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J. Bryol. (1986) 14, 269-280

Ecological factors relating to morphological variation in theaquatic moss Rhynchostegium riparioides (Hedw.) C. Jens.

JOHN D. WEHR and BRIAN A. WHITTON

University of Durham

INTRODUCTION

Many species of aquatic moss are morphologically very variable (Warnstorf et al. ,1914; Watson, 1919; Lodge, 1959) and recent taxonomic works (Smith, 1978, Crum& Anderson, 1981; Ireland, 1982) give examples where confusion may result fromsuch variability. During the course of a study of metal accumulation by field popu-lations of Rhynchostegium riparioides (Wehr & Whitton, 1983a, b) it became evi-dent that this species exhibits a wide range of morphologies in different types oflotic habitat. The present study was planned to run in parallel with that on metalaccumulation because, at least in Northern Europe, R. riparioides is perhaps themost useful of all species for monitoring heavy metal pollution in rivers. The princi-pal objectives were to reveal the extent of morphological variation, to identifythose characters which were stable over a broad range of ecological conditions andto determine which environmental factors might be related to the variability.

R. riparioides is one of the most common aquatic mosses in rivers in Europe(Nyholm, 1954-1969; Smith, 1978; Watson, 1981) and perhaps also some othertemperate regions (Smith, 1978). It tolerates a wide variety of ecological conditions(Watson, 1919; Empain, 1978) and was present in 68.1 % of 1055 one kilometrelengths of river surveyed in England, Scotland and Wales by N. T. H. Holmes(personal communication). The ecology of the species is reviewed in more detail byWehr & Whitton (1983a).

MATERIALS AND METHODS

Materials for the present study were collected at the same time as the samples ofwater and moss taken for chemical analysis. (Wehr & Whitton, 1983a). Details oflocation, environment, sampling and analytical methods appear elsewhere and aregiven here only briefly.

Water and R. riparioides populations were sampled from 105 reaches (each10 m in length) on 71 different streams and rivers in a broad region of northernEngland (Fig. 1); all samples were collected between May and September 1981.The following variables were measured for the water at each site: pH, total alkalinity,Na, K, Mg, Ca, Mn, Fe, Zn, Cd, Ba, Pb, NH4~N, N03- N, reactive P04-P,F, Cl, S04-S, All elements were measured in the filtrable fraction, a 0.2 fJ-mNucleopore filter being used for metals and a No. 2 porosity sintered glass funnelfor anions. Samples of the moss were taken from at least five separate boulders orother microhabitats within a reach, sampling being restricted to submerged plants

269

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270 JOHN D. WEHR AND BRIAN A. WHITION

Fig. 1. Location of sampling sites forRhynchostegium riparioides in northern England.

I Alston Moor 10 sitesII West Allendale 5 sites

III East Allendale 4 sitesIV Derwent Valley 4 sitesV Weardale 17 sites

VI Durham Coalfield 9 sitesVII Teesdale 8 sites

VIII Lower Tees 2 sitesIX Arkengarthdale 12 sitesX Swaledale 13 sites

XI Holme Valley 3 sitesXII Mersey Catchment 8 sites

XIII Ribble Catchment 3 sitesXIV Lake Disrict 7 sites

in areas with the greatest current velocity. Herbarium specimens for the presentstudy were amalgamated from the entire collection of each reach.

Gametophyte characters were used exclusively for analysis, since sporophytesare usually present only during late autumn and winter (Wehr & Whitton, 1983b).Furthermore, preliminary observations had shown that many characteristics of thesporophyte (colour, length, texture of seta, capsule and operculum shape) wereessentially uniform. Fifteen gametophyte characters were scored for each of the 105populations (Table 1). The variables chosen were all essentially continuous, butbecause of difficulties in quantification, some (e.g. robustness) were ranked andallocated to a particular category subjectively. Quantifiable variables weremeasured on a minimum of 20 leaves or whole plants (depending on the character)and means calculated where needed.

Data were compiled on computer (Northumbrian Universities Multiple AccessComputer (IBM 360/370) running under the Michigan Terminal System) and solu-tions calculated using the MIDAS statistical package (Fox & Guire, 1976). Wherenecessary, data were normalized using the most appropriate transformations priorto statistical treatment (Statistical Research Laboratory, 1976; Wehr & Whitton,

Table 1. Characters used in analysing variation among popula-tions of Rhynchostegium riparioides (details of character states

are given in Table 2)

Plants

Maximum plant lengthMaximum leafy axis lengthMaximum unbranched lengthMean weight of 2-cm apical tipRelative 'robustness'Plant colourProportion of plant leafyBranching pattern

Leaves

Mean lengthMean mid-leaf widthWidth/length ratioLeaf shapeMargin denticulationNerve length (proportion)Angular cell shape

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VARIATION IN RHYNCHOSTEGIUM RIPARIOIDES 271

1983a). Populations were classified using a cluster analysis to determine whetherdistinct morphologies exist using the Clustan II package (Wishart, 1978). Severalalgorithms were tested. Most gave broadly similar solutions, but the one whichgave the most meaningful and distinct clusters was based on Ward's method withstandardized measurements in Euclidean distance. Standardized scores enabledboth ranked and continuous data to be used in this solution. In order to examinethe relationship between ecological· characters and morphological variation, con-tinuous characters of the moss populations were correlated against environmentalvariables measured in the streams from which they were collected.

RESULTS AND DISCUSSION

Variation in plantsThe maximum length of a shoot from an individual population (Fig. 2) ranged from5.0 to 22.0 cm, the upper values exceeding the maxima quoted in various floras (lO-IS cm: Szafran, 1963; Lawton, 1971; Smith, 1978; Watson, 1981). However, veryfew shoots were collected which exceeded the standard deviation of the mean(10.3 ± 2.5 cm). This limited variation also characterized the lengths of the leafyportion of the plant, although more than half of the populations were devoid ofleaves in one half or more of their length (Table 2). Side branches were producedwithin the first 5 cm of the apex in more than 75% of all populations.

In contrast, 'robustness', a subjective ranking which includes the rigidity of thestem and how strongly the leaves diverge, varied markedly. Some plants were rigidwhen removed from the water, while others were flaccid. Such variation perhapscauses much of the confusion mentioned by authors. This robustness index can berelated to plant weight, which is quantified more exactly. The mean dry weight of a2-cm apical fraction of the moss ranged from 0.55 to 3.26 mg, a factor of nearly six.

Variation in leavesAverage leaf length (Fig. 2) varied by a factor of less than two, but the distributionof this measure was strongly skewed above the mean. Most floras which includeleaf dimensions (Lawton, 1971; Iwatsuki & Mizutani, 1972; Gangulee, 1978; Crum& Anderson, 1981; Watson, 1981; Ireland, 1982) indicate a leaf range (1.5-2.5 mm)the maximum of which is actually less than the mode found here. Measurements ofwidth and the width/length ratio were more normally distributed and were withinthe ranges given in all floras examined. One character, the shape of angular cells,was consistently rectangular (never inflated) in all populations.

Leaf shape was typically ovate (often broadly so), but a few populations had dis-tinctly lanceolate leaves (Table 2). This was usually found in the less robust,smaller-leaved plants. In 77% of the populations the leaves were denticulate almostto the base and nearly all had nerves which extended up more than three-fourths ofthe length of the leaf. There were exceptions, however, with consistently weaknerves, narrower leaf shape and little or no indication of teeth at the margin.

Biometric study of variationMeasured variation was considered as a whole by use of cluster analysis. Two of theoriginal. 15 characters were excluded from analysis. Angular cell shape was constant

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272 JOHN D. WEHR AND BRIAN A. WHITTON

30

20

10

max. plant length

x=10'3

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x= 4 ..1

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x= 6'4

6 8 10 12 14 16 1820 22 em 2 4 6 8 10 em 2 4 6 8 10 12 14 16 em

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20 leafx =0'49

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Fig. 2. Frequency distributions of continuous characters of leaves and plants of Rhynchostegiumriparioides. 'Node' refers to the maximum unbranched length.

throughout all populations. Colour was omitted because it was found that thedarker shades were related to (and probably caused by) bound Mn and Fe, andhence were not a characteristic of the plants themselves.

The analysis revealed four groups of approximately equal dissimilarity (Fig. 3).Between these four, the larger plants, classified into Groups A and B, were mostclosely related, while Groups C and 0 were joined at greater dissimilarity level.Plant size clearly played an important role in discriminating between morphologicaltypes. Linked with this· analysis, an ordination identified several of the originalmorphological variables which had the strongest loadings along the first three com-ponent axes (70% of total variance among populations). These were axis length,primary node length, leaf length and width; and also branching pattern, leaf shapeand margin denticulation.

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VARIATION IN RHYNCHOSTEGIUM RIPARIOIDES 273

Table 2. Distribution of discrete characters in 105 populationsof Rhynchostegium riparioides, expressed as percentage

frequency

Character State Percentage

PlantsRobustness Robust 21

Moderately robust 34Intermediate 29Flaccid 8Moderately flaccid 8

Colour Pale to bright green 33Medium to dark green 51Brown to black 16

Proportion of < 1/3 14axis leafy 1/3-1/2 42

>1/2 44

Branching type Parallel 11Intermediate 92Dendroid 1

LeavesShape Ovate 14

Elliptical 81Lanceolate 5

Proportion < 10% 5denticulate 10-33% 2

>33-66% 20>66% 73

Proportionate < 1/2 1nerve length 1/2-3/4 7

>3/4 92

Angular cell shape Rectangular 100Rhombic 0Oval/inflated 0

Diagnostics at the four cluster level indicated that Groups A and B containeddistinctly heavier and longer plants than those in C or D; A and B also had longerand broader leaves. While C and D plants were smaller and less robust, these twowere fairly dissimilar from each other. Group C had long, but narrower leaves andranged from intermediate to moderately robust. These plants were typically moredensely branched than members of all other groups. Group D plants were small toaverage-sized, with a flaccid to intermediate texture; leaves were smaller thanaverage, with a short nerve and weak denticulation: Average (2 cm) tip weight ofplants in Group D (0.96 mg) was half that in Group A (1.98 mg) and only a thirdthe average weight of the highly robust plants in Group B (2.94 mg). Two extremesof this gradation in robustness are shown in Fig. 4.

A· feature of this analysis is that while the larger clusters are fairly distinct,members within a cluster have varying (and often weak) levels of internal simi-larity. This indicates that although morphological 'types' may. exist, individual

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274 JOHN D. WEHR AND BRIAN A. WHITION

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AVARIATION IN RHYNCHOSTEGIUM RIPARIOIDES

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Fig. 4. Examples of shoots from two extremes of robust ("A") and flaccid ("D") morphologies ofRh ynchostegium riparioides.

populations cannot be identified easily as a member of that group. These resultssuggest that forms or varieties that have been recognized in this species are notsufficiently distinct to be useful in practice. More than a half a century ago,Warnstorf et ale (1914) complained that it was a ' ... thankless task ... ' to unravelthe numerous forms and varieties which have been included within this species.However, a few later floras still make such separations (Szafran, 1963; Pavletic,1968).

Environmental factorsThe streams, rivers and other lotic habitats from which populations of R.riparioides were collected differed greatly in their chemical composition. Asummary of the data of Wehr & Whitton (1983a), which is relevant to the presentpaper, is given in Table 3. These data indicate the broad range of environments inwhich this species occurs. The rivers range from sites with quite soft water (ninesites < 10 mg 1- I Ca) to calcareous ones. The pH spectrum is also broad, althoughfew sites were found to have a pH < 7.0. Many of the soft water sites were locatedin the Mersey Catchment (region XII, Fig. 1) and several streams in Weardale(region V) and Allendale (regions II & III). As sites included small upland streams,industrially polluted rivers and even a sewage effluent, it is not surprising that con-centrations of reactive orthophosphate-P and nitrate-N spanned more than threeorders of magnitude. A number of sites were also polluted by heavy metals, both

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276 JOHN D. WEHR AND BRIAN A.WHITION

Table 3. Chemistry of 105 sites with Rhynchostgium riparioides(For details, see Methods)

Variable Units Minimum Mean Maximum

pH 6.8 7.8 8.7Total alkalinity meq I-I 0.10 1.42 7.12Na mgl-I 2.6 12.4 87.8K mgl-I 0.08 2.04 14.8Mg mgl-I 0.72 6.29 60.0Ca mgl-I 3.72 32.4 90.4Mn mgl-I <0.004 0.049 0.75Fe mgl-I <0.002 0.14 0.58Zn mgl-I <0.006 0.122 1.62Cd lJ,.gl-1 0.06 0.47 3.32Ba mgl-I <0.02 0.17 0.74Pb mgl-1 0.0010 0.011 0.178NH4-N lJ,.gl-1 <5.0 96.4 1990N03-N lJ,.gl-1 7.5 1360 31900P04-P lJ,.gl-1 < 1.5 72.9 3180504-5 mgl-1 1.30 12.1 80Si mgl-1 0.64 2.42 9.9F mgl-1 0.025 0.26 1.30CI mgl-I 5.2 18.1 155

from mining and industrial sources. Further discussion of the ecology of this specieshas been given previously (Wehr & Whitton, 1983a, b).

During the course of the study it became apparent that populations with certainmorphological features were characteristic of certain types of stream. Robust formswith larger leaves and longer stems were observed most frequently in springs andupland streams of calcareous districts, particularly in Arkengarthdale andSwaledale (regions IX and X, Fig. 1). The opposite extreme, with smaller leavesand a flaccid texture, was found mostly in larger rivers, especially those whichmight be regarded as eutrophic (elevated Na, NH4-N, reactive P04-P). This en-vironmental pattern was not, however, entirely consistent. For instance, a distinctlynon-robust population was collected from an unpolluted, soft water stream in theLake District. The groups identified by cluster analysis do have some evidence ofgeographical trends. The smallest, non-robust Group D consists of populations ofwhich 800/0 are from streams and rivers west of the Pennines (i.e. regions XII-XIV,Fig. 1). In contrast, only two populations from these western regions were groupedin either clusters A or B (40/0 of total).

The significance of the relationship between morphological variation andenvironment was considered further by correlating all continuous morphologicalvariables (plus proportionate length of marginal denticulation of the leaf) with thechemical data (Table 4). A derived variable, the ratio of aqueous Na:Ca, was in-cluded in the correlations. Very few of the environmental variables were signifi-cantly correlated with variations in shoot length, leafy length of the primary axis.The characteristics which correlated most strongly were the average weight of a2 cm tip, leaf length and marginal denticulation. All three characters were nega-tively correlated (highly significantly) with aqueous Na, NH4-N, N03-N and

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VARIATION IN RHYNCHOSTEGIUM RIPARIOIDES 277

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278 JOHN D. WEHR AND BRIAN A. WHITION

reactive P04-P. Although preliminary observations had noted robust, large-leavedplants in many calcareous districts, correlation of morphological characters withaqueous Ca were mostly nonsignificant. However, correlations with the ratio ofNa:Ca were all greater than for Na alone. The heavy metals Zn, Cd, Ba and Pbapparently had little effect on plant morphology, even though several sites weregrossly polluted.

General commentsIt is hardly surprising that a species which is ecologically so widespread should bemorphologically variable. This has been observed widely among terrestrial mosses(Longton, 1981; Schofield, 1981). It is of course impossible to decide whether thevariation is genotypic or ecotypic without experimental studies such as reciprocaltransplants. Many of the habitats where R. riparioides occurs have been profoundlyinfluenced by the activities of man. R. riparioides does, however, contrast with theother species of moss which occurs most widely in European rivers, Fontinalisantipyretica (Say & Whitton, 1983), for robust plants of the latter are common innutrient-rich waters.

Comparison with other speciesIn view of the importance of Rhynchostegium riparioides for monitoring purposes,some practical comments are included to aid in identification. Several characterswere found to be relatively stable and here are apparently independent of a broadrange of ecological conditions. In our experience, taxonomic confusion is most fre-quent between R. riparioides and either Amblystegium riparium or Hygrohypnumochraceum. Table 5 gives a summary of 'typical' characters based on this study,supplemented by several floras. Among gametophytic characters in Rhynchostegiumriparioides the most constant is the presence of rectangular chlorophyllose cells in

Table 5. Comparison of 'typical' characteristics of gametophytes and sporophytes of Amblystegiumriparium, Hygrohypnum ochraceum and Rhynchostegium riparioides

Amblystegium Hygrohypnum RhynchostegiumCharacter riparium ochraceum riparioides

PlantsTexture flaccid-intermediate flaccid robust or rigid'Leafiness' leafy throughout leafy throughout denuded belowLeaf arrangement divergent ± complanate imbricate imbricate

LeavesShape lanceolate or ovate - ovate - oblong ovate to broadly

lanceolate ovateMargin entire entire or minutely strongly denticulate

denticulateCosta single; 112-3/4 length single or double; ~ 112 length single; >3J4IengthAngular cells pellucid inflated, auriculate chlorophyllose

SporophytesSeason spring - summer ?; very rare autumn - winterOperculum conical conical rostrate

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VARIATION IN RHYNCHOSTEGIUM RIPARIOIDES 279

the angles of the leaves. These are unlike those of either of the other mossesmentioned above. Another constant character (not part of the analysis) is the leafarrangement in Rhynchostegium riparioides which is radially symmetrical evenwhen the plants are flaccid, while in Amblystegium riparium it is sub-complanate tocomplanate. A useful character of Hygrohypnumochraceum which removes anypossibility of confusion with flaccid plants of Rhynchostegium riparioides is the pre-sence (in the former) of inflated hyaline cells in the epidermis of the stem (viewedin cross-section); this character is considered diagnostic by North Americantaxonomists (e.g. Crum & Anderson, 1981; Ireland, 1982).

Some characters, even when variable, may still be considered diagnostic.Narrower leaves which lack denticulation cannot be regarded as a fully reliabletaxonomic character in Amblystegium riparium, since these occur occasionally inRhynchostegium riparioides. However, leaves which are broadly ovate and havestrong marginal denticulation never occur in the former, so are positive charactersfor R. riparioides. Finally, when present the sporophyte, with its rostrate operculum,is also distinct from other similar aquatic species.

SUMMARY

A study was made of morphological variation in Rhynchostegium riparioides from 105sites in 71 different streams and rivers; 15gametophyte characters were scored for eachpopulation. Nineteen water-chemistry variables were also measured· at each site.Variation was observed in the size and robustness of the plants, dimensions and shapeof the leaves, the degree of denticulation and relative length of the nerve. Clusteranalysis revealed broad morphological groups, with low internal similarity. These re-sults do not support the recognition of subspecific taxa. Correlations with environmen-tal variables showed that plants were significantly less robust, smaller-leaved and withweaker denticulation when collected from nutrient-rich water. In view of the widemorphological range shown by this species and its importance for monitoring heavymetals, a comparison with other widespread aquatic mosses is given.

ACKNOWLEDGMENTS

This research was carried out under contract DG R/480/571 (Bryophytes formonitoring river water quality) from the Department of the Environment, U.K.We are grateful for assistance in the field from I. G. Burrows and D. A. Donaldsonand for stimulating discussion with Dr A. Empain (formerly University of Liege,now Jardin Botanique de I'Etat Meise, Belgium) and Dr P. J. Say (formerly Uni-versity of Durham, now Northern Environmental Consultants Ltd, Consett).Thanks also go to R. C. Bailey (University of Western Ontario, London, Canada)who provided considerable assistance with statistical analysis. Professor W. B.Schofield (University of British Columbia, Canada) provided many usefulcomments on the manuscript.

REFERENCES

Crum, H. A. & Anderson, L. E. (1981). Mosses of Eastern North America. Columbia University Press,New York.

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280 JOHN D. WEHR AND BRIAN A. WHIITON

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Fox, D. J. & Guire, K. E. (1976) Documentation for MIDAS. 3rd edn, Statistical Research Laboratory,University of Michigan, Ann Arbor.

Gangulee, H. C. (1978). Mosses of Eastern India and Adjacent Regions. A Monograph. Fascule 7.Hypnobryales. Published by the author, Calcutta, India.

Ireland, R. R. (1982). Moss Flora of the Maritime Provinces. National Museum of Natural Sciences.Publications in Botany, No. 13. National Museums of Canada, Ottawa, Canada.

Iwatsuki, Z. & Mizutani, M. (1972). Coloured Illustrations of Bryophytes of Japan. Osaka, (InJapanese).

Lawton, E. (1971). Moss Flora of the Pacific Northwest. Nichinan, Japan.Lodge, E. (1959). Effects of certain cultivation treatments on the morphology of some British species of

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moss Bryum argenteum Hedw. J. Bryol. 11, 501-520.Nyholm, E. (1954-1969). Illustrated Moss Flora of Fennoscandia. II. Musci. Lund.Pavletic, Z. (1968). Flora Mohovina Jugoslavije. Zagreb, Yugoslavia.Say, P. J. & Whitton, B. A. (1983). Accumulation of heavy metals by aquatic mosses. 1: Fontinalis

antipyretica Hedw. Hydrobiologia 100, 245-260.Schofield, W. B. (1981). Ecological significance of morphological characters in the moss gametophyte.

Bryologist 84, 149-165.Smith, A. J. E. (1978). The Moss Flora of Britain and Ireland. Cambridge University Press, Cambridge.Statistical Research Laboratory (1976). Elementary Statistics Using MIDAS. 2nd edn, Statistical Re-

search Laboratory, University of Michigan, Ann Arbor.Szafran, B. (1963). Bryophyta I. Musci - Mchy. In: Starmach, K. (ed.), Flora Slodkowodna Polski, Tom

16. Warszawa.Warnstorf, C., Monkmeyer, W. I. & Schiffner, V. (1914). Die Susswasser-Flora Deutschlands, Oster-

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J. D. WEHR (Present address.) Department of Biological Sciences, Fordham University, Larkin Hall,Bronx, New York, 10458, U.S.A ..

B. A. WHllTON (To whom reprint requests should be sent.) Department of Botany, University ofDurham, South Road, Durham DHI 3LE.