nena masthead
NENA Home Staff & Editors For Readers For Authors

Benthic Algae in Episodically Acidified Pennsylvania Streams
Sarah E. MacDougall, Hunter J. Carrick, and David R. DeWalle

Northeastern Naturalist, Volume 15, Issue 2 (2008): 189–208

Full-text pdf (Accessible only to subscribers.To subscribe click here.)

 

Access Journal Content

Open access browsing of table of contents and abstract pages. Full text pdfs available for download for subscribers.



Current Issue: Vol. 30 (3)
NENA 30(3)

Check out NENA's latest Monograph:

Monograph 22
NENA monograph 22

All Regular Issues

Monographs

Special Issues

 

submit

 

subscribe

 

JSTOR logoClarivate logoWeb of science logoBioOne logo EbscoHOST logoProQuest logo

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. Literature Cited Baker, J.P., J. VanSickle, C.J. Gagen, D.R. DeWalle, W.E. Sharpe, R.F. Carline, B.P. Baldigo, P.S. Murdoch, D.W. Bath, W.A. Kretser, H.A. Simonin, and P.J. Wigington. 1996. Episodic acidification of small streams in the northeastern United States: Effects on fish populations. Ecological Applications 6(2):422–437. Battarbee, R.W., D.F. Charles, S.S. Dixit, and I. Renberg. 1999. Diatoms as indicators of surface water acidity. Pp. 85–127, In E.F. Stoermer and J.P. Smol (Eds.). The Diatoms: Applications for the Environmental and Earth Sciences. Cambridge University Press, Cambridge, UK. 482 pp. Biggs, B.J.F. 1987. Effects of sample storage and mechanical blending on the quantitative- analysis of river periphyton. Freshwater Biology 18(2):197–203. Brock, T.D. 1973. Lower pH limit for existence of blue-green-algae: Evolutionary and ecological implications. Science 179:480–483. Carrick, H.J., and A.D. Steinman. 2001. Variation in periphyton biomass and species composition in Lake Okeechobee, Florida (USA): Distribution of algal guilds along environmental gradients. Archiv Für Hydrobiologie 152(3):411–438. 2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 203 Carrick, H.J., F.J. Aldridge, and C.L. Schelske. 1993. Wind influences phytoplankton biomass and composition in a shallow, productive lake. Limnology and Oceanography 38(6):1179–1192. Cattaneo, A., and M.C. Amireault. 1992. How artificial are artificial substrata for periphyton? Journal of the North American Benthological Society 11(2):244–256. Clesceri, L.S., A.E. Greenberg, and A.D. Eaton (Eds). 1998. Standard Methods for the Examination of Water and Wastewater, 20th Edition. United Book Press, Inc., Baltimore, MD. DeNicola, D.M. 2000. A review of diatoms found in highly acidic environments. Hydrobiologia 433(1–3):111–122. DeWalle, D.R., B.R. Swistock, and W.E. Sharpe. 1995. Episodic flow-duration analysis: A method of assessing toxic exposure of Brook Trout (Salvelinus fontinalis) to episodic increases in aluminum. Canadian Journal of Fisheries and Aquatic Sciences 52(4):816–827. Dillard, G.E. 1966. The seasonal periodicity of Batrachospermum macrosporum Mont and Audouinella violacea (Kuetz.) Ham in Turkey Creek, More County, North Carolina. Journal of Elisha Mitchell Science Society 82:204–207. Dozier, B.J., and P.J. Richerson. 1975. An improved membrane filter method for the enumeration of phytoplankton. Verhandlungen der International Vereinigung fur Theoretische and Angewandte Limnologie 19:1524–1529. Gagen, C.J. 1991. Direct effects of acidic runoff episodes on the distribution and abundance of fishes in streams of the Northern Appalachian Plateau. Ph.D Dissertation. Pennsylvania State University, University Park, PA. 133 pp. Gensemer, R.W., and R.C. Playle. 1999. The bioavailability and toxicity of aluminum in aquatic environments. Critical Reviews in Environmental Science and Technology 29(4):315–450. Genter, R.B. 1995. Benthic algal populations respond to aluminum, acid, and aluminum- acid mixtures in artificial streams. Hydrobiologia 306(1):7–19. Herrmann, J., E. Degerman, A. Gerhardt, C. Johansson, P.E. Lingdell, and I.P. Muniz. 1993. Acid-stress effects on stream biology. Ambio 22(5):298–307. Hirst, H., F. Chaud, C. Delabie, I. Juttner, and S.J. Ormerod. 2004. Assessing the short-term response of stream diatoms to acidity using inter-basin transplantations and chemical diffusing substrates. Freshwater Biology 49(8):1072–1088. John, D.M., B.A. Whitton, and A.J. Brook (Eds). 2002. The Freshwater Algal Flora of the British Isles: An Identification Guide to the Freshwater and Terrestrial Algae, Cambridge University Press, Cambridge, UK. Kaeser, A.J., and W.E. Sharpe. 2001. The influence of acidic runoff episodes on Slimy Sculpin reproduction in Stone Run. Transactions of the American Fisheries Society 130(6):1106–1115. Kahl, J.S., J.L. Stoddard, R. Haeuber, S.G. Paulsen, R. Birnbaum, F.A. Deviney, J.R. Webb, D.R. Dewalle, W. Sharpe, C.T. Driscoll, A.T. Herlihy, J.H. Kellogg, P.S. Murdoch, K. Roy, K.E. Webster, and N.S. Urquhart. 2004. Have US surface waters responded to the 1990 Clean Air Act Amendments? Environmental Science and Technology 38(24):484A–490A. Keener, A.L. 2003. The effects of doubling limestone sand applications in two southwestern Pennsylvania streams. M.Sc. Thesis. The Pennsylvania State University, University Park, PA. 33 pp. Kimmel, W.G. 1999. The effects of acidic deposition on aquatic ecosystems in Pennsylvania. Pp. 17–12, In W.E. Sharpe and J.R. Drohan (Eds.). Proceedings of the 1998 PA Acidic Deposition Conference. Environmental Resources Research Institute, University Park, PA. 266 pp. 204 Northeastern Naturalist Vol. 15, No. 2 Kimmel, W.G., D.J. Murphey, W.E. Sharpe, and D.R. DeWalle. 1985. Macroinvertebrate community structure and detritus processing rates in two southwestern Pennsylvania streams acidified by atmospheric deposition. Hydrobiologia 124(2):97–102. Kinross, J.H., P.A. Read, and N. Christofi. 2000. The influence of pH and aluminium on the growth of filamentous algae in artificial streams. Archiv Für Hydrobiologie 149(1):67–86. Komrek, J., and K. Anagnostidis. 2005. Cyanoprokaryota: Oscillatoriales. Swasserflora von Mitteleuropa. Vol 2:3. Spektrum, Italy. 800 pp. Krammer, K., and H. Lange-Bertalot. 1986. Bacillariophyceae 1. Teil: Naviculaceae. Susswasserflora von Mitteleuropa. 1:4. Gustav Fisher, Verlag, Jena, Germany. 876 pp. Krammer, K., and H. Lange-Bertalot. 1988. Bacillariophyceae 2. Teil: Epithemiaceae, Bacillariaceae, Surirellaceae. Susswasserflora von Mitteleuropa. 2:4. Gustav Fisher von Mitteleuropa, Verlang, Jena, Germany. 610 pp. Krammer, K., and H. Lange-Bertalot. 1991a. Bacillariophyceae 3. Teil: Centrales, Fragilariaceae, Eunotiaceae, Achnanthaceae. Susswasserflora von Mitteleuropa. 3:4. Gustav Fisher von Mitteleuropa, Verlang, Jena, Germany. 598 pp. Krammer, K., and H. Lange-Bertalot. 1991b. Bacillariophyceae 4. Teil: Achnanthaceae, Kritische Erganzungen zu Navicula (Lineolatae) und Gomphonema. Susswasserflora von Mitteleuropa. 4:4. Gustav Fisher von Mitteleuropa, Verlang, Jena, Germany. 437 pp. Ledger, M.E., and A.G. Hildrew. 2005. The ecology of acidification and recovery: Changes in herbivore-algal food web linkages across a stream pH gradient. Environmental Pollution 137(1):103–118. LeFevre, S.R., and W.E. Sharpe. 2002. Acid stream water remediation using limestone sand on Bear Run in southwestern Pennsylvania. Restoration Ecology 10(2):223–236. Lowe, R.L., and Y. Pan. 1996. Benthic algal communities as biological monitors. Pp. 705–739, In R.J. Stevenson, M.L. Bothwell, and R.L. Lowe. (Eds.). Algal Ecology: Freshwater Benthic Ecosystems. Academic Press, NY. Maurice, C.G., R.L. Lowe, T.M. Burton, and R.M. Stanford. 1987. Biomass and compositional changes in the periphytic community of an artificial stream in response to lowered pH. Water Air and Soil Pollution 33(1–2):165–177. Mulholland, P.J., J.W. Elwood, A.V. Palumbo, and R.J. Stevenson. 1986. Effect of Stream Acidification on Periphyton Composition, Chlorophyll, and Productivity. Canadian Journal of Fisheries and Aquatic Sciences 43(10):1846–1858. Pagano, T.A. 1990. Evaluation of a method to fractionate aluminum in surface waters and interpretation of the speciation of aluminum in streams during episodes of acidic runoff. M.Sc. Thesis. Pennsylvania State University, University Park, PA. 122 pp. Parent, L., M. Allard, D. Planas, and G. Moreau. 1986. The effects of short-term and continuous experimental acidification on biomass and productivity of running water periphytic algae. Pp. 28–41, In B.G. Isom, S.D. Dennis, and J.M. Bates (Eds.). Impact of Acid Rain and Deposition on Aquatic Biological Systems. Vol. 928. American Society for Testing and Materials, Philadelphia, PA. Passy, S.I. 2006. Diatom community dynamics in streams of chronic and episodic acidification: The roles of environment and time. Journal of Phycology 42(2):312–323. Passy, S.I., I. Ciugulea, and G.B. Lawrence. 2006. Diatom diversity in chronically versus episodically acidified Adirondack streams. International Review of Hydrobiology 91(6):594–608. 2008 S.E. MacDougall, H.J. Carrick, and D.R. DeWalle 205 Patrick, R., and C.R. Reimer. 1966. Diatoms of the United States: Exclusive of Alaska and Hawaii. 1:2. The Academy of Natural Science, Philadelphia, PA. 671 pp. Planas, D. 1996. Acidification effects. Pp. 497–530, In R.J. Stevenson, M.L. Bothwell, and R.L. Lowe (Eds.). Algal Ecology: Freshwater Benthic Ecosystems. Academic Press, NY. 753 pp. Prescott, G.W. 1962. Algae of the Western Great Lakes Area. W.M.C. Brown Company Publishers, Dubuque, IA. 977 pp. Prescott, G.W., H.T. Croasdale, and W.C. Vinyard. 1975. A Synopsis of North American Desmids: Part II. Desmidiaceae: Placodermae, Section 1. University of Nebraska Press, Lincoln, NE. Schindler, D.W. 1988. Effects of acid-rain on fresh-water ecosystems. Science 239(4836):149–157. Shannon, C.E. 1948. A mathematical theory of communication. Bell Systems Technical Journal 27:379–423. Sheath, R.G., and J.M. Burkholder. 1985. Characteristics of softwater streams in Rhode Island. II. Composition and seasonal dynamics of macroalgal communities. Hydrobiologia 128(2):109–118. Stevenson, R.J., M.L. Bothwell, and R.L. Lowe. 1996. Benthic algae communities as biological monitors. Pp. 705–733, In R.J. Stevenson, M.L. Bothwell, and R.L. Lowe (Eds.). Algal Ecology: Freshwater Benthic Ecosystems. Academic Press, NY. 753 pp. Sweeney, J.S., W.E. Sharpe, and D.R. DeWalle. 1999. Base cation export from Appalachian Plateau LTM watersheds in Pennsylvania. Pp. 45–61, In W.E. Sharpe, and J.R. Drohan (Eds.). Proceedings of the 1998 PA Acidic Deposition Conference. Vol. 1. Environmental Resources Research Institute, University Park, PA. 266 pp. Van Dam, H., A. Mertens, and J. Sinkeldam. 1994. A coded checklist and ecological indicator values of freshwater diatoms from the Netherlands. Netherlands Journal of Aquatic Ecology 28:117–133. VanDerWerff, A. 1955. A new method of concentrating and cleaning diatoms and other organisms. Verhandlungen der International Vereinigung für Theoretische and Angewandte Limnologie 12:276–277. VanSickle, J., J.P. Baker, H.A. Simonin, B.P. Baldigo, W.A. Kretser, and W.E. Sharpe. 1996. Episodic acidification of small streams in the northeastern United States: Fish mortality in field bioassays. Ecological Applications 6(2):408–421. Verb, R.G., and M.L. Vis. 2005. Periphyton assemblages as bioindicators of minedrainage in unglaciated western Allegheny Plateau lotic systems. Water, Air, and Soil Pollution 161:227–265. Wehr, J.D., and R.G. Sheath. 2003. Freshwater Algae of North America: Ecology and Classification. Academic Press, New York, NY. 939 pp. Wetzel, R.G., and G.E. Likens. 2000. Limnological Analyses. 3rd Edition. Springer, New York, NY. 429 pp. Whitford, L.A., and G.L. Schumacher. 1973. A Manual of the Fresh-water Algae. Sparks Press, Raleigh, NC. 337 pp. Wigington, P.J., D.R. DeWalle, P.S. Murdoch, W.A. Kretser, H.A. Simonin, J. VanSickle, and J.P. Baker. 1996a. Episodic acidification of small streams in the northeastern United States: Ionic controls of episodes. Ecological Applications 6(2):389–407. Wigington, P.J., J.P. Baker, D.R. DeWalle, W.A. Kretser, P.S. Murdoch, H.A. Simonin, J. VanSickle, M.K. McDowell, D.V. Peck, and W.R. Barchet. 1996b. Episodic acidification of small streams in the northeastern United States: Episodic Response Project. Ecological Applications 6(2):374–388. 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.