patterns of epipelic algal distribution in an acidic adirondack lake

146 LEE W. WILCOX AND GARY L. FLOYD

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O'Kelly, C. J. & Floyd, G . F. 1984. Flagellar apparatus absolute orientations and the phylogeny of the green algae. BioSystems

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J. South Afr. Bot. 26:167-319.

J. Phycol. 24, 146-152 (1988)

PATTERNS OF EPIPELIC ALGAL DISTRIBUTION IN AN ACIDIC ADIRONDACK LAKE'

Deborah A. Roberts and Charles W. Boylen2 Department of Biology and Fresh Water Institute, Rensselaer Polytechnic Institute, Troy, New York 12181

ABSTRACT

The biovolume and species composition of epipelic algae along sediment depth gradients were sampled seasonally in a n acidic oligotrophzc lake in the Adzrondack Park in ;Vew York State. The epipelic algal community of Woods Lake (Herkimer Co., NU) was dominated by diatoms and cjanobacteria. Distinct depth zonation patterns of com- munitj composition were evident. Total algal biouolume increased with depth due to a dense cjanobacterial mat on the sediments in deeper water (5-8 m). This mat was dominated by a single species of cyanobacteria, Hapalo- siphon pumilus (Kutz.) Kirchner, whzch accounted f o r the late summer maximum in total biovolume at 7 m. The shallower (1-4 m) epipelic communities were dominated bj diatoms, which showed a spring maximum in total biouolume and were dominated by Fragilaria acidobiontica Charles, Navicula tenuicephala Hust. and N. subtilissima Cl.

Kej index words: acidGcation; Bacillariophyeae; cyanobacteria; Cyanophjceae; depth; epipelic algae; Hap- alosiphon; zonatzon

Recent studies suggesting that benthic algal communities are favored by conditions in acidified lakes have raised questions about the extent of this phe- nomenon, types of communities affected, and factors controlling the distribution of benthic algae in soft water lakes in general. Although some studies have investigated the development of benthic algae in shallow littoral zones of acidic lakes (Lazarek 1980, Stokes 1981, Turner et al. 1987), few have specifi- cally addressed habitats at greater depths, and the habitat of the epipelon has been neglected.

Early reports (Hultberg and Grahn 1976, Hen- drey and Vertucci 1980, Lazarek 1980,1982, Stokes 198 1) indicated that extensive mats of cyanobacteria

' Accepted: 4 Januar j 1988. * Address for reprint requests.

develop in clear-water acidic lakes; however, sub- sequent research has not substantiated these obser- vations. A survey of 20 lakes in New Hampshire by Stevenson et al. (1 985) found that the distribution of cyanobacteria was negatively correlated with pH, but few examples of extensive mat formation were observed. Recent work by Turner et al. (1987), showed no evidence that populations of cyanobacteria were favored by acidification.

Previous studies have not investigated the seasonal distribution of epipelic algal communities in clear- water acidic lakes or artificially acidified lakes. The purpose of the present study was to characterize the community composition of the epipelic algae of a small acidic Adirondack lake and to delineate patterns in species composition and biovolume of the living component of naturally occurring benthic algae related to t w o environmental gradients. The first gradient was depth and the second was the com- plex of factors associated with seasonal variation.

MATERIALS AND METHODS

S tud j szte. Woods Lake (43"52' N, 7 l"58' W) is 25 ha in surface area and has a maximum depth of 1 1.5 m (Fig. 1). This lake has been studied extensively as representative of small, chronically acidic lakes in the Adirondack region of New York State (Tetra Tech 1983, Davis et al. 1986, Driscoll et al. 1986). Beginning in 1984, a multidisciplinary study, the Lake Acidification Mitigation Project (LAMP), was initiated at Woods Lake. A detailed char- acterization of the variation in water chemistry of Woods Lake prior to base addition has been published (Driscoll et al. 1986).

Collection and processing of algal samples. Samples were collected monthly along transect A (Fig. 1) from May to August and in October, 1984. Three depths along transect A were sampled (1, 4, and 7 m). On the sampling dates for the benthic algae, light profiles were measured at 1-m intervals using a LlCOR Lambda (LI 185) submersible photometer. At each depth, nine replicate intact cores (35 mm diam) were collected by hand using SCUBA. In the boat, the bottom stopper was pushed up into the core tube to collect the upper layer of sediment. The top 1.5 cm of each of three cores was pooled to form three replicate pooled samples from each depth (with the exception of May 1984, when nine unpooled cores were collected at each depth). During September

EPIPELIC ALGAL COMMUNITIES 147

WOODS L A K E

.2 .3 .4 .5

KILOMETERS

Depth contours in f e e t

FIG. 1. Bathymetric map of Woods Lake, Herkimer Co., NY. The location of the water chemistry sampling site (W) and the three transects sampled for benthic algae (A, B and C) are shown.

1984, transects A, B and C were sampled at l-m depth intervals from 1-8 m. At each depth one intact core was collected and the top 1.5 cm removed.

The sediment/algal slurry was brought up to 100-200 mL total volume with distilled water and preserved in a solution of 1.5% glutaraldehyde, 1.5% formaldehyde, and 0.05 M sodium caco- dylate buffer. A 0.2-1.0 mL subsample of this suspension was mounted in commercial corn syrup on a glass slide (Stevenson 1984). One slide, containing an appropriate cell density, was counted from each sample at 1000 x magnification using a Leitz Ortholux microscope. Only cells with intact protoplasm were counted. An average of two transects per slide were counted, until a total of approximately 300 organisms were found. I f more than 50% of the counts belonged to one taxon, then counting was continued until at least 100 individuals of all other taxa combined were found. For filamentous cyanobacteria each 10 pm segment was counted as one cell.

Raw counts were converted to biovolume data by calculating a cell volume for each form identified. These cell volumes were calculated from measurements of 25 or more cells (except in the case of extremely rare forms) and formulae for the volume of the shape that best matched the shape of the cell. These included cylinder, sphere, ellipsoid, three-dimensional end-to-end trian- gles or end-to-end cones (Stevenson et al. 1985).

All statistical analyses were conducted with the statistical package SPSSx (SPSS 1983, Norusis 1985) and a simple Fortran pro-

W 2 3 0 rn 8

100

80

60

40

20

Diatoms Cyanobacteria

FIG. 2. Percent of total epipelic algal biovolume represented by diatoms and cyanobacteria at three depths in Woods Lake. Mean values for May-October 1984. Vertical bars represent f 1 SE (n = 5).

gram to help sort data on an IBM (308 1 D) computer at Rensselaer Polytechnic Institute, or with a multivariate statistical package (M.V.S.P.) for the IBM-PC (Kovach 1986). Analysis of variance (ANOVA) and Tukey, pairwise comparisons of log-transformed biovolume data were used to identify differences in community abundance and total biovolume for various taxa between depths and sampling dates. Evenness and Shannon diversity (Pielou 1977) were calculated for the average log abundance data from the monthly samples collected along transect A.

RESULTS

Phjsical f chemical measurements. Temperature and oxygen profiles showed that Woods Lake was weakly stratified from July through September, with the thermocline at a depth of approximately 10 m. Av- erage summer light penetration (June-October) at 1 , 4 and 7 m depths were 65, 33 and 16% of surface intensity, respectively. Since the three main sampling depths were above the thermocline throughout 1984, pH and acid neutralizing capacity (ANC) of the water did not vary with depth (Driscoll et al. 1986). The surface water pH ranged from 4.8-5.2 from May to October. The ANC of the epilimnetic water averaged -3 peq.L-' and increased from -8 to +4 peq.L-' over the summer season. Total A1 concentration decreased from 9.8 pmo1.L-' to 3.1 pmol. L-'. The average water column DOC concentration was 158 pmo1.L-' (Driscoll et al. 1986).

Seasonal distribution of algal communities. Average algal biovolume from the surface sediments of Woods Lake ranged from 5.4 x lo5 to 6.7 x lo7 prn3.mm-* during the sampling period. Throughout this period, cyanobacteria (Cyanophyceae) and diatoms (Ba- cillariophyceae) dominated the epipelon (Fig. 2).

DEBORAH A. ROBERTS AND CHARLES W. BOYLEN 148

N E f 107

s 3 g 106

0 m

W' I

.

May Jun Jul Aug Sep act

DATE

FIG. 3 . Variation in total epipelic algal biovolume at 1, 4 and 7 m depths in Woods Lake, May-October 1984. Vertical bars represent f l SE (n = 3).

Chlorophyceae contributed an average of less than 5% of the biovolume throughout the study in spite of the observation that dense communities of Mou- geotia spp. were noted along the shoreline in shallow water.

Diatoms made up 50-70% of total biovolume on sediments at the 1 and 4 m depths whereas they were < 15% of the biovolume at the 7 m sample site on transect A (Fig. 2). Cyanobacteria were dominant at 7 m (90%) and were less abundant at the shallower sites (<40% at 1 and 4 m).

Variation in algal biovolume was significant between sampling dates for several taxonomic groups including total cyanobacteria, total diatoms and Na- vicula tenuicephala Hust. (= Stauroneis gracillima Hust.), as well as in the sum of the biovolume of all algal species. The seasonal variation in total algal biovolume at each depth for 1984 is shown in Figure 3. Biovolume varied significantly with depth, date and an interaction between date and depth (P < 0.05, ANOVA). Total biovolume at 7 m increased (P < 0.05) between May and June and decreased (P < 0.05, Tukey allowances) between August and October. Total biovolume at 1 m decreased (P < 0.05) between May and July and decreased (P < 0.05) at 4 m between early summer (May and July) and fall.

The biovolume of cyanobacteria at the 1 m depth was highest in May but reached a maximum in late summer at both 4 and 7 m depths, where significant (P < 0.05) declines occurred between August and October similar to the pattern seen in total algal biovolume described above. A single species of cyanobacteria, Hapalosiphon pumilus (Kutz.) Kirchner, contributed more than 80% of the biovolume at 7 m throughout the study. The other two major groups of cyanobacteria, the Oscillatoriaceae and Anacystis spp. showed no distinct seasonal patterns.

The seasonal effect on the total biovolume of diatoms was significant (P < 0.05) only at the l and 4 m depths. Epipelic diatom standing crop was highest

B w 0

Jun Jul Sep

Other

Melosira spp.

Eunotia SDP.

Pinnularia s p p .

Fragilaria acidobiontica

Navicula subtilisSIma

Navicula tenuicephala

FIG. 4. Community composition of the diatoms at A) 1 m, B) 4 m, and C ) 7 m in Woods Lake, May-October 1984. Percent of total diatom abundance of each taxon is plotted for each sampling date.

in early summer at both 1 and 4 m depths. Diatoms controlled the seasonal pattern of total algal biovolume at these depths.

The variation in relative abundance of dominant diatom species with season at each site is illustrated


TABLE 1 . nities along transect A, May-October 1984.

Date Depth Diversitv Evenness No. of species

Diversity and ewnnPss indices for epipplic sample commu-

May 1 2.414 0.487 31 4 2.285 0.453 33 7 0.997 0.203 30 1 2.369 0.524 23 4 1.928 0.410 26

J U "

7 2.169 0.438 31 1 3.496 0.713 30 4 2.316 0.477 29

Ju l

7 0.893 0.207 20 A% 1 3.167 0.710 22

4 1.455 0.310 26 7 0.658 0.150 21

Oct 1 2.559 0.551 25 4 2.408 0.540 22 7 2.269 0.458 31

in Figure 4. The most abundant diatom species at the shallower sites was N. tenuicephala. Fragilaria acidobiontica Charles had high relative abundance values at all depths in May 1984 and decreased slightly over the growing season. Melosira spp. increased in relative abundance over the summer season at all of the depths sampled. Neither Eunotia spp. nor Pin- nularia spp. showed any significant variation in biovolume with sampling date.

Depth distribution of algal communities. Except in May, total algal biovolume was significantly lower (P < 0.05, Tukey allowances) at 1 and 4 m than at 7 m (Fig. 3). Although average biovolume was highest at 7 m throughout 1984, the diversity of this community was consistently lower than at the other two depths. The numerical dominance (80% average relative abundance) of a single species (H. pumilus) caused low values of diversity and evenness of the 7 m community on most dates (Table 1).

Total cyanobacterial biovolume was significantly (P < 0.05) higher at 7 m than at 1 or 4 m. This was due to the abundance of H. pumilus at 7 m. The Oscillatoriaceae showed a different depth distribution with higher abundances at 4 m than at the other depths. Anacystis spp. showed no distinct pattern with depth; the 4 m biovolumes were slightly but not significantly higher than at 1 or 7 m (P > 0.05).

The depth distributions of the most common taxa averaged over all three transects (A, B and C) for September (Fig. 5 ) showed similar results to the data from the averages of monthly samples along transect A. Cyanobacteria contributed most of the total abundance with the branched filamentous form, H. pumilus, dominant below 5 m. Oscillatoriaceae were most abundant at the 3-5 m depth zone whereas the most common coccoid forms of the genus An- acjstis showed two abundance peaks at 4 and 7 m.

Diatom biovolume, averaged over all dates for transect A, was higher at 4 m than at 1 or 7 m depths (P < 0.05, Tukey allowances). Depth distribution of

H

Abundance, 10000/mm2

H

Abundance, S00/mm2

Depth distribution of selected taxa in Woods Lake, September 1984. Average abundance for the three transects are plotted (n = 3).

FIG. 5.

total diatom abundance averaged over the three transects from September shows a similar distribution with depth (Fig. 5). Individual species distributions of the more abundant species are also presented in Figure 5 . Eunotia spp. were more abundant at deeper sites, peaking at 7 m. Although slightly higher abundances of Eunotia spp. were observed in shallower depths in May and June on transect A, biovolume of Eunotia spp. was significantly higher (P < 0.05) at 7 m than at other depths. F. acidobiontica Charles and N. tenuicephala showed peak abundances at 4 m depths in September (Fig. 5), as well as having significantly higher abundance at 4 m than at 1 or 7 m from May to October on transect A (ANOVA, P < 0.05). Xavicula subtilissiina C1. was also a relatively abundant species with lower numbers at 4 m and peaks at 2-3 m and 6 m depths during September 1984.

DISCUSSION

The epipelic algal community of Woods Lake was dominated by cyanobacteria and diatoms throughout the growing season in 1984. Filamentous green algae were a minor component of the epipelic algal samples throughout the study. Similar to many other acidic clear-water lakes on the Precambrian Shield in North America (Stokes 198 1, Singer et al. 1983, Turner et al. 1987), and lakes in sensitive regions of Scandinavia (Hultberg and Grahn 1976, Lazarek

150 DEBORAH A. ROBERTS AND CHARLES W. BOYLEN

1982), dense “clouds” of filamentous green algae, primarily Mougeotia spp., were observed to develop around the shoreline and particularly on the stems of macrophytes in the shallow littoral zone in early summer. The sampling strategy of this study was not designed to quantify this community since the sediment cores, even at the shallowest depth (1 m), were collected approximately 20 m from shore and in an area of little macrophyte cover. Accordingly, fila- ments of Mougeotia spp. were rarely encountered in the samples.

Shallow and deeper zones were clearly distin- guishable on the basis of community composition structure and abundance. The depth distribution of communities was similar on all transects sampled around the lake. From a depth of 6 to 8 m the bottom is covered with a nearly continuous mat of H. pumilus. H. pumilus forms globular colonies up to 2 cm in diameter producing a patchy, fuzzy carpet. This species was also found by Conway et al. (1 980) to be abundant in Woods Lake in 1979. Extensive development of Hapalosiphon mats was also observed in three other clear-water lakes with summer pH values less than 5.0 in the Adirondack mountain region of New York, Herkimer Co. (Silver Lake, Clear Lake, and Little Rock Pond) (Roberts, unpubl.). Lazarek (1982) reported a mat of H.fontinalis in an acidic lake in Sweden. Stevenson et al. (1985) reported that one of the acidic lakes in New Hamp- shire had a significant population of Hapalosiphon in shallow water. H. pumilus is generally characterized as an acid bog species, common in the shallow mar- gins of swamps (Prescott 1962). It was recorded from five sites in bogs in Ontario by Flensburg and Spar- ling (1973). Apparently the growth of this species is favored by acidic conditions, either in naturally acidic dystrophic water or in clear-water, acidified lakes.

Variation in total biomass with depth can in part be explained by species zonation. Total biomass increased with depth, primarily due to the dense mat of H. pumilus at 7 m. The 4 m depth had a higher total biovolume than 1 m, mostly due to the higher biovolume at 4 m during August (Fig. 3). Several other studies have also indicated that the shallowest depths do not always have the highest standing crops of benthic algae (Gruendling 1971, Round 1981, Stevenson and Stoermer 198 1). A number of factors may vary along a depth gradient which influences total biomass distribution as well as community composition. Light is one of the most obvious factors that varies with depth. In addition to the reduction of light intensity with depth, the spectral quality of light also will have been altered as it passes through water. Other factors such as vertical temperature differences, chemical gradients, sediment quality, grazing pressure and wave action may play a role in some lakes (Stockner and Armstrong 197 1, Round 198 1, Stevenson and Stoermer 198 l), although di- rect experimental evidence is rare. Of these factors, it is likely that light and/or sediment quality played

a major role in the depth distribution of biomass in Woods Lake. Since all samples were taken above the thermocline, temperature and water chemistry were not major factors in vertical distribution differences over the course of the study. Wave action can disrupt surface sediment communities in shallow water (Gruendling 197 1, Stevenson and Stoermer 198 1); in Woods Lake, however, the shallowest site (1 m) was 20 m from the western shore. The prevailing wind direction and short fetch makes the probability of wave disturbance minimal.

The lower light level at 7 m may have enhanced the competitive ability of H. pumilus over other algal taxa. Stevenson et al. (1985) also found the distribution of this species to be patchy with a narrower niche breadth than other cyanobacteria. In Woods Lake H. pumilus seems to be limited to lower light conditions.

Oscillatoriaceae were not found in deep zones. The mid-depth (4-6 m) distribution of the Oscilla- toriaceae in Woods Lake was similar to that reported by Round (198 1) in Lake Itasca, Minnesota. Steven- son et al. (1985) found this group to have a weak negative correlation with depth in 20 low alkalinity lakes in New Hampshire. Dense mats of Oscillato- riaceae have been reported from shallow water (< 2 m) in several other studies (Lazarek 1982, Turner et al. 1987, Roberts, unpubl. data) indicating that this is normally a high light, shallow water flora.

The vertical distribution patterns of epipelic diatoms have been better studied than those of other algal taxa. Some of these data are from the Great Lakes (Stevenson and Stoermer 1981, Kingston et al. 1983) and describe a different flora, common in more alkaline conditions and sandy substrates. These studies as well as the work of Stockner and Arm- strong (1971) and Stevenson et al. (1985), indicate that a clear vertical zonation of species is common among diatoms.

The majority of abundant diatom species in Woods Lake had peak distributions in shallow water (1 or 4 m) including Navicula tenuicephala, N . subtilissima, and Fragilaria acidobiontica. Navicula spp., in both the present study and that of Stevenson et al. (1985), primarily inhabited shallow littoral sediments. N. subtilissima has been described from other shallow acidic systems (Flensburg and Sparling 1973). In September, this species had its lowest abundance at the depth which the two most common species (N. tenuicephala and F. acidobiontica) had peak distributions (Fig. 5 ) which could indicate competition or simply a difference in depth distribution patterns based on different habitat variables. The distribution of the total biovolume of diatoms was controlled by the relatively large biovolume of F. acidobiontica and the high abundances of N. tenuicephala.

Eunotia spp. was the only diatom taxon to have a significantly higher abundance with depth. Cells were commonly seen attached to Hapalosiphon sp. fila- ments from the 7 m samples. This is an interesting


assemblage, since similar to H. puinilus (Prescott 1962), Eunotia spp. (Patrick and Reimer 1966) are found commonly in dystrophic (humic) water, and both inhabited a discrete depth zone in Woods Lake.

The distinct depth zonation in community composition, particularly between 7 m and the shallower depths, can account for the different seasonal patterns in total algal abundance observed at these depths. Total algal biomass on the sediments showed a spring maximum at 1 and 4 m depths, which is typically observed when a community is dominated by diatoms (Godward 1937, Round 1953, 1960, 1972, Jorgenson 1957, Castenholz 1960, Greund- ling 197 1, Stockner and Armstrong 197 1, Hickman 1978, DeNicola 1986). In contrast to the studies cited above, mainly from temperate, neutral or alkaline lakes, no fall maximum in total biovolume or diatom biovolume was observed in Woods Lake. No sudden appearance or disappearance of common species between dates was observed, which indicates that the seasonal patterns were adequately characterized by monthly samples.

The overall seasonal pattern of diatom biomass was a function of the combined independent variation in only a few of the most abundant species. Analogous to depth distribution patterns, the size of F. acidobiontica and numbers of X . tenuicephala controlled the general seasonal pattern at 1 and 4 m. Most species showed synchronous peaks in biovolume at both 1 and 4 m, although the magnitude of the peaks varied. It is unlikely that a single factor is responsible for the observed seasonal periodicity. Changes in temperature were cited as a major factor in controlling seasonal succession (Round 198 1, Sea- burg and Parker 1983); however, non-optimal light conditions, changes in day length and seasonal fluc- tuation in nutrient/chemistry conditions may also play a role. The peak in F. acidobiontica and N. tenuicephala occurred during the period of major epilimnetic temperature increase (< 10" C to 20" c). F. acidobiontica had an earlier peak which also corre- sponded with the spring maximum in total aluminum concentration in the water. Although separat- ing the effects of temperature, grazing and chemical changes is not possible, the occurrence of the max- imal growth period of F. acidobiontica during the early spring supports the findings of Charles (1 986) that this species successfully competes for resources during periods of high aluminum concentration.

Although total biovolume in May at the shallow sites was at a maximum, biovolume at 7 m increased more slowly and reached its peak in late summer due to the growth pattern of H. pumilus. A late summer maximum in cyanobacterial growth was also observed for Oscillatoriaceae in a neutral lake in Ontario, Canada (Turner et al. 1987). Oscillatori- aceae made up a portion of the filamentous matrix on the sediments in the shallow depth zones in the lake but did not form thick mats observed in other studies.

The present-day epipelic diatom flora is dominated by two species, N. subtilissima and F. acidobiontica, which in the sedimentary record have only re- cently been part of the lake's flora (Davis et al. 1986). These taxa are abundant in Adirondack lakes with water pH values less than 5.5 and high total aluminum concentrations (Charles 1985, 1986). Other diatom taxa that have been present throughout the lake's history are widely distributed with respect to organic acid content of water. This is evidence that the community composition of diatoms has changed in recent history. It is likely also to be the case for other taxa for which there is no sedimentary record. The composition of the epipelic flora of clear-water acidic lakes may depend not only upon present-day lake chemistry, but also on the pH and chemical history of the system.

The research presented is part of a Ph.D. dissertation submitted to the Biology Department of Rensselaer Polytechnic Institute by D. A. Roberts and was supported in part from a grant from the United Parcel Service. We thank R. Jan Stevenson, R. Singer, M. Osgood, K. Miller and L. Ginsburg for their assistance with this work.

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J . PhycQl. 24, 152-157 (1988)

T H E INTRAMEMBRANOUS PARTICLE PROFILE OF T H E PARAMYLON MEMBRANE DURING PARAMYLON SYNTHESIS IN E U G L E N A (EUGLENOPHYCEAE)'

John Z. Kiss,' Aurea C. Vasconcelos and Richard E. Triemer Department of Biological Sciences and Bureau of Biological Research, Rutgers University

Piscataway, New Jersey 08855-1059

ABSTRACT

Paramylon is the P-I,3-glucan storage product in euglenoid algae. It is afibrous crystal that occurs as membrane-bound granules in the cjtosol. The role of the sur- rounding membrane in paramylon synthesis was investigated by the use of freeze-etch electron microscopy. When Euglena gracilis Klebs strain Z (Pringsheim) cells were frozen in supercooled liquid nitrogen, the fracture plane primarily was through the paramjlon membrane. '4 large intramembranous particle (IMP, mean diam range 5.6-6.5 nm) and a small IMP (mean diam range 9.6-

10.3 nm) were predominant in both PF (protoplasmic fracture) and EF (exoplasmic fracture) faces of the paramjlon membrane. During paramnylon synthesis induc- tion, the ratio of small to large IMPS increased in both fracture faces. The IMP density decreased in both fracture faces concomitant to paramjlon sjnthesis increase. These changes in IMP profile and density suggest that the paramjlon membrane is involved in the synthesis of paramylon.

Key index words: Euglena gracilis; freeze-etch; intramembranous particle; paramylon sjnthesis; supercooled liquid nitrogen

' Accepted: 8 Januarj 1988. * Present address and address for reprint requests: Department

of Botany, Ohio State University, 1735 Neil Avenue, Columbus, Ohio 43210-1293.

One of the distinctive features of euglenoid algae is paramylon, their storage carbohydrate. Paramy- lon is a @- 1,3-glucan that occurs as membrane-bound

patterns of epipelic algal distribution in an acidic adirondack lake

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