
two reasons. First, the calculations were
made by using average benthic abundances
per volume of sediment in the top 10 cm
of the streambed. Because abundances there
were slightly higher than in deeper sediment
layers (Palmer 1990b), this would yield a
smaller percentage than if calculations were
made using average benthic abundances per
volume of sediment based on the entire (O50 cm) streambed. Second, the day-night
sampling of the drift and benthos indicated
that the drift nets may have seriously underestimated total drift. The number of animals lost from the streambed at night was
much higher than the nighttime increase in
drift (Fig. 3). Drift nets apparently did not
capture all animals exiting the sandy bottom, probably because they were anchored
5 cm above the bottom and thus did not
capture the large numbers of meiofauna that
may be transported near the bed.
Although percent drifting is a useful indication of the approximate magnitude of
drift, comparisons between bottom fauna
and the drift (in this and other papers) should
be interpreted with caution becausenets may
sample the drift inefficiently and because
the origin of the animals caught in drift nets
is usually unknown, In my study, only the
dominant benthic habitat-the main sandy
channel-was sampled for benthos, yet individuals captured in drift nets were likely
to have been advected into nets from areas
other than where benthic samples were collected. Indeed, the species identifications of
drift vs. bottom samples indicated that at
least rotifers and copepods were advected
in from other areas. Even though the dominant benthic habitat is the sandy channel,
debris dams are present along much of the
streambank and they may be important
habitats for meiofauna, perhaps contributing substantially to the drift (see Benke et
al. 1986 for macrofauna).
Evidencefor active andpassive control over
drift-The present study provides a starting
point for examining the factors controlling
drift of lotic meiofauna (Table 4). There is
evidence that both active and passive mechanisms operate. The positive relationship
between drift and current velocity in Goose
Creek suggests that animals may enter the
water column by passive entrainment as flow
increases. Passive entrainment has been
demonstrated for marine meiofauna (Hagerman and Rieger 198 1; Palmer and Gust
1985) and there is also evidence that the
drift of lotic macrofauna increases as flow
increases (Brittain and Eikeland 1988). Passive entry into the water for Goose Creek
meiofauna during the daytime was also supported by the flume experiments in which
critical entry thresholds (9-l 2 cm SS’)could
be identified. Interestingly, these thresholds
were less than the entrainment threshold for
the sandy bottom, probably reflecting differences in lift and drag experienced by an-
338
Palmer
Table 4. Summary of evidence that stream meiofauna can actively influence transport based on results of
the field and flume studies. This active influence may involve various behaviors which affect the transport
process at several stages (e.g. entry into the water, exit, time spent in transport). Support found for an active
component-(+); no evidence for an active component-(-);
no data available-NA..
Diel patterns*
Dl-if-t
Sediment
Sinking rate?
(live > dead)
Drift distance*
(dead > live)
Drift threshold4
(live Z dead)
Densitydependent
driflll
* Fig. 3.
t Palmer 1990~2.
:z* 1.
1)Text.
.
imals and sediments due to their different
specific gravities, sizes, and shapes. Regardless of the underlying cause, this difference
in thresholds is an important result because
it implies that whenever the streambed is
eroded (e.g. during spates), meiofauna may
also be transported.
Despite evidence that passive mechanisms do operate, drift in streams and in
marine environments clearly is not purely
passive (Walters and Bell 1986; Allan et al.
1986). My findings that some meiofauna
exhibited diel patterns in drift, differences
in live vs. dead sinking rates, shorter drift
distances for live than dead, and drift
thresholds which differed for live vs. dead
show that some lotic meiofauna can actively
influence drift entry or exit (Table 4), just
as appears true for some stream macrofauna
and some marine fauna. The strong diel drift
pattern observed for Goose Creek meiofauna was less surprising for chironomids than
for copepods and rotifers because diel patterns #arewell known for stream macrobenthos, and chironomids are merely juvenile
macrofauna. This diel pattern suggeststhat
selective pressures favoring nighttime over
daytime entry for macrofauna (e.g. predator
avoidance) may operate for stream meiofauna as well. Schram et al. (1990) also found
diel platterns in stream drift for rotifers and
copepods. They made the important point
that drift patterns were quite variable between species of rotifers and, thus, there is
a need to focus more effort at the species
level in such studies. Perhaps the differences
between dates in the diel drift patterns of
rotifers found here (Fig. 3) could be explained if species-level data were available.
The meiofaunal behaviors responsible for
high nighttime drift are unknown. Animals
may actively leave the bottom, swimming
up into the water at night or may increase
their susceptibility to passive transport by
moving closer to the sediment-water interface at night. Indeed, the fact that migrations within the sediment may influence
transport is supported by several studies
showing that some meiofauna move down
into the sediments as flow increases (see
Palmer et al. 1992). Such migrations may
also be responsible for the differences in live
vs. dead drift thresholds observed for some
of the Goose Creek fauna.
The behavioral basis for the shorter drift
distances of live vs. dead copepods may include differences in the posture assumed by
live and dead animals while being transported, result from downward swimming of
drifting animals, or be due to active re-entry
into the water after initial contact with the
bottom. Little is known about postures assumed during settlement; however, there is
evidence from marine studies that the supply of meiofaunal-sized organisms to the
bottom is a flow-domlinated process (i.e. not
controlled by swimming) because faunal
swim speeds are substantially lower than
near-bottom flow speeds (Butman 1987;
Palmer 1990a) and th[e animals settle where
they are hydrodynamically retained (Butman 1987; Eckman 1990). Even if settlement is passive, Mullineaux and Butman
(199 1) have shown that active control over
final settlement site can be altered by behaviors that come into play immediately
postsettlement.
The compelling evidence that stream
Lotic meiofauna drift
meiofauna may actively enhance nighttime
drift contrasts with what has been found for
marine meiofauna in environments with
swift flows similar to those experienced by
lotic meiofauna. Active nighttime entry by
meiofauna has never been found in openchannel, high-flow marine environments
where, instead, animals burrow down or
cling to substrates to avoid transport (reviewed by Palmer 1988). There has been
speculation that behaviors which decrease
water-column transport may actually be advantageous in such systems. Suspension into
the water may lead to transport to unsuitable habitats (Palmer and Gust 1985), expose the animals to pelagic predators
(D’Amours 1988), and interfere with feeding (D. Thistle pers. comm.). Active nocturnal entry by meiofauna has been found
in seagrass beds (Walters and Bell 1986)
where flow is greatly reduced and predation
risk may be decreased due to high structural
complexity of the habitat.
Significance of mei0faunal transport in
streams-I have shown that extremely large
numbers of meiofauna are transported in
the water column in Goose Creek but that
these drifters represent a small proportion
of the total meiofauna in the sediment. Despite the potential problems with such proportion calculations (see above), this percentage is comparable to what has been
reported for lotic macrofauna and for marine meiofauna (Williams 1980; Palmer
1988). The small proportion (<O.lO%)
should not be taken to mean that drift is
unimportant. Because these are quantifications of the percentage of bottom fauna
in the water at an instant in time, others
(e.g. Wilzbach and Cummins 1989) have
pointed out that the cumulative effect is
nonetheless great: 24 h of drift over a unit
area can reach ten or even a hundred times
benthic density.
The transport of lotic meiofauna should
have great significance for both meiofauna
population dynamics and streamwide community structure. First, drift allows meiofauna to colonize new and denuded habitats, which is of particular importance in
streams that are often subjected to frequent
and intense flooding (Townsend and Hildrew 1976). Given the prevalence of asexual
reproduction by many freshwater meiofau-
339
na (e.g. rotifers), only a few individuals may
be required for invasion of a habitat. Second, water-column transport is the primary
dispersal mechanism for meiofauna (Palmer 1988) and, as such, has important genetic
consequences. Others have shown that there
is generally an inverse relationship between
the duration of the pelagic phase for dispersing marine larvae and the degree of genetic differentiation between intraspecific
populations (seeBurton and Feldman 1982).
For sexually reproducing meiofauna (e.g.
copepods), water-column transport from one
stream habitat to another surely enhances
gene flow, even if only a small fraction of
the population is moving. For example, genetic homogeneity across a number of demes
is theoretically ensured even at extremely
low dispersal rates (e.g. one migrant per
deme per generation; Smith 1989). Such
dispersal rates are certainly achieved by
drifting meiofauna.
Third, from a community perspective,
meiofauna that have entered the water are
easily available as food for filter-feeding
macroinvertebrates and drift-feeding fish
(e.g. D’Amours 1988). Marine studies have
shown that even when only a small fraction
(%) of a meiofaunal population is eaten, the
fauna may still be extremely important in
overall community trophodynamics: because of the vast numbers of meiofauna,
they can represent a major portion of the
diets of predators (e.g. meiobenthic copepods made up > 80% of the diet of juvenile
chum salmon in a sixth-order stream in
Canada; Sibert et al. 1977). Although we do
not yet know which lotic macrofauna or fish
depend primarily on meiofauna for food,
we do know that stream meiofauna serve as
prey for these larger animals (O’Doherty
1988; Vadas 1990).
This study represents the first examination of both the significance of meiofaunal
drift and the extent to which entry into the
water column in lotic environments is influenced by active or passive processes.
Clearly both processes influence drift, often
in complex ways (Table 4). No simple dichotomy between active and passive entry
or exit exists. Evidence for passive entry
came from a general increase in drift with
flow and from the presence of critical flow
thresholds for drift. Evidence for active en-
340
Palmer
try included nighttime increases in drift and,
special reference to meiobenthic species. Mar. Biol.
31: 139-156.
for copepods, the ability to influence return
to the bottom once in the drift. The general GOODMAN, K. S. 1980. The estimation of individual
dry weight and standing crop of harpacticoid coabsence of such active entry in fast-flowing
pepods. Hydrobiologia 72: 253-259.
marine systems suggests that the relative HAGERMAN,G. M., AND R. M. RIEGER. 1981. Discost-benefit ratio of transport may be higher
persal of benthic meiofauna by wave and current
action in Bogue Sound, North Carolina, U.S.A.
in the more open marine systems than in
Mar. Ecol. Publ. Staz. Napoli 2: 245-270.
streams. The importance of meiofaunal drift
L. S., AND C. A. BUTMAN. 199 1. Initial
in terms of abundance and biomass was great MULLINEAUX,
contact, exploration, and attachment of barnacle
in this study, as it generally is in marine
cyprids settling in flow. Mar. Biol. 110: 93-104.
environments and for lotic macrofauna. ODEN, B. J. 1979. The freshwater littoral meiofauna
in a South Carolina reservoir receiving thermal
There are both ecological and evolutionary
effluents. Freshwater Biol. 9: 29 I-304.
consequences associated with this waterE. C. 1988. The ecology of meiofauna
column transport. These consequences have @DOHERTY,
in an Appalachian headwater stream. Ph.D. thesis,
important implications not only for underUniv. Georgia. 113 p.
standing meiofauna population dynamics PALMER,M. A. 1988. Dispersal of marine meiofauna:
A review and conceptual model explaining passive
but for the study of trophic transfers from
transport and active emergence with implications
the benthic to the pelagic realm.
for recruitment. Mar. Elcol. Prog. Ser. 48: 8 l-9 1.
1990a. Understanding the movement dynamics of a stream-dwelling meiofauna commuALMN, J. D. 1987. Macroinvertebrate drift in a Rocky
nity using marine analogs. Stygologia 5: 67-74.
Mountain stream. Hydrobiologia 144: 26 l-268.
--.
1990b. Temporal and spatial dynamics of
---,
A.S. FLECCKERANDN.C.MCCLINTOCK. 1986.
meiofauna within the hyporheic zone of Goose
Diel epibenthic activity of mayfly nymphs, and its
Creek, Virginia. J. N. Am. Benthol. Sot. 9: 17-25.
nonconcordance with behavioral drift. Limnol.
--,
A.E. BELY,AND K. E. BERG. 1992. Response
Oceanogr. 32: 1057-1065.
of invertebrates to lotic disturbance: A test of the
BENKE,A.C.,R.J. HUNTER,ANDF. K. PARRISH. 1986.
hyporheic refuge hypothesis. Oecologia 89: 182194.
Invertebrate drift dynamics in a subtropical black--AND G. GUST. 1985. Dispersal of meiofauna
water river. J. N. Am. Benthol. Sot. 5: 173-190.
in ‘a turbulent tidal creek. J. Mar. Res. 43: 179---,
K.A. PARSONS,AND~.M. DHAR. 1991. Pop210.
ulation and community patterns of invertebrate
drift in an unregulated coastal plain river. Can. J. PENNAK,R. W., AND J. V. WARD. 1986. Interstitial
faunal communities of the hyporheic and adjacent
Fish. Aquat. Sci. 48: 81 l-823.
groundwater biotopes of a Colorado mountain
----?
T.C.VAN ARSDALL,JR., lack D.M. GILLESPIE.
stream. Arch. Hydrobiol. Suppl. 74(3), p. 3561984. Invertebrate productivity in a subtropical
396.
blackwater river: The importance of habitat and
QUIGLEY, T. F., AND M. A. NALEPA. 1983. Abunlife history. Ecol. Monogr. 54: 25-63.
dance and biomass of the meiobenthos in nearBRITTAIN,J. E., AND T. J. EIKEL~~ND.1988. Invertcshore Lake Michigan with comparisons to the
brate drift-a review. Hydrobiologia 166: 77-9 3.
macrobenthos. J. Great Lakes Res. 9: 53oL547.
BURTON,R. S., AND M, W. FELDMAN. 1982. PopuSANDLUND,0. T. 1982. The drift of zooplankton and
lation genetics of coastal and estuarine invertemicrozoobenthos in the River Strandaclva, westbrates: Does larval behavior influence population
ern Norway. Hydrobiologia 94: 33-48.
structure?, p. 537-55 1. In V. S. Kennedy [ea.],
SAS INSTITUTE,INC. 1985. SAS user’s guide: StatisEstuarine comparisons. Academic.
tics. Version 5 ed.
BUTMAN, C. A. 1987. Larval settlement of soft-sedSCHRAM, M. D., A. V. BB~OWN,
AND D. C. JACKSON.
iment invertebrates: The spatial scales of pattern
1990. Diel and seasaNnadrift of zooplankton in
explained by active habitat selection and the
a headwater stream. Am. Midl. Nat. 123: 135emerging role of hydrodynamical
processes.
143.
Oceanogr. Mar. Biol. Annu. Rev. 25: 113-l 65.
&BERT, J., T. J. BROWN, M. C. HEALEY, AND B. A.
D'AM~uRs, D. 1988. Vertical distribution and abunKASK. 1977. Detritus-based food webs: Exploidance of natant harpacticoid copepods on a vegtation by juvenile chum salmon (Oncorhynchus
etated tidal flat. Neth. J. Sea Res. 22: 16l-l 70.
keta). Science 196: 649-650.
ECKMAN,J. E. 1990. A model of passive settlement
SMITH, J. M. 1989. Evolutionary genetics. Oxford.
by planktonic larvae onto bottoms of differing
STRAYER,D. 1985. The benthic micrometazoans of
roughness. Limnol. Oceanogr. 35: 887-90 1.
Mirror Lake, New Hampshire. Arch. Hydrobiol.
ELLIOTT,J. M. 197 1. The distances travelled by driftSuppl. 72(3), p. 287-126.
ing invertebrates in a Lake District stream. OecoTOWNSEND,C. R., AND A. B. HJLDREW. 1976. Field
logia 6: 350-379.
experiments of the drifting, colonisation and continuous redistribution of stream benthos. J. Anim.
GIERE.,0. 1975. Population structure, food relations,
and ecological role of marine oligochaetes with
Ecol. 45: 759-773.
References
---.
Lotic meiofauna drift
VADAS,R. L., JR. 1990. The importance of omnivory
and predator regulation of prey in freshwater fish
assemblages of North America. Environ. Biol.
Fishes 27: 285-302.
WALTERS,K., AND S. S. BELL. 1986. Diel patterns of
active vertical migrations in seagrass meiofauna.
Mar. Ecol. Prog. Ser. 34: 95-103.
WILLIAMS,D. D. 1980. Invertebrate drift lost to the
sea during low flow conditions in a small coastal
stream in western Canada. Hydrobiologia 75: 25 l254.
341
WILZBACH,M. A., AND K. W. CUMMINS. 1989. An
assessment of the short term depletion of stream
macroinvertebrate benthos by drift. Hydrobiologia 185: 29-39.
Submitted: 27 December 1990
Accepted: I1 September 1991 .
Revised: 3 October 1991