2008 NORTHEASTERN NATURALIST 15(2):189–208
Benthic Algae in Episodically Acidified
Pennsylvania Streams
Sarah E. MacDougall1,2,*, Hunter J. Carrick1, and David R. DeWalle1
Abstract - Benthic algal assemblages were surveyed in five Pennsylvania streams in
order to examine trends in biomass and taxonomic groups along an acidification gradient.
Acidification severity of each stream was estimated from total dissolved aluminum
(AlTD), and discharge regression equations were established for each stream using
historic data from the EPA Episodic Response Program. Benthic algae were sampled
once after an acidic episode in April and again during August base flows. Chlorophyll
decreased with increasing episodic severity in April, thus acid episode severity
and associated aluminum toxicity may have influenced algal biomass. Cyanophyta
biovolume decreased with increasing severity, while Bacillariophyta, Chlorophyta,
and Rhodophyta showed no distinct patterns. A diatom, Eunotia exigua, exhibited a
parabolic response in April over the AlTD gradient. Episodic acidification severity may
influence algal biomass and composition in streams.
Introduction
Acidic deposition has been a long-standing ecological problem in the
northeastern United States causing widespread surface-water acidification
(Schindler 1988). Trends of improved water chemistry illustrate the effectiveness
of the 1990 amendments to the Clean Air Act in reducing sulfuric
dioxide and nitrous oxide emissions, the primary agents causing acidic
atmospheric deposition. From 1991 to 2002, northeastern streams exhibited
decreasing trends in sulfate concentrations and increasing trends in acidneutralizing
capacity (ANC) (Kahl et al. 2004).
Despite these long-term trends of ameliorating water quality, storm events
and snowmelt continue to cause episodic acidification, defined as short-term
drops in ANC (<0 μeqL-1) and pH (≤5 s.u.), in Pennsylvania mountain
streams (Kahl et al. 2004, Wigington et al. 1996a). The US Environmental
Protection Agency initiated the Episodic Response Project in 1988 in order
to enhance understanding of the occurrence, nature, and ecological effects of
episodic acidification (Wigington et al. 1996b). Streams in poorly buffered
regions of Pennsylvania and New York were selected in order to encompass
a range of episodic acidification severities. As with most episodic acidification
studies, the biological research stemming from the Episodic Response
Project focused on the adverse effects on fish and macroinvertebrates (Baker
et al. 1996, Kimmel et al. 1985, VanSickle et al. 1996). Given the extensive
research on episodic acidification, the streams in the Episodic Response
Project provided a logical site for investigating the little-known effects of
1School of Forest Resources, The Pennsylvania State University, University Park,
PA, 16802. 2Current address - ENSR, 2 Technology Park Drive, Westford, MA,
01886. *Corresponding author - smacdougall@ensr.aecom.com.
190 Northeastern Naturalist Vol. 15, No. 2
acid episodes on the base of the food chain, specifically benthic algae.
Benthic algae are common acidification indicators and play an important
role in aquatic food webs as well as biogeochemical cycling (Lowe
and Pan 1996). The few studies looking at acidification effects in streams
simulated rapid pH shifts and compared biomass and composition changes
to circumneutral conditions. Parent et al. (1986) compared the short- and
long-term biomass response to acid pulses in an artificial stream channel
and observed only slight changes in biomass in the short term. Hirst et al.
(2004) observed rapid taxonomic shifts due to large pH changes when transplanting
substrates from circumneutral streams to acidic streams. Diatom
assemblages on tiles removed from circumneutral streams and placed in
acidic streams quickly (within 3 days) became an acidobiontic assemblage
dominated by Eunotia exigua Brebisson. The reciprocal transplant yielded
a slower transition to a more circumneutral assemblage, dominated by Achnanthidium
minutissimum Kutzing, that took 9 days.
Only recently has episodic acidification been studied in a natural setting
with differing types of acidity. Passy (2006) and Passy et al. (2006) investigated
the effects of chronic vs. episodic acidity on benthic algal composition
and diversity using two of the Episodic Response Project streams in the New
York Adirondacks. The studies found that temporal factors and environmental
factors equally influenced the taxonomic composition of the episodically
acidified stream and that diversity was lower in the episodically acidified
stream compared to a chronically acidified stream. This study expands upon
Passy’s study and focuses on the effects of acid episode severity on benthic
algal biomass and composition using the 5 Pennsylvania streams from the
Episodic Response Project. The specific objectives of the study are: 1) to
determine an index to quantify episodic acidification severity in the 5 Pennsylvania
Episodic Response Project streams using historical chemistry data,
2) to compare differences in benthic algal biomass and taxonomic groups
among streams and between acid episodes and base-flow conditions, and
3) to identify trends in benthic algal biomass and taxonomic groups along
the proposed severity gradient in these streams.
Methods
Watershed and stream reach characteristics
Benthic algal assemblages were studied in 5 temperate, northern Appalachian
Plateau second-order streams in Pennsylvania: Baldwin Creek, Benner
Run, Linn Run, Roberts Run, and Stone Run (Fig. 1). The streams were located
on unglaciated terrain with shale and sandstone bedrock, and located
in watersheds ranging in size from 535 to 1156 ha (Wigington et al. 1996b).
The forest canopy of the undisturbed deciduous mixed forests was dominated
by Quercus spp. (oak), Acer spp. (maple), and Prunus serotina Ehrh. (black
cherry). The differing geologies of the watersheds contributed to the range
of acid-buffering capacities of the streams. Benner Run and Baldwin Creek
were underlain by Pocono Group rocks (Burgoon and Shenango sandstone),
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 191
which generally produced higher stream ANCs. Pottsville and Allegheny
sandstones of Stone Run, Roberts Run, and Linn Run watersheds contributed
more acidic groundwater to these streams (Wigington et al. 1996b). Baldwin
Creek and Linn Run were also influenced by Loyalhanna limestone in the
Mauch Chunk formation and thus had somewhat elevated base-flow ANC.
Geologic differences also caused aluminum to mobilize in Linn Run and
Stone Run at higher ANC and pH levels (Wigington et al. 1996a).
Development of Severity Criteria
The episodic acidification severity gradient for the 5 streams was based
on published stream discharge-AlTD regression equations developed from
the USEPA Episodic Response Project data (1988–1990), with sample sizes
of 287 to 414, depending on the stream (DeWalle et al. 1995). A discharge of
0.1 m3 s-1 km-2 was chosen to establish a severity gradient among the streams
because the maximum AlTD concentrations during episodes were predicted
at this discharge according to the regression equations. At discharges above
0.1 m3 s-1 km-2, the predicted AlTD reached a plateau or decreased slightly
with increasing discharge for all of the streams. The maximum predicted
AlTD concentrations during acid episodes reflected the greatest differences
in water quality among the streams and hence were used to characterize the
stream’s overall severity of acid episodes. The measured AlTD values in this
study were not used as indicators of acidification severity because they arose
from a single acid episode and during base flow, and thus were not as representative
of longer-term impacts of acid episodes as the predicted AlTD.
Benthic algal and water quality sampling
Figure 1. Location of the study sites on the Pennsylvania Northern Appalachian
Plateau.
192 Northeastern Naturalist Vol. 15, No. 2
Benthic algae were sampled in each of the 5 high-elevation streams
(>700 m above sea level) within 15 m upstream or downstream of the outlet
gauging stations where water chemistry samples have been collected for
the Episodic Response Project (1988–1990) and continued to be collected
for EPA long-term monitoring (1991–present). Algal samples were collected
twice in the 5 streams: spring (April 5–7, 2005) during acidic high
discharges, and in summer (August 20–21, 2005) during the less acidic base
flows. Discharges were calculated from continuous stream-stage hydrograph
records and stage/discharge rating curve equations (Sweeney et al. 1999,
Wigington et al. 1996b). April sampling occurred on the falling limb of a
peak flow after a rain event. The duration of the April event ranged from
9 to 13 days depending on the stream location. During April, the study
streams had maximum discharges ranging from 0.124 to 0.215 m3 s-1 km-2.
August samples were collected during base-flow conditions with maximum
discharges ranging from only 0.007 to 0.013 m3 s-1 km-2.
For each date-stream combination, benthic algae were collected from both
natural and artificial substrata. Six unglazed clay tiles were affixed to concrete
blocks that had been placed in the streams 4 weeks prior to each sampling. The
tiles provided a uniform substrate to compare monthly net biomass accumulation
rates and to gauge the relative turnover rate of biomass on natural rock
substrate in the streams. The 4-week colonization period reflected the recommended
time interval for representative biomass estimates on artificial
substrate (Cattaneo and Amireault 1992). A standard area of 64 cm2 was
scraped with razors and toothbrushes from the 6 clay tiles and from 10 rocks
randomly collected from the streambed for analysis of the benthic algal assemblage
biomass. Some of the rocks sampled for each stream had moss
attached (0–70% of rocks depending on the stream), but were included in the
10 rock replicates in order to provide a representative sample. Three of
the 10 rock samples that were collected for biomass analysis were additionally
analyzed for composition. Scraped rocks were placed downstream after being
scraped in order to prevent re-sampling the same rocks.
On each sampling interval, triplicate water temperatures were measured
with an Oaklon Temp 5 Thermometer. Leaf-area index (LAI) measurements
were taken one time per stream in April and September with a LAI-2000 Plant
Canopy Analyzer. August LAI values were not obtained due to equipment errors,
and September values were used instead. Grab samples of stream water
were collected on the sampling day and subsequently analyzed using standard
methods for pH, ANC, SO4
2-, NO3
-, and total phosphorus (Clesceri et al. 1998).
AlTD was analyzed with a Perkin Elmer 5100 furnace using the 3500A AA
spectrometric method with 0.10-μm filtration (Clesceri et al. 1998).
Analysis of algal chlorophyll
Benthic algal slurries collected from each rock and tile were diluted
to a similar volume and individually homogenized with a blender (Biggs
1987). Epiphyton were detached from the moss scraped from the sample
during blending as well. Chlorophyll-a concentrations were measured from
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 193
subsamples concentrated onto filters (0.3-μm Whatman EPM 2000) and
extracted in 50:50 90% acetone-DMSO. Total chlorophyll-a concentrations
were determined using a Turner Fluorometer, model 10-AU (Carrick et al.
1993). Chlorophyll-a was not corrected by subtracting phaeophyton values.
Parts of moss present in samples scraped from moss-covered rocks were
removed from the filter in order to not bias chlorophyll-a estimates. All
samples were preserved for enumeration with 1% of Lugol’s solution and
1% of formalin (Wetzel and Likens 2000).
Analysis of algal taxonomic composition
Three of the ten benthic algal samples were randomly chosen to be
enumerated from each stream-date combination using a stratified counting
procedure. Approximately 500 cells were enumerated in a procedure
similar to that described by Carrick and Steinman (2001). A subsample was
pipetted into a Palmer-Maloney counting chamber and allowed to settle for
5 minutes. Large algal taxa (colonial and filamentous forms) were enumerated
by scanning the entire chamber under low magnification (100x total
magnification). Small algal taxa (unicellular forms) were enumerated by
scanning random fields under high magnification (400x). When necessary,
additional aliquots were concentrated onto Millipore nitrocellulose filters to
prepare semi-permanent slides in order to obtain a cell count of >500 (Dozier
and Richerson 1975). Finally, diatoms enumerated in the counting chamber
were identified on slides that were prepared from cleaned subsamples using
the hydrogen peroxide/potassium dichromate method (VanDerWerff 1955)
and mounted with Zfrax (W.P. Dailey, Philadelphia, PA). Despite a constant
effort in enumerating cells using the counting chamber and the slides, 17%
of the samples had <500 cells counted due to algal sparseness. All algae
were identified using a Leica DMR research microscope. The algal taxonomy
references used for Bacillariophyta were Patrick and Reimer (1966)
and Krammer and Lange-Bertalot (1986, 1988, 1991a, 1991b); Whitford
and Schumacher (1973), Prescott (1962), and Prescott et al. (1975) were
used for Chlorophyta; Komrek and Anagnostidis (2005) was used for Cyanophyta;
and Wehr and Sheath (2003) was used for Rhodophyta.
Cellular biovolumes were derived for each taxon using the surface-area
calculation feature of the Northern Eclipse microscope imagery software
(MVIA, Monaca, PA). Cell breadth was estimated using half of cell widths
for soft algae and observed girdle widths for diatoms. These estimates seem
reasonable given the significant correlation between chlorophyll-a concentrations
(mg m-2) and algal biovolume concentrations (μm3 cm-2) for the samples
enumerated when natural log transformed (r = 0.74; p < 0.001, n = 30).
Data analysis
Linear and quadratic regression analyses were used to compare biomass,
taxonomic groups, and species diversity trends across the episodic
acidification gradient. Chlorophyll-a, absolute biovolume, and diversity were
regressed against predicted AlTD concentrations. The most appropriate regres194
Northeastern Naturalist Vol. 15, No. 2
sion model was determined by comparing the coefficient of determination, R2,
for both linear and quadratic trend lines. Trend lines with the largest R2 were
chosen to represent the response. Cell biovolumes were transformed in order
to improve homogeneity of residual scatter with natural log (ln[value + 1]).
Chlorophyll-a was used as a proxy for biomass and transformed with
natural log (ln[value + 1]) for the mean comparisons. A one-way fixed factor
ANOVA was used to test differences of the mean biomass on rocks and
tiles among the streams for each date. Pairwise comparisons were performed
to compare chlorophyll-a biomass using the Bonferroni simultaneous test.
Independent separate variance t-tests were performed to test differences in
chlorophyll-a biomass between dates for each stream.
Benthic algal taxa were categorized into phyla and groups with acidity
preferences. These groups included acid-tolerant diatoms, acid-intolerant
diatoms, filamentous green algae, cyanobacteria, and red algae. Diatoms
were also classified into acidity preference groups according to Van Dam
(1994) where acidobiontic diatoms had an optimum pH of <5.5 and acidophilous
diatoms predominately occurred at pH < 7 . The acid-tolerant diatoms
incorporated both the acidobiontic and acidophilous diatoms for this
study. Differences in the absolute biovolume of these categories among
streams and between dates were tested using a two-way ANOVA. Species
present in all streams were also compared with a two-way ANOVA regression,
and mean comparisons were performed on SPSS version 13.0 (SPSS,
Inc., Chicago, IL) and Minitab 13.32 (Mintab, Inc., State College, PA). We
measured diversity, using the Shannon-Weiner diversity index (Shannon
1948), and taxon richness, which was defined as the total number of taxa
observed in a sample.
Results
Episodic acidification chemistry
The 5 Appalachian streams exhibited an episodic acidification severity
gradient as a result of ranking of the predicted AlTD concentrations. Baldwin
Creek exhibited the least severe episodic acidification. Benner Run, Roberts
Run, Stone Run, and Linn Run had increasingly severe acidic episodes
(Fig. 2). Differences in water chemistry between the sampling dates demonstrated
that the stream water chemistry corresponded to the respective flow
regimes (Table 1). For example, the median pH of the April samples taken
during high discharges was 5.29, whereas the median pH of the base-flow August
samples was 6.21. Furthermore, the mean ANC in the 5 streams was also
lower in April (1.34 μeq L-1) than in August (56.96 μeq L-1). However, concentrations
of phosphorus (0.006–0.014 mg L-1) were similar among streams and
dates. Nitrate concentrations varied among streams due to differences in atmospheric
inputs and other factors, with concentrations being lowest in Stone
Run and highest in Linn Run (0.11–2.54 mg L-1).
Variation in algal chlorophyll
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 195
Benthic algal biomass, as chlorophyll-a, on natural stream rocks varied
nearly one order of magnitude among the 5 streams (Table 1). In April, the
streams had significantly different chlorophyll-a concentrations, ranking
highest to lowest in the following order: Baldwin Creek, Benner Run, and
Roberts Run, Stone Run, and Linn Run (F = 54.3, df = 4, p < 0.01). Linn
Run had significantly lower chlorophyll-a concentrations on rocks than the
other streams in April (Bonferroni adjusted p-values of <0.01). In August,
the streams also had significantly different chlorophyll-a concentrations, but
ranked in a different order from highest to lowest chlorophyll-a concentration:
Benner Run, Roberts Run, Stone Run, Baldwin Creek, and Linn Run
(F = 13.82, df = 4, p < 0.01). Linn Run had significantly less chlorophyll-a
Figure 2. The episodic
acidification severity gradient
established from AlTD values
derived from discharge-AlTD
regression equations (DeWalle
et al. 1995) at a common
discharge of 0.1 m3 s-1 km-2.
Higher predicted AlTD values
indicate more severe episodic
acidification.
Table 1. Physical and chemical parameters as well as total chlorophyll-a (TChl) estimates measured
during the acid episodes in April when benthic algae were sampled. Units of measure:
maximum discharge = m3s-1km-2, ANC = μeq L-1, AlTD = mg L-1, SO4
2- = mg L-1, NO3
- = mg L-1,
and Tchl = mg m-2.
Water Mean Mean
Stream temp. Maximum rock tile
by month (oC) LAI dischargeA pH ANC AlTD SO4
2- NO3
- TchlB TchlC
April
Baldwin 8.11 2.34 0.215 5.93 18.70 0.034 9.98 1.73 25.30 1.76
Benner 6.70 0.00 0.172 5.92 10.60 0.027 4.91 0.40 27.81 0.34
Roberts 10.20 2.55 0.124 4.94 -9.68 0.125 7.47 0.31 20.70 3.65
Stone 7.50 0.00 0.197 4.94 -12.80 0.190 7.94 0.11 7.44 0.48
Linn 4.20 0.90 0.140 5.29 -0.14 0.037 8.76 1.29 0.54 0.03
August
Baldwin 19.40 4.38 0.010 6.65 109.0 0.018 8.37 1.06 4.42 1.64
Benner 20.39 2.08 0.009 6.21 32.1 0.057 3.86 0.60 22.56 7.86
Roberts 17.50 4.99 0.007 5.83 25.8 0.058 6.64 0.60 13.55 0.32
Stone 17.80 3.16 0.013 5.75 16.9 0.045 6.39 0.30 5.69 1.22
Linn 18.60 2.15 0.008 6.46 101.0 0.024 9.08 2.54 1.16 0.18
AMaximum discharge in month prior to sampling.
Bn = 10.
Cn = 6.
196 Northeastern Naturalist Vol. 15, No. 2
on rocks in August than the other streams except Baldwin Creek (Bonferroni
adjusted p-values < 0.01). The overall mean biomass was usually higher in
April than in August. In particular, Baldwin Creek had significantly more
biomass on rocks in April than in August (t = 7.40, df = 11, p < 0.01). Conversely,
Linn Run had significantly less biomass on rocks in April than in
August (t = 2.90, df = 17, p = 0.01).
Tiles generally had lower chlorophyll-a biomass than rocks had for both
dates. For each stream, there was significantly lower chlorophyll-a biomass
on tiles than on rocks (p < 0.05). In April, mean chlorophyll-a concentrations
on tiles ranged from 0.03 mg m-2 (Linn Run) to 3.65 mg m-2 (Roberts Run).
August chlorophyll-a concentrations on tiles were similarly low compared
to quantities on rocks and ranged from 0.18 mg m-2 (Linn Run) to 7.86 mg
m-2 (Benner Run). Biomass on tiles represented a monthly net accumulation
rate. Accumulation rates reflected the cumulative result of colonization
rates, growth rates, and grazing rates because loss rates were not measured.
Benthic algal chlorophyll-a on rocks decreased with increasing predicted
AlTD when both sampling dates were included (Y = 24.39 - 64.17x, R2 =
0.40, df = 9, p = 0.05) (Fig. 3). This decline was strong in April (p = 0.06,
R2 = 0.74), but no significant trend was evident in August alone. The low
chlorophyll-a mean in Baldwin Creek during August appeared to be an outlier,
and it was lower than the April estimate. There was no significant trend
for chlorophyll-a on tiles over the predicted AlTD gradient.
Variation in algal taxonomic composition
A total of 62 algal taxa representing four phyla were identified from the 5
study streams. Chlorophyta (green algae), Cyanophyta (cyanobacteria), and
Bacillariophyta (diatoms) were present in all streams (Fig. 4, Appendix A).
Only one genus of Rhodophyta (red algae), Audouinella, inhabited Baldwin
Creek and Roberts Run. Benner Run had the highest relative biovolume of
filamentous green algae for both dates. Some of the filamentous green algae
present included Bulbochaete sp., Microspora tumidula Hazen, Mougeotia
sp., and Ulothrix tenerrima Kuetzing. Baldwin Creek had the greatest
Figure 3. Regression of
predicted AlTD and mean total
chlorophyll-a (Tchl mg m-2) for
rocks during the April sampling
(Y = 35.7 - 98.0x, R2 = 0.90, p
= 0.01). Each point represents
the mean total chlorophyll-a
from ten rocks. Error bars represent
one standard error.
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 197
biovolume of cyanobacteria for both dates. Cyanobacteria biovolume was
significantly different among streams (F = 18.22, df = 29, p < 0.01) and higher
in April than August (F = 6.42, df = 29, p = 0.02). Stone Run and Roberts
Run had the highest relative biovolume of acid-tolerant (acidobiontic and
acidophilic) diatoms for both sampling dates, 99.5% and 94.8% respectively.
Biovolume of acid-tolerant diatoms was higher in April compared to August
for all streams (F = 3.57, df = 29, p < 0.1). A significant interaction occurred
between stream and date for the 2-way ANOVA analysis of non-acid-tolerant
diatoms and this complicated the interpretation of results. The diatom E.
exigua was the dominant algal species in Roberts Run, constituting 55%
of total algal biovolume in April. Diatom species commonly found in the
streams included E. exigua, Frustulia rhomboides (Ehrenberg) De Toni, and
Achnanthes marginulata Grunow. E. exigua, an acidobiontic species, was
significantly different among streams (F = 6.02, df = 29, p = 0.002), but not
between sampling dates (F = 2.79, df = 29, p = 0.110). Generalist species
(species with a pH tolerance range of >3 pH units) dominated Linn Run in
Figure 4. Percent
biovolume of algal
phyla found in
each study stream
during the a)
April sampling of
acidic high flows
and b) August
sampling of lessacidic
base flows.
A c i d - t o l e r a n t
diatoms refer to
acidobiontic and
acidophilic species.
198 Northeastern Naturalist Vol. 15, No. 2
April, constituting 53% of total algal biovolume.
Predicted AlTD provided a quantitative estimate of episodic acidification
severity in order to relate biovolume to a severity gradient. There was
a declining linear trend in cyanobacteria biovolume across the gradient of
increasing severity of episodic acidification (p < 0.1; Fig. 5a). While diatoms
as a group did not show a significant trend with AlTD, some species did. E.
exigua exhibited a significant parabolic response over the gradient, with a
peak at moderately high aluminum concentrations in April, but not in August
(Fig. 5b). Other taxonomic groups or taxa present in all of the streams failed
to exhibit significant trends across the severity spectrum. Also, no significant
trend was evident for diversity or taxon richness over the severity gradient.
Discussion
Episodic acidification severity
Total dissolved aluminum was chosen to characterize episodic acidifi-
cation severity because, given its pH-dependent solubility, it served as an
indicator for metal toxicity to aquatic organisms as well as ANC decline.
Numerous studies on Episodic Response Project streams demonstrate
the effect of Al toxicity on fish during acid episodes. Baker et al. (1996)
observed depressed trout populations and the absence of other acid-sensitive
fish species in streams with pH less than 5 to 5.2 and aluminum concentra-
Figure 5. a) Regression of natural
log transformed cyanobacteria
biovolume (Biovol μm3
cm-2) against predicted AlTD
for all sample dates (Y = 13.8
- 15.8x, R2 = 0.35, p = 0.07).
b) Regression of natural log
transformed of Eunotia exigua
biovolume against predicted
AlTD for April. (Y = -21.38 +
378.1x + -855.2 x 2, R2 = 0.97,
p = 0.03).
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 199
tions exceeding 110–200 μg L-1. Kaeser and Sharpe (2001) suggested that
elevated aluminum and hydrogen ion concentrations disrupted acid-sensitive
Cottus cognatus Richardson (Slimy Sculpin) reproduction in Stone Run.
VanSickle et al. (1996) supported these findings using bioassays to associate
Salvelinus fontinalis Mitchill (Brook Trout) mortality with elevated AlIM
(inorganic monomeric). Gagen (1991) showed that duration of exposure as
well as the concentration of aluminum affected the extent of fish mortality
in the study streams. Although comparatively less is known about aluminum
toxicity to algae compared to fish, Gensemer and Playle (1999) present an
extensive review of the aluminum toxicity on algae.
AlTD was also chosen as an indicator of episodic acidification severity
because all 5 streams exhibited strong correlations between Al-ANC and
Al-pH during the Episodic Response Project (Wigington et al. 1996b). Inorganic
monomeric aluminum (AlIM) is the species of aluminum most toxic
to aquatic biota (Gensemer and Playle 1999), but AlTD provided a good
estimate of AlIM in the study streams due to the strong overall linear relationship
between AlTD and AlIM in Episodic Response Project data (Pagano
1990). The relationships in Baldwin Creek and Benner Run were weaker, but
only Benner Run had a significant portion of organic monomeric aluminum
(AlOM), especially at low AlTD concentrations. However, all of the streams
had relatively low dissolved organic carbon that can potentially bind and
decrease aluminum bioavailability (DeWalle et al. 1995). Therefore, AlTD
was deemed to be an effective indicator of episodic acidification severity for
this study.
Factors influencing algal biomass
Our data suggest that the severity of episodic acidification contributed to
differences in the quantity of algal biomass on natural rocks among streams.
Chlorophyll-a decreased with increasing severity in April after an acid episode,
but did not change in August during base flow. Hence, acid episodes may
limit biomass accrual in streams severely affected by episodic acidification.
Parent et al. (1986) hypothesized that short-term acidification may negatively
affect biomass accrual after observing slight decreases in benthic algal biomass
4 hours following the acidification of an artificial stream. Acidity effects
on benthic algae may be similar to the effects on phytoplankton, which include
growth inhibition, reduced nutrient uptake, and lower photosynthetic rates
with aluminum exposure (Gensemer and Playle 1999). In fact, benthic algal
biomass decreased following exposure to total dissolved aluminum treatments
of 0.5 mg L-1 in an experimental stream with a pH of 4.8 (Genter 1995). Another
study with water quality more similar to the stream conditions in April in
this study (pH of 5.5 and AlTD concentrations of 0.2 mg L-1) also demonstrated
that adding aluminum to already depressed pH treatments could result in
larger decreases in biomass than in treatments with only low pH (Kinross et al.
2000). Therefore, aluminum toxicity effects may contribute to lower biomass
accrual observed in the study streams with higher aluminum concentrations
during acid episodes. However, the effects of acid episodes may be shortlived,
given that no trend was found in August.
200 Northeastern Naturalist Vol. 15, No. 2
Other environmental and biological factors, such as nutrients, grazing
pressure, light, temperature, flow velocity and substratum interactions may
influence the observed biomass trends given that the mean biomass on rocks
was lower in August than in April (Planas 1996). Large between-stream
differences were not observed for most physical-chemical factors, such as
water temperature, maximum stream flow, and phosphorus concentrations
(Table 1), although differences in canopy cover were evident among streams.
Very little data exist regarding the trophic structure of these streams. In
several studies, Linn Run was found to have low densities of the shredderdominated
macroinvertebrate population, especially during acid episodes
(Keener 2003, Kimmel 1999, LeFevre and Sharpe 2002). However, macroinvertebrate
grazing may contribute to differences in biomass among the
streams given the low biomass estimates despite little canopy cover in April.
Passy (2006) suggested that both environmental and temporal factors, such
as competition and herbivory, controlled diatom community composition
in communities in an episodically acidified stream in the New York Adirondacks.
This pattern may also be true for this study and may explain the
observed biomass trends. Therefore, episodic acidification severity is only
one factor likely influencing benthic algal assemblages.
Factors influencing algal taxonomic composition
Variation in algal composition among samples indicates a phylum- and
species-specific nature of the algal response to episodic acidification severity
and other environmental factors. For instance, cyanobacteria biovolume
exhibited the strongest trend among the algal phyla and was significantly
higher in April following the acid episode than in August during base-flow
conditions. The findings in these streams correspond with historical observations
that filamentous cyanobacteria species tend to be intolerant of acidic
conditions (Brock 1973, Ledger and Hildrew 2005, Maurice et al. 1987,
Mulholland et al. 1986, Passy 2006).
Diatoms as a phylum did not demonstrate any overall trend over the
severity gradient. This finding may be the result of their diverse acidity
optima (Van Dam et al. 1994). There were significantly more acid-tolerant
diatoms after the acid episode in April than during summer base flow. In
particular, the acidobiontic diatom E. exigua exhibited a parabolic trend that
peaked at moderate AlTD concentrations. This result suggests that E. exigua
found optimal conditions in streams with moderate aluminum concentrations
(≈0.15–0.25 mg L-1) during acid episodes. The lower biovolume of E. exigua
in streams having more severe acid-episodes suggests that the aluminum
concentrations during acid episodes in the severely affected streams may be
greater than the tolerance of the species. Passy (2006) also found E. exigua
to be a dominant species in an episodically acidified stream, and the population
declined with increased AlIM:AlOM ratios and low pH. Since the AlOM
was found to be low in the Pennsylvania study streams during the Episodic
Response Project, this study’s findings of decreased E. exigua at high aluminum
concentrations is similar to Passy’s observations in the Adirondack
stream with AlIM concentrations reaching approximately 0.40 mg L-1.
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 201
Interestingly, E. exigua is commonly found in metal-enriched acid mine
drainage sites with much greater aluminum concentrations than those predicted
to occur during acid episodes (Verb and Vis 2005) and is considered
to be acidification-tolerant (Battarbee et al. 1999, DeNicola 2000, Van Dam
et al. 1994, Verb and Vis 2005). The ability of E. exigua to withstand low
pH and high aluminum concentrations suggests that the relative abundance
of E. exigua should have been greater rather than lesser in streams with
more severe acid episodes, such as Linn Run. Interestingly, other species
commonly found in streams with heavy metals, such as Microthamnion kuetzingianum
Naegeli in Kuetzing and Microspora tumidula Hazen (John et al.
2002), were also present in the moderately-inflicted streams, Roberts Run
and Stone Run, but not in the most severe stream, Linn Run. The presence of
species indicative of metal stress, such as E. exigua, M. kuetzingianum, and
M. tumidula, suggests that chemical stress is greatest in the study streams
with moderately severe acid episodes. However, the autoecology of these
species is primarily based upon surveys conducted in acid mine drainage and
chronically acidified systems.
Episodic acidification appears to be a more complex acidification phenomenon
because of the short-term nature of acid pulses. For instance, the
duration of acid episodes may influence benthic algal composition
in the study streams as it did with fish mortality (Gagen 1991). Although
Linn Run has the most severe predicted acid episodes, it also has relatively
high pH and ANC during base flow. The dominance of algal species with
wide pH tolerances in Linn Run during April (environmental generalists)
may allow the assemblage to withstand the pH fluctuations from base flow
conditions to acidic conditions during episodes. Species with lower pH optimums
and higher metal tolerances, such as E. exigua, may not be able to
colonize the most severely affected stream, Linn Run. Slow colonization
rates inferred from the low biomass accumulation on tiles over one month
may account for the slow response of the benthic algal community during
high flows. Biomass growing on the tiles represents monthly net accumulation
rate because the tiles were placed in the streams one month prior to algal
sampling (Stevenson et al. 1996). Colonization rates are assumed from the
biomass accumulation rates of tiles, although the influence of macroinvertebrate
grazing effects is uncertain. The tile biomass was significantly lower
than rock biomass for all of the streams, thus colonization rates of algae in
the streams are assumed to be slow. Therefore, the frequency, duration, and
severity of episodic acidification may all contribute to the composition patterns
observed among the streams.
Stream acidity likely contributes to the presence of some filamentous
green algal genera commonly found in acidic waters, such as Bulbochaete,
Microspora, Mougeotia, and Ulothrix. However, there was no significant
increasing trend in filamentous green algae in the study streams over the
episodic acidification severity gradient. This finding is contrary to what was
expected given that filamentous green algae commonly increased following
experimental acidification (Herrmann et al. 1993, Mulholland et al. 1986,
Parent et al. 1986). Hence, the lack of trend found with the filamentous green
202 Northeastern Naturalist Vol. 15, No. 2
algae over the severity gradient may reflect the complex interplay of site
variables, such as nutrients, light, flow velocity, and chemical stress.
Differences in light conditions may explain the presence of red alga in
two streams of differing episodic acidification severity: Baldwin Creek, the
stream with the least severe acid episodes, and Roberts Run, a moderately
disturbed stream. Dense Tsuga candensis (L.) Carrière (hemlock) similarly
shade both of these streams at the study sites, as reflected by relatively high
LAI values (2.34–2.55 in spring and 4.39–4.99 in summer), and thus these
streams have the lowest light conditions. The tendency of Audouinella spp.
to be present in lower light conditions further supports the hypothesis that
light conditions may have been influential in determining algal composition
in the study streams (Dillard 1966, Sheath and Burkholder 1985).
Conclusions
Overall, the study provided initial insights into the dynamics of the benthic
algal assemblage in streams with varying levels of episodic acidification
severity. Historical AlTD-discharge relationships provide a simple yet effective
means to quantify the severity of episodic stream acidification. Using
the proposed severity gradient, aluminum toxicity seemingly has negative
effects on algal biomass. Trends within algal phyla vary, likely due to the
numerous in situ environmental factors. This benthic algal survey provides a
basis for further study to unravel the enigmatic dynamics of episodic acidification on benthic algal assemblages.
Acknowledgments
We wish to extend thanks to Penn State Institutes for the Environment and the
School of Forest Resources for their support. US EPA LTM provided the primary
funding for the study with the assistance of Penn State Limnology Lab. Chemistry
data were analyzed in the Penn State Institutes for the Environment Water Quality
Laboratory. Jeffrey Johanson assisted with algal identification in a Humboldt Field
Research Institute field course.
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206 Northeastern Naturalist Vol. 15, No. 2
Appendix A. List of taxa present in the study streams.
Phyla Taxon StreamA
Bacillariophyta Achnanthes daui (ploenesis) Foged Benner (Aug)
Linn (Apr)
Bacillariophyta Achnanthes lanceolata (Brebisson) Grunow Baldwin (Aug)
Benner (Aug)
Bacillariophyta Achnanthes marginulata Grunow Baldwin
Benner
Linn (Apr)
Roberts
Stone
Bacillariophyta Achnanthes minutissima Kutzing Linn (Aug)
Bacillariophyta Achnanthes sp. A Linn (Apr)
Bacillariophyta Anomoeoneis vitrea (Grunow) Ross Benner (Aug)
Bacillariophyta Aulacoseira spp. Stone (Aug)
Bacillariophyta Cocconeis placentula Ehrenberg Linn (Aug)
Stone (Aug)
Bacillariophyta Diatoma mesodon (Ehrenberg) Kutzing Baldwin
Benner
Roberts (Apr)
Linn (Aug)
Stone (Aug)
Bacillariophyta Diatoma vulgaris Bory Linn (Apr)
Bacillariophyta Eunotia bilunaris (Ehrenberg) Mills Roberts (Aug)
Bacillariophyta Eunotia exigua (Brebisson) Rabenhorst Baldwin (Aug)
Benner
Linn
Roberts
Stone
Bacillariophyta Eunotia formica Ehrenberg Roberts (Apr)
Bacillariophyta Eunotia implicata Norpel, Lange-Bertalot Benner
& Alles Linn (Aug)
Bacillariophyta Eunotia incisa Gregory Benner
Stone (Aug)
Bacillariophyta Eunotia muscicola var. tridentula Norpel & Stone (Apr)
Lange-Bertalot Roberts (Apr)
Bacillariophyta Eunotia pectinalis var. ventricosa Grunow Benner (Aug)
Bacillariophyta Eunotia subarcuatoides Alles, Norpel & Roberts Stone
Lange-Bertalot
Bacillariophyta Eunotia sp. A Linn
Bacillariophyta Fragilaria delicatissima (W. Smith) Benner (Aug)
Lange-Bertalot Linn (Aug)
Bacillariophyta Fragilaria leptostauron (Ehrenberg) Hustedt Baldwin (Apr)
Bacillariophyta Fragilaria virescens Ralfs Stone (Aug)
Bacillariophyta Fragilaria spp. Linn (Apr)
Bacillariophyta Frustulia rhomboides (Ehrenberg) De Toni Benner
Linn
Roberts
Stone
2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 207
Phyla Taxon StreamA
Bacillariophyta Gomphonema angustatum (Kutzing) Rabenhorst Baldwin (Apr)
Stone (Apr)
Benner (Aug)
Stone (Aug)
Bacillariophyta Gomphonema gracile Ehrenberg Benner (Aug)
Bacillariophyta Gomphonema insigne Gregory Benner (Aug)
Linn (Aug)
Bacillariophyta Gomphonema sp. A Linn (Aug)
Bacillariophyta Meridion circulare (Greville) Agardh Benner (Aug)
Stone (Aug)
Bacillariophyta Navicula cryptocephala Kutzing var. Linn (Apr)
cryptocephala
Bacillariophyta Navicula radiosa var. parva Wallace Baldwin (Aug)
Benner
Bacillariophyta Pinnularia abaujensis (Pantocsek) Ross Baldwin
Stone (Aug)
Bacillariophyta Strauroneis anceps Ehrenberg 1843 Baldwin (Aug)
Stone (Aug)
Bacillariophyta Tabellaria flocculosa (Roth) Kutzing Benner (Apr)
Linn
Roberts
Stone
Chlorophyta Bulbochaete spp. Benner
Stone (Aug)
Chlorophyta Chaetophora elegans (Roth) C.A. Agardh Stone (Apr)
Roberts (Apr)
Chlorophyta Closterium cornu var. cornu Ehrenberg Benner (Aug)
Chlorophyta Closterium cornu var. minor Irenee-Marie Roberts (Aug)
Chlorophyta Closterium moniliferum (Bory) Ehrenberg Baldwin (Aug)
Chlorophyta Closterium spetsbergense var. laticeps Gronblad Benner (Aug)
Chlorophyta Cosmarium spp. Benner
Linn
Roberts (Apr)
Stone
Chlorophyta Cylindrocapsa geminella Wolle Linn (Aug)
Chlorophyta Cylindrocapsa spp. Benner
Chlorophyta Microspora tumidula Hazen Stone (Apr)
Chlorophyta Microthamnion kuetzingianum Naegeli Stone
Roberts (Apr)
Chlorophyta Mougeotia laetevirens (Braun) Wittrock Benner (Aug)
Chlorophyta Mougeotia sp. A Stone (Aug)
Chlorophyta Mougeotia sp. B Benner (Aug)
Chlorophyta Stichococcus subtilis (Kutzing) Klercker Benner (Apr)
Chlorophyta Ulothrix tenerrima Kutzing Benner
Cyanophyta Calothrix spp. Benner (Aug)
Cyanophyta Geitlerinema sp. A Benner
Roberts (Apr)
Stone
208 Northeastern Naturalist Vol. 15, No. 2
Phyla Taxon StreamA
Cyanophyta Geitlerinema sp. B Benner
Roberts (Apr)
Stone (Apr)
Cyanophyta Geitlerinema sp. C Baldwin (Apr)
Cyanophyta Jaaginema spp. Baldwin
Cyanophyta Leptolyngbya sp. A Stone (Apr)
Cyanophyta Leptolyngbya sp. B Baldwin (Apr)
Cyanophyta Leptolyngbya spp. Roberts (Aug)
Cyanophyta Phormidium sp. A Baldwin (Apr)
Cyanophyta Pseudanabaena spp. Baldwin
Benner (Aug)
Roberts (Apr)
Stone
Cyanophyta Spirulina spp. Benner (Aug)
Cyanophyta Tychonema spp. Benner (Aug)
Rhodophyta Audouinella spp. Baldwin
Roberts
A(Apr) indicates the taxon was present only in April and (Aug) indicates it was present
only in August.