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2009 SOUTHEASTERN NATURALIST 8(2):305–316
Spatial Distribution of Epiphytic Diatoms on Lotic
Jessica M. Knapp1,2,3,* and Rex L. Lowe1,2
Abstract - In stream ecosystems, bryophytes greatly increase substrate heterogeneity
and support a high density and diversity of lotic primary producers, such as
epiphytic algae. However, there is little information about how the spatial distribution
and density of epiphytic diatoms varies with respect to bryophyte morphology.
This study examined epiphytic diatom communities from the contrasting bryophyte
morphologies of mosses and liverworts. We predicted that mosses, with morphologies
that create more crevices, would have a higher density of epiphytic diatoms than
liverworts, with leaves highly exposed to the turbulence of the stream current. Six
species of bryophytes (two mosses and four liverworts) were collected from streams
in the Great Smoky Mountains National Park, and 37 species of epiphytic diatoms
were identified on these bryophytes. Diatom density was significantly higher on the
adaxial leaf surface of mosses compared to the abaxial leaf area (ANOVA, df = 29,
P < 0.001). There was no difference in diatom density on either the adaxial or abaxial
leaf surfaces of liverworts, and these diatom densities were statistically identical to
the density observed on the abaxial surface of moss leaves. The findings of our study
support our hypothesis that the morphology of mosses, comprised of leafy whorls,
provides a greater level of protection from disturbance than the open, fl at nature of
leafy liverworts. These findings emphasize that differences in microscale habitats
can result in varying diatom distribution and density that may be critical to food-web
interactions, such as grazing.
Diatoms are key components of primary production in stream ecosystems,
and the structural complexity of the available substrata is pivotal in
the determination of diatom density and community structure (Eminson
and Moss 1980). Bergey (1999) demonstrated that diatoms growing on
etched glass rods were present in higher densities in the etched crevices
than any other part of the glass rod. Diatoms have also been shown to inhabit
crevices on rock surfaces and sand grains in higher abundance than
on the flat, more exposed regions of the substrata (Krejci and Lowe 1986,
Round 1981). These studies suggest that different structural features of
microscale habitats, such as those of bryophytes, may influence diatom
community dynamics and growth patterns, especially in streams where
available substrata can vary considerably.
1Department of Biological Sciences, Bowling Green State University, Bowling Green,
OH 43403. 2University of Michigan Biological Station, Pellston, MI 49769. 3Current
address - Large Pelagics Research Center, University of New Hampshire, Spaulding/
Rudman Halls, 46 College Road, Durham, NH 03824 - 2618. *Corresponding author
306 Southeastern Naturalist Vol. 8, No. 2
Without a quiescent microhabitat, diatoms are susceptible to a variety of
disturbances, such as grazing, desiccation, scour, or fl ooding (Bergey 1999,
Biggs 1996, Krejci and Lowe 1986), and aquatic bryophytes have the potential
to provide diatoms with the refugia required for proliferation despite
disturbances (Resh et al. 1988). Although bryophytes are subjected to abrasion
from mobile substrata, such as sand and rocks, they are seldom directly
removed from the substratum by fast currents or fl ood events and have been
reported to provide protection for loosely attached organisms such as diatoms
(Power and Stewart 1987; Suren 1991, 1996; Suren and Winterbourn
1992; Suren et al. 2000). Water velocity is decreased within the bryophyte
thallus, creating a protected habitat for epiphytic diatoms (Sand-Jensen and
Mebus 1996, Suren 1991, Suren et al. 2000). To date, little work has been
done directly examining how the variability in the structure of bryophytes,
i.e., leafy liverworts versus whorled mosses, shapes the distribution and
density of diatoms within and between the different morphologies.
Mosses are characterized by a radially symmetric, leafy gametophyte
(Crum and Anderson 1981, Schofield 1985). Leaf phyllotaxy is spiral, and
the leaves are arranged around the stems in 3 or more rows with broad insertion.
Leaves are 3 to many ranked and not lobed. Most leaves possess a
midrib, and all leaves on one plant are alike. In contrast, the liverwort plant
body growth form can be thalloid, or fl attened, or can consist of a stem with
oppositely or alternately arranged leaves. The leaves vary in shape, never
have a midrib, and are only 1 cell layer in thickness (Ammons 1940). These
contrasting morphologies are likely to provide varying degrees of protection
and refugia for diatoms and thus may contribute to the structure and community
dynamics of the base of the food web in stream ecosystems.
This study explored how diatom densities varied with differing bryophyte
morphology in 3 streams in the Great Smoky Mountains National
Park. The density of diatoms was compared on the abaxial and adaxial surfaces
of both mosses and leafy liverworts. We predicted that mosses, with
morphologies that create more crevices, would have a higher density of
epiphytic diatoms than liverworts, with leaves highly exposed to the turbulence
of the stream current.
The Great Smoky Mountains National Park (GSNP) straddles the
border between North Carolina and Tennessee and contains the largest
old-growth forest in the eastern United States (Fig. 1). Algal samples were
collected in the spring (19–20 May 2004) and fall (22–23 October 2004)
from 3 streams: an unnamed tributary herein referred to as Stream A, the
Little Pigeon River, and the Little River. Streams were chosen based on accessibility
and presence of bryophytes.
2009 J.M. Knapp and R.L. Lowe 307
Table 1.Information about collections made at Stream A (SA), Little Pigeon River (LPR), and
the Little River (LR) across all sampling dates. “Submerged bryophyte coverage” and “exposed
bryophyte coverage” are the average estimated coverages of bryophytes across both sampling
dates, below and above the water surface, respectively. “Samples collected” is the number of
species of bryophyte samples collected at each river.
% % bryophyte Samples
Elev. Stream overstory coverage collected
River Date GPS (m) order coverage Submerged Exposed May Oct
SA 05/19/04, N 35°42.398, 1150 1 80 80 20 2 2
10/22/04 W 83°19.536
LPR 05/19/04, N 35°42.409, 1150 4 15 50 50 2 3
10/22/05 W 83°19.457
LR 05/20/04, N 35°36.966, 650 4 70 30 70 3 3
10/23/06 W 83°39.662
Stream A is a first-order tributary of the Little Pigeon River located on
the Ramsay Cascades Trail. The Little Pigeon River is a fourth-order stream,
and the sampling location was located 2 km upstream from Stream A on the
Ramsay Cascades Trail. The Little River collection site was located in a
fourth-order stream off of the Middle Prong Trail (Table 1).
At each collection site, GPS coordinates and elevation were recorded
using a Garmin eTrex Legend GPS unit (Garmin International Inc.,
Olathe, KS). Temperature was recorded using a digital thermometer, and
pH was measured using EMD colorphast pH-indicator strips (range = 2.5
to 10 pH units, sensitivity of 0.2 to 0.3 pH units; EMD Chemicals Inc.,
Figure 1. Location of the Great Smoky Mountains National Park in the southeastern
United States , and the position of the Little River, the Little Pigeon River and its
tributary Stream A.
308 Southeastern Naturalist Vol. 8, No. 2
Gibbstown, NJ). Bryophyte coverage and overstory coverage were visually
estimated (Table 1). Stream current velocity was not measured, as
this study focuses on the microhabitat between bryophyte leaves and not
on the general stream velocity.
To characterize the general water chemistry of each site, 500-mL water
samples were collected 10–15 cm below the water’s surface (Csuros 1994).
Samples were frozen within 24 hours of collection and were sent to the
University of Michigan Biological Station for analysis. Water samples were
filtered and tested for total fixed nitrogen (NO3
-N) and soluble reactive
3-P) using the cadmium reduction method and the ascorbic
acid reduction method, respectively (Eaton et al. 1995).
At each stream, 2 types of samples were collected for each bryophyte
species. An undisturbed/epiphytic sample (herein referred to as
the epiphytic sample) was collected to examine the natural distribution
of diatoms on the bryophyte. For this sample, 3 representative samples
of each bryophyte species were collected randomly from the stream. To
preserve the spatial arrangement of the diatoms on bryophyte leaves, the
epiphytic samples were placed into bags and transported upright in a box
to minimize agitation. An epipelic sample was also collected to determine
which diatom taxa were actually associated with the sediment rather than
with the bryophyte leaves. For the epipelic sample, a composite of each
bryophyte species was collected by sampling an area of 6 cm2 from 3 to
5 randomly selected rocks covered with the target bryophyte species. The
sample bag was then gently shaken to remove the sediment loosely attached
to the bottom of the bryophyte patch sampled. The bryophyte was
removed after the sediment settled and the remaining sediment was kept
Sample processing and analysis
Bryophyte samples were preserved with 50% glutaraldehyde within
24 hours of collection. Bryophytes were identified to species (Crum and
Anderson 1981) at the University of Tennessee. The epiphytic samples
were assigned an arbitrary code to ensure blind analysis, and a small section
was randomly selected and removed from the thallus for examination
with a Hitachi S-2700 scanning electron microscope (SEM; Hitachi Ltd.,
Tokyo, Japan). Epiphytic samples were dehydrated with an ethanol series
using a Samdri 780A critical point dryer (Tousimis Research Corp.,
Rockville, MD), mounted on aluminum stubs, and sputter coated with
10 nm of AuPd (Postek et al. 1980). General bryophyte morphology was
digitally recorded at 35–70 times magnification (Fig. 2). Spatial arrangement
and densities of diatoms were determined from 10 images each
of abaxial and adaxial surfaces of leaves chosen randomly and taken at
1000x magnification. In order to determine the surface area of images
at different tilts, an image of a 10-μm grid was taken at 1000x with a tilt
2009 J.M. Knapp and R.L. Lowe 309
of zero degrees. From this image, the surface area of each image was calculated
using basic trigonometry. Leaves did not always lie parallel to the
SEM stub, so there was some error (both over- and underestimations) associated
with the calculated surface areas.
Diatoms were subsampled from the epipelic samples by homogenizing
and then removing 25 mL of the diatom/sediment slurry, which was boiled
Figure 2. Scanning electron micrographs documenting the differences in plant
morphology of each bryophyte species. Mosses are Platyhypnidium riparioides
(a) and Fontinalis dalecarlic (b). Liverworts are Jubula pennsylvanic (c),
Scapania undulata (d), Porella pinnata (e), and Marsupella emarginata (f). Note the
midrib on the mosses and the broad fl at leaves on the liverworts. The adaxial surface of
(a) is marked with an arrow and abaxial is marked with a double arrow. Bars are 500 μm.
310 Southeastern Naturalist Vol. 8, No. 2
with nitric acid to remove organic matter (Round et al. 1990). Permanent
slides were made with Naphrax® for examination using light microscopy. Diatoms
were identified to species (Krammer and Lange-Bertalot 1986, 1988,
1991a, 1991b) using differential interference contrast on an Olympus BX51
light microscope (LM; Olympus, Melville, NY) at 1000x magnification under
oil immersion. Samples were counted until the relative frequencies of the
dominant taxa did not change with counting additional fields of view (about
All statistical analyses were run using JMP (5.1, SAS Institute Inc., Cary,
NC). Analysis of variance (ANOVA; α = 0.05) was used to examine the relationship
between bryophyte surface and diatom density while considering
whether the bryophyte was a leafy liverwort or a moss (herein referred to as
bryophyte type). Individual plants were nested within bryophyte type, and
bryophyte type and leaf surface were crossed factors. Upon determination
of significance, t-tests were used to compare leaf surface at each bryophyte
type (α = 0.05). Since there was only one comparison at the second level,
there is no need for a Bonferroni correction.
Elevation at the 3 sites ranged from 650 to 1150 m (Table 1). Submerged
bryophyte coverage, defined as bryophytes on rocks below the
water surface, was variable across the 3 sampling sites, ranging from 30
to 80% coverage of the streambed. Exposed bryophyte coverage, defined
as bryophytes on rocks above the water surface, was also variable, ranging
from 20 to 70% coverage. Temperatures were variable between sites
and sampling dates, ranging from 11 to 15 °C with the lowest recordings
occurring in May (Table 2). The pH levels in the streams were consistently
acidic throughout the study. Stream nutrient levels were variable
both between streams and between collection dates with Stream A having
the lowest total fixed nitrogen (NO3
-N) and soluble reactive phosphorus
3-P; SRP) levels. In the May collections, NO3
-N ranged from 18.0
μg/L to 300 μg/L, and SRP ranged from 1.1 μg/L to 13.0 μg/L. In the
Table 2. Temperature, pH, and water chemistry data from Stream A (SA), Little Pigeon River
(LPR), and Little River (LR) collection sites. The pH measurement reported is the level that
was recorded at both sampling events.
Temp (°C) NO3
- (ppb) PO4
River May October pH May October May October
SA 13.0 14.0 5 18.0 3.7 13.0 11.3
LPR 11.1 12.0 5 300.0 131.3 1.1 0.8
LR 15.0 14.0 5 126.0 3.0 3.9 2.9
2009 J.M. Knapp and R.L. Lowe 311
October collections, the NO3
-N ranged from 3.0 μg/L to 131.3 μg/L, and
SRP ranged from 0.8 μg/L to 11.3 μg/L (Table 2).
Community composition and distribution
Six species of bryophytes were identified: 2 mosses—Platyhypnidium
riparioides (Hedwig) Dixon and Fontinalis dalecarlica B.S.G; and 4 leafy
liverworts—Scapania undulata (L.) Dumortier, Jubula pennsylvanica
(Stephani) Evans, Porella pinnata L., and Marsupella emarginata (Ehrhart)
Dumortier. The bryophyte morphology documented with SEM (Fig. 2) was
variable in a manner consistent with known leaf arrangement of both leafy
liverworts and mosses (Schofield 1985).
Using LM and SEM, 19 genera and 37 species of epiphytic diatoms were
identified. Most species observed were prostrate forms (fl at against the substrate),
such as Cocconeis placentula. There was little variation in the diatom
species identified between bryophyte types and between leaf surfaces. Although
many of the taxa observed growing directly on the bryophyte leaves
were also observed in the epipelic samples, additional diatom species were
found only associated with the bryophyte sediments (e.g., Frustulia rhomboides
(Ehrenberg) De Toni).
Overall, the epiphytic diatom density varied between 0 and 231
diatoms·mm-2 with Eunotia rhomboidea, present in the highest density
(Table 3). Diatom densities were significantly different with respect to bryophyte
type and leaf surface (ANOVA, df = 29, P < 0.001; Fig. 3, Table 4).
Diatom density was significantly higher on the adaxial surface of the moss
leaves than on both the abaxial leaf surface of mosses and either leaf surface
of liverworts (Fig. 4, Table 4).
As predicted, the mosses (whorled leaves with more crevices) had a significantly higher density of epiphytic diatoms than the leafy liverworts (fl at
Table 3. Epiphytic diatom densities according to diatom species. Species reported here occurred
on more than one bryophyte sample. Total diatom density for the abaxial and adaxial surfaces
for each species was calculated by dividing the total number of diatoms for each surface by the
total area captured for the equivalent surface (mm2).
Diatom species Adaxial Abaxial
Achnanthidium appalachianum Camburn et. Lowe 39 29
Achnanthidium minutissimum (Kützing) Czarnecki 4 4
Cocconeis placentula Ehrenberg 15 1
Cymbella sp. 2 0
Decussata placenta (Ehrenb.) Lange-Bert. et Metzeltin 2 0
Diadesmis sp. 23 19
Eunotia praerupta Ehrenberg 1 1
Eunotia rhomboidea Hustedt 231 144
Meridion alansmithii Brant 13 6
Planothidium lanceolatum (Brébisson) Lange-Bertalot 47 16
312 Southeastern Naturalist Vol. 8, No. 2
exposed leaves). Within mosses, diatom densities were significantly higher
on the more protected adaxial surface of the moss than the more exposed
Results from this study suggest that the significant difference in diatom
density between mosses and liverworts is partially due to the protected environment
created by moss morphology. The adaxial surface of moss leaves
is positioned close to the stem forming a shelter in which epiphytic diatoms
Table 4. ANOVA table for examining the relationship between diatom density and leaf surface
while considering bryophyte type. Effects test table is below the ANOVA table showing significant differences between diatom density on leaf surfaces with respect to bryophyte type. An
individual is each piece of moss that was examined using SEM. Bryophyte type is nested within
individual, and leaf surface and bryophyte type are crossed factors.
Source df Sum of squares Mean square F Ratio Prob > F
Model 29 0.00005012 0.0000017 7.6792 <0.0001
Error 270 0.00006076 2.25E–07
Total 299 0.00011088
b. Effects test
Source df Sum of squares F Ratio Prob > F
Bryophyte type 1 0.00000135 5.9863 0.0151
Adaxial vs. abaxial 1 0.00000283 12.5715 0.0005
Individual (bryophyte type) 13 0.00003921 13.4030 <0.0001
Adaxial vs. abaxial*bryophyte type 1 0.00000189 8.4100 0.0040
Adaxial vs. abaxial*individual (bryophyte type) 13 0.00000513 1.7519 0.0508
Figure 3. Diatom density (least squares means) on the adaxial and abaxial leaf surfaces
of mosses (diamond) and leafy liverworts (square). Diatom density on the adaxial
leaf surface on mosses is significantly higher than that on the abaxial leaf surface on
mosses and either leaf surface on liverworts.
2009 J.M. Knapp and R.L. Lowe 313
are protected from disturbances (e.g., scour, desiccation, etc.). This shelter is
not provided by the abaxial surface of moss leaves or by either leaf surface
on leafy liverworts. Furthermore, epiphytic diatoms on bryophytes are also
protected because bryophytes increase substrate stability by decreasing the
drag of the rocks on which they are growing (Suren et al. 2000).
This contrasting distribution of diatom densities between different
leaf surfaces and different bryophyte types was consistent across streams,
regardless of the variability in stream conditions. The elevated nitrogen
levels during the May collection, relative to the October collection are
consistent with nitrogen peaks observed as a result of the spring snowmelt
(Campbell et al. 2000) and did not appear to affect diatom distribution or
Diatom community composition similarities between leaf surfaces
within and across streams may reflect the influences of grazing pressure
Figure 4. Scanning
depicting the high
variation in diatom
density between the
adaxial leaf surface
(top) and the abaxial
leaf surface (bottom)
of mosses. Both images
the moss Fontinalis
dalecarlica. Bars are
314 Southeastern Naturalist Vol. 8, No. 2
in addition to the acidic nature of streams in the GSMNP. The most
abundant taxon, E. rhomboidea, is acidophilic and often associated with
mosses. Many of the other identified taxa are acidophilic and/or grazer resistant
(Lowe 1974). Bryophytes harbor higher densities of invertebrates
than any other stream substratum (Brusven et al. 1990, Suren 1991),
which may explain the abundance of grazer-resistant algal taxa observed
in this study.
High densities of epiphytic diatoms on bryophytes may influence
food-web dynamics, especially in the bryophyte-rich streams in the
GSMNP, where nutrients are low and bryophytes shape much of the lotic
landscape. Bryophyte-associated differences in diatom community structure,
density, and spatial distribution may result in different degrees of
food resources available to grazers. Conditions such as pollution or disturbance
that result in a decline or shift in the types of bryophytes would
directly affect the periphyton population density and thus food-web interactions.
Further studies should include experimental disturbances in
controlled environments to determine the effect of different disturbances
on diatom spatial distribution and composition.
We thank Mike Grant and the University of Michigan Biological Station for
water chemistry data, David Smith and the University of Tennessee at Knoxville
for bryophyte identification, and the field assistants who helped with sample collection.
This work was funded by a US NSF grant (0315979) to R.L. Lowe. Portions
of this manuscript were completed while J.M. Knapp was supported by Bowling
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