Aquatic hyphomycete communities in the river Teign. IV. Twig colonization

Myca/. Res. 95 (4): 413-420 (1991)

Printed in Great Britain

413

Aquatic hyphomycete communities in the river Teign. IV. Twig colonization

C. A. SHEARER Department of Plant Biology, University of Illinois, Urbana, Illinois

61801

USA.

J. WEBSTER Department of Biological Sciences, University of Ereter, EX4 4PS, U.K.

Corticated and decorticated twigs of alder and oak were placed at five sites along a gradient of stream order and pH in the river Teign, Devon, England. After 2 and 6 months of submersion. twigs were examined directly and then placed individually in bubble chambers to simulate stream conditions. The water was membrane filtered and aquatic hyphomycete conidia on filters were identified and quantified. The mean number of conidia per twig varied from 0 to over 700000/48 h incubation period and was generally greater from samples at downstream than from upstream sites. Only three species, HelisCU5 lugdunensis, Mycocentrospora acerina, and Tricladium splendens occurred frequently throughout the river. Longitudinal distribution patterns varied among species with the majority of species occurring from the third site and downstream. Species replacements occurred over time. Species diversity and H' diversity varied among sites and peaked at the penultimate site rather than increasing linearly in a downstream direction. More species of aquatic Hyphomycetes were detected with bubble chamber incubation than with direct examination and moist chamber incubation; the converse was true for Ascomycetes.

Studies on the taxonomy and ecology of aquatic Hyphomycetes have traditionally centred on their growth on allochthonous deciduous tree leaf litter in streams and rivers, following the pioneering investigations by Ingold (1942, 1943 a, b). Later work made use of submerged leaves of various sorts (Kaushik & Hynes, 1971; Barlocher & Kendrick, 1974; Suberkropp & Klug, 1974, 1976; Barlocher, 1982; Chamier & Dixon, 1982; Shearer & Lane, 1983; Chamier, Dixon & Archer, 1984; Suberkropp, 1984; Shearer & Webster, 1985 a, b; Gonczo1, 1989). Their occurrence on wood has been studied relatively little Oones & Oliver, 1964; Archer & Willoughby, 1969; Jones & Stewart, 1972; Willoughby & Archer, 1973; Lamore & Goos, 1978; Sanders & Anderson, 1979; Shearer & von Bodman, 1983). The teleomorphs of these fungi, although occasionally reported from leaves, e.g. Abdullah, Descals & Webster (1981) and Descals, Fisher & Webster (1984), have been, for the most part, found on wood partially submersed or stranded along the banks of streams, or on wood incubated in moist chambers (Webster & Descals, 1979). In a study of the longitudinal and temporal distribution of aquatic Hyphomycetes in the river Teign in Devon, England (Shearer & Webster, 1985 a, b), packs of alder (Alnus glutinosa (L.) Gaertn.) leaves were submerged in the river at selected points starting from its origins in open moorland above the tree line, extending downriver to regions where the banks were densely overhung by deciduous trees. Variations in the colonization of the leaves were followed in relation to site and to period of immersion. In this study we have extended the

investigation to the colonization of short lengths of corticated and decorticated alder and oak twigs in the same river.

MATERIALS AND METHODS Study sites Five sites were chosen on the river Teign, starting in the treefree moorland gathering ground, and extending to wellwooded stretches downstream. Site I: Walla Brook, Scorhill Down, SX 653871. This site, used in an earlier study, has been described previously (Shearer & Webster, 1985 a). It is on the banks of Walla Brook, about 200 m upstream of Walla Brook footbridge. Site II: Scorhill Tor, SX 6598696. Downstream of Site I, Walla Brook joins the North Teign river which then descends rapidly over granite boulders. The banks of the watercourse are bordered by occasional trees of Sorbus aucuparia L., Salix cinerea L., Betula pendula Roth., Corylus avellana L., Quercus petraea (Mattuschka) LeibL and Fagus sylvatica L. Streamside herbaceous and shrubby vegetation includes Luzula sylvatica (Huds.) Gaud., Calluna vulgaris (L.) Hull, Vaccinium myrtillus L. and Blechnum spicant (L.) Roth. The river (in this stretch called the North Teign) falls between large granite boulders over a gravelly or stony bottom with little instream vegetation. The boulders form effective traps for fallen tree branches and leaves. Site II on the north bank of the river below Scorhill Tor is shaded by species of Betula and Salix. Unfortunately, some of the samples were lost at this site because of fraying of the cord which attached samples to the bank.

414

Aquatic hyphomycetes on twigs Site III: Glassy Steps, North Park near Gidleigh, about 300 m downstream of the footbridge crossing the river, SX 672855. Between Sites II and III, the river has flowed for about 1'5 km through dense woodland dominated by Q. petraea and a larch plantation. Immediately upstream of Site III, the most abundant trees are Q. petraea, F. sylvatica, C. avellana, Rhododendron ponticum L., S. aucuparia and [lex aquifolium L. The dominant herbaceous plants bordering the river are L. sylvatica and V. myrtillus. The rapidly-flowing river passes over large boulders which trap tree trunks and branches. The bed of the river is rocky or gravelly with little or no attached macrophyte growth. At Site III, the river is fairly level and flows between large boulders which trap tree branches. The bed is gravelly with little macrophyte growth. Site IV: Middle Leigh Ford, Leigh, SX 683879. This site is on the west bank of the river, 100 m upstream of the stepping stones and ford. It is about 1 km upstream of Holystreet Manor (Site II in the study of Shearer & Webster, 1985 a). Immediately above Site IV, the banks are overhung by coppiced A. glutinosa, C. avellana L. and bushes of R. ponticum L. There are occasional large old trees of Q. petraea, Fraxinus excelsior L., Acer pseudoplatanus L., F. sylvatica L., and Alnus sp. Many of these older trees are densely covered with ivy (Hedera helix L.), whose leaves make a significant contribution to the allochthonous leaf litter. Bankside vegetation also includes Rubus fruticosus Agg. and L. sylvatica. The bed of the river is gravelly, with occasional large boulders which trap tree branches. Some of the older trees had fallen into the river upstream of the collecting site. The river is wider and less rapidly flowing than at the upstream sites. Site V: Steps Bridge, SX 805882. The sampling site, on the south bank of the river in Bridford Wood, near Dunsford, Devon, was some 500 m downstream of Steps Bridge (Map reference SX 805882). At this point the river had been flowing for almost 11 km through dense overhanging mixed woodland. Immediately above Steps Bridge is a large weir constructed of boulders lying obliquely across the entire width of the river. The bed of the river is rocky with some stones and gravel. There are also occasional large boulders so that there are many places where litter, including fallen branches, is trapped. On both sides of the river there are overhanging mature trees, predominantly of Acer pseudoplatanus, Alnus glutinosa, Fagus sylvatica, Fraxinus excelsior, Quercus robur L., Betula pendula Roth. and shrubs such as C. avellana and Ilex aquifolium L., The riparian herb layer is made up largely of Luzula sylvatica. The rocks forming the river bed support the growth of several mosses including Fontinalis antipyretica L. No attached angiospermous macrophytes were observed. The flow of the water is turbulent, even at low flow rates and there are numerous riffle areas and shallow and deep pools. Preparation of samples

Lengths of twigs of alder (A. glutinosa) and oak (Q. petraea) approximately 7'5 cm long and 1-1'5 cm diam were cut from a single tree of each in August 1986. Half of the twigs were stripped of bark, and a hole about 2 mm diam was drilled near to the end of each twig. The twigs were threaded together

with monofilament nylon cord in groups of 10; 5 with and 5 without bark. Each pack of 10 twigs was lashed to a masonry brick, and tethered by means of a cord and tent peg to the riverbank. Samples were immersed in the river on 20 Aug. 1986 and recovered on the following dates: Site 1-2 Oct. 1986 and 3 Feb. 1987; Site II - 5 Oct. 1986 and 3 Feb. 1987; Site III - 8 Oct. 1986 and 10 Feb. 1987; Site IV - 8 Oct. 1986 and 13 Feb. 1987; Site V - 15 Oct. 1986. Additional samples to be taken at each site in Sept. 1987, together with the Feb. 1987 sample at Site V were lost. Upon recovery, bricks with attached twigs were placed in separate polythene bags. In the laboratory, the twigs were immediately examined using a dissecting microscope (x 50 magnification). Fructifications, e.g. apothecia or masses of conidia, were scraped off, and where pOSSible, identified. Notes were made of the state of the twigs, e.g. whether they had been degraded by insect larvae. The twigs were then placed separately in bubble chambers consisting of vertical glass tubes 180 mm long and 18 mm diam. The bottom of each tube was closed by a bung through which passed a tube connected internally to an aquarium aeration stone. The tubes containing the twigs were topped up with distilled water and a gentle stream of compressed air was forced through the aeration stone to create a flow of air bubbles over the twigs. The tubes were aerated in a constant temperature room at 15°C for 2 d. After the period of aeration, water from the column was filtered (Millipore membrane filters, 8 J.lm pore size) and the filters were fixed with lactic acid containing either cotton blue or acid fuchsin. Squares (1 cm 2 ) cut from the filters were mounted on microscope slides in lactic acid with cotton blue and examined for the conidia of aquatic Hyphomycetes, which were identified and quantified. After incubation in aeration tubes, twigs were incubated in moist chambers prepared by lining the bottom of plastic lunch boxes with wet paper towels. The twigs were incubated for up to a year at 17° under an alternating light/dark cycle of 8/16 h. Twigs were examined at 3 month intervals for the presence of fungal fruiting structures. Cultures prepared from ascospores of ascomata were made in order to attempt to link teleomorphs with anamorphs. RESULTS

The mean number of conidia of aquatic Hyphomycetes per twig varied from to over 700,000/48 h (Fig. 1). Conidia of some species from decorticated twigs submerged at Site IV for 6 months were too numerous to count. After 2 months submersion, fewer conidia were produced per twig at Sites I (for alder) and Sites I and II (for oak) than downstream sites. These differences, however, were reduced after 6 months of submersion (Fig. 1). After 2 months submersion, conidial production was greatest at Site III, but differences among sites decreased after 6 months of submersion (Fig. 1). Generally, more aquatic hyphomycete conidia were produced from decorticated than corticated wood after 2 months of submersion, but this effect was masked by the abundance on corticated wood of Heliscus lugdunensis, an early colonizer (Fig. 2). After 6 months of submersion, aquatic hyphomycete

°

C. A. Shearer and

J.

415

Webster

Total conidia production

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Fig. 1. Mean number of conidia of aquatic Hyphomycetes produced per twig (n = 5) for corticated and decorticated twigs of alder and oak after 2 and 6 months of submersion at each sampling site. " no conidia; MD, lost sample; TNTC, conidia too numerous to count; WJ, corticated; . , decorticated. Fig. 2. Mean number of conidia of aquatic Hyphomycetes less those of Heliscus lugdunensis produced per twig (n = 5) for corticated and decorticated twigs of alder and oak after 2 and 6 months of submersion at each sampling site. " no conidia; MD, lost sample; TNTC, conidia too numerous to count; WJ, corticated; . , decorticated. Figs 3, 4 Mean number and relative frequency of conidia of Heliscus lugdunensis produced per twig (n = 5) on corticated and decorticated twigs submerged for 2 and 6 months at each sampling site. " no conidia. Fig. 3. Alder twigs. Fig. 4. Oak twigs. WJ, corticated; . , decorticated.

conidial production on corticated twigs increased. An exception to this pattern occurred for alder twigs submerged for 2 months at Sites II and III (Fig. 2). The number of species found (Table I) was greatest at Sites III and IV while species number and H' diversity calculated for those species with a relative mean frequency of conidia per twig> 0'01 was highest at Site IV after 2 months of submersion (Table 2), Since 6-month samples at Site V were lost and conidia from twigs at Site IV were too numerous to count, comparable data could not be obtained for the 6months sample (Table 3). Clearly, though, after 6 months, species number was high at Site IV and conidial frequencies

were distributed among a greater number of species than at Sites I-III. Longitudinal distribution patterns varied among species. Only two species, H. lugdunensis (Figs 3, 4) and Tricladium splendens (Fig. 5) occurred frequently at all sites. Mycocentrospora acerina occurred at Sites I-IV (Fig. 9) and probably occurred at Site V also. The most common pattern was absence at Sites I and II and presence at Sites III, IV and V (Figs 6, 8). Frequently-occurring species with this type of pattern were Clavariopsis aquatica, Tumularia aquatica, Dimorphospora foliicola and Tricladium chaetocladium. Distribution patterns of several species displayed a seasonal or successional

Aquatic hyphomycetes on twigs

416

Table 1. Species found on corticated (C) and decorticated (D) twigs of alder and oak at each sampling site after two and/or six months of submersion

Species

Sampling sites I

II

ALD

OAK

ALD

OAK

ALD

OAK

ALD

D CD CD

CD

CD D CD

CD

CD

CD D D

CD

IV

III

V

OAK

ALD

OAK

Deuteromycetes

Alalospora acuminala Ingold Alatospora sp. 1 Anguillospora crassa Ingold Anguillospora longissima (Sacc. & Syd.)

D

C D

D CD

D D D

CD CD

Ingold

Articulospora tetracladia Ingold Composporium sp. I' Casaresia sphagnorum' Fragoso Clavariopsis aquatica de Wild. Culicidospora aquatica Petersen Cylindrocarpon ianthothele Wollenw. Dendrospora sp. Dimorphospora foliicola Tubaki Flagellospora ?curvula Ingold Fusarium sp. Goniopila monticola (Dyko) Marv. &

C D

C D

CD

CD

C

C CD

D

D CD

C

CD D CD CD CD D

D CD CD CD CD CD

CD

CD CD

CD CD CD D CD D CD D

CD C CD D CD D

CD D CD

CD D CD

CD

CD

CD

D

0 CD

C CD

CD

CO

CD

CB

D

C D

CD

CD

Descals

Heliscina ?campanulala Marv. Heliscus lugdunensis SacCo & Therry CD Infundibura adhaerens' Nag Raj & Kend-

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CD

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rick

Lunulospora curvula Ingold Menispora sp" Mycocenlrospora acerina (Hartig)

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Deighton

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Phialocephala sp" Sporidesmium hyalosperma' (Corda) Hughes

Trichoderma sp" Tricladium chaetocladium Ingold Tricladium giganteum Iqbal Tricladium splendens Ingold Triscelophorus monosporus Ingold Tumularia aquatica (Ingold) Descals & Marv. Varicosporium sp. Pycnidial form I'

Sigmoid form 1 Sigmoid form 2

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Ascomycetes

Massarina sp. Nectria discophora' (Mont.) Mont. Nectria lugdunensis' Webster Niptera excelsior (Karsten) Dennis Hydrocina chaetocladia Scheuer

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16

14

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15

13

21

21

26

D 22

15"

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, Found by direct examination or examination after moist chamber incubation only. " Low species numbers may reflect loss of the 6-months sample.

effect, i.e. presence at 2 months but absence at 6 months (Figs 7, 8) and vice versa (Fig. 6). Non-aquatic hyphomycete spores were present in the twig spora after 2 months submersion, but numbers dropped to very low or undetectable levels after 6 months of submersion. Spores of non-aquatic species were not distinctive and usually could not be identified. There was little evidence of distinct substrate preferences. Only eight of 39 species occurred on only one wood type

(Table 1). Five of the eight species were found only once, thus their occurrence on only one wood type could be just as well a matter of chance as of substrate preference. Sigmoid form 2, and Nectria discophora occurred more frequently on oak than alder and may indeed be consistently associated with one wood type. There was high reproducibility among the five replicates of a single sample. Generally, for those species whose conidia occurred with a relative mean frequency 0-01 or greater, if the species occurred on one twig, it occurred on

417

C. A. Shearer and J. Webster

Table 2. Relative mean frequencies (n = 5) of conidia and number of species per twig' and H' diversity at each sampling site on decorticated twigs of alder and oak submerged for 2 months Sampling sites I

II

ALD

OAK

ALD

OAK

ALD

OAK

0'57

0'41

0'23

0'15

0'54

0'32

0'60

0'27

0'58

0'68

0'01 0'05 0'33 0'33 0'02 0'02 0'01

0'01 0'59

0'18 0'18

Species

ALD

OAK

ALD

Alatospora acuminata

0'67 0'14 0'1I 0'03 0'03

0'91

0'02

0'02 0'01 0'07

0'14

Branched, unknown Sigmoid, unknown

Heliscus lugdunensis Tricladium giganteum Lunulospora curvula Tricladium splendens Clavariopsis aquatica Tricladium chaetocladium Goniopila monticola

OAK"

0'09 0'10

Sigmoid, form 2 Alatospora sp. I

Dimorphospora foliicola No. of species H' Diversity

5 0'89

4 0'40

V

IV

III

4 1'03

MD'

2 0'68

3 0'82

8 1'57

0'28 0'12

0'03 7 1'17

0'06 0'03 0'01 0'02 5 0'97

4 1'03

• Only species with relative frequencies of 0'01 and greater are included. " Sample lost. e MD, missing data.

all five. Rarely did a species produce many conidia on only one or two twigs of a 5-replicate sample. Frequency of occurrence on twigs, therefore, was not a useful parameter in evaluating fungal community structure in that most species in a given community would have 100 %frequency of occurrence. More aquatic Hyphomycetes were detected with bubble chamber incubation than with direct examination and moist chamber incubation (Table 1), Conversely, aquatic species of Ascomycetes were detected more frequently by examination directly and after moist chamber incubation than by bubble chamber incubation.

DISCUSSION Submerged twigs supported the sporulation of a wide variety of aquatic Hyphomycetes and large numbers of conidia were generated from colonized wood. Since wood is decomposed much more slowly than leaves (Shearer & von Bodman, 1983) and its input is less seasonal than that of leaves, it provides a more temporally constant substratum than deciduous leaves. thus wood in streams may be important in the long-term maintenance of populations of aquatic Hyphomycetes. The relative contributions of fungi and bacteria to the degradation of lignocellulose can affect the structure of detritus-based food webs through the differential selection for fungivorous or badivorous animals (Benner, Moran & Hobson, 1986). The role of aquatic Hyphomycetes in decomposing leaves (Suberkropp & Klug, 1980; Suberkropp, Arsuffi & Anderson, 1983; Suberkropp & Arsuffi, 1984; Chamier, 1985) and as a food source for invertebrates (Biirlocher, 1985) has been documented. Less is known, however, about the nature and extent of their role on wood. That aquatic fungi occur and sporulate abundantly on submerged wood suggests that they are able to use it as a 27

carbon source. Few studies, however, have documented their ability to degrade wood. Jones (1981) found that four of six species of aquatic Hyphomycetes caused weight loss in birch, beech and balsa wood test blocks. Gunasekera, Webster & Legg (1983) found that six aquatic and three aeroaquatic hyphomycete species were able to degrade oak and pine wood blocks, and Zare-Maivan & Shearer (1988) found that five species of aquatic Hyphomycetes grown on bark and sapwood blocks of ash and cottonwood caused weight-loss, but only 3 of the 5 formed soft-rot cavities. Additional studies are necessary to determine the extent to which species of aquatic Hyphomycetes are able to degrade wood, the types of particulate fractions that result from their activities, and the extent of their use as a food substrate by invertebrates. A comparison of species found in this study of twigs with a parallel study of leaves at the same sites (Shearer & Webster, 1985 a, b) indicates that wood supports a variety of ascomycete teleomorphs that leaves do not. A similar pattern was seen for twigs (Shearer & von Bodman, 1983) and leaves (Metwalli & Shearer, 1989) in Jordan Creek, Illinois. The presence of teleomorphs on wood may be related to longevity of the substrate and/or nutritional factors. Wood may be important as a site for genetic recombination in species of aquatic Hyphomycetes with sexual states. Substratum specificity was not detected among frequently occurring species of aquatic Hyphomycetes; generally such species occurred on both wood types. This finding is in agreement with that of a study that found little substratum specificity for species of aquatic Ascomycetes on wood (Shearer & von Bodman, 1983). Bark, however, apparently retarded the establishment of most aquatic Hyphomycetes, except Heliscus lugdunensis, Lunulospora curvula and Dimorphospora foliicola, as conidial numbers were much lower on corticated than decorticated wood. Heliscus lugdunensis occurred relatively more frequently than other aquatic hyphoMYC 95

Aquatic hyphomycetes on twigs

418 Clavariopsis aquatica

Tricladium splendens

6 Oak (2 months)

6 Alder (2 months)

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Figs 5-8. Mean number of conidia produced per twig (n = 5) on corticated and decorticated twigs of alder and oak submerged for 2 and 6 months at each sampling site, " no conidia; MD, lost sample; TNTC, conidia too numerous to count. Fig. 5. Tricladium splendens, Fig. 6. Clavariopsis aqua/ica, Fig. 7. Lunulospora curvuJa, Fig. 8. Goniopila monticoJa; 8, corticated; •. decorticated,

mycete species on corticated than decorticated wood and may be well-adapted to corticolous substrata (Figs 3. 4). In support of this idea. Zare-Maivan & Shearer (1988) found that this species caused greater loss of weight in bark than in sapwood. The major discontinuity in aquatic hyphomycete community structure. i.e. diversity. H' diversity. and species composition. occurred between Sites III and IV. This is in agreement with the longitudinal differences found for aquatic hyphomycete communities on leaves in the same river (Shearer & Webster, 19850). Although Sites I and II lack substantial riparian trees. Sites III. IV and V are in forested areas. Thus vegetational differences between the sites may not account for differences in community structure between Sites I-III and IV-V. Stream pH, however. ranges between 5'4 and 5'7 at Sites I-III and changes I pH unit to 6'6 at Site IV and may affect species diversity. Using literature values. Barlocher & Rosset (1981)

found that the number of fungal species occurring in a stream as a function of the stream pH forms a unimodal curve with a maximum at a pH of 6'7. Data from this study agree with this model in that the highest species diversity occurred at Site IV with a pH of 6'6 and with lower diversity at pH's below (Sites I-III) and above (Site V). Chamier (1987) found that aquatic hyphomycete species numbers on grass. alder and oak leaf packs generally were lower in upland streams of low pH (pH 4'9-5'5) than in lowland streams with a higher pH (pH 6'8). An exception to this pattern was an upland tree-lined stream with a pH of 6'6 in which only 2-3 species colonized leaf packs. In her discussion she suggested that stream location (upland v. lowland) may be more important than pH and vegetation in influencing species numbers. Longitudinal distribution patterns varied among species and were similar to those found for aquatic hyphomycete species on leaves in the same river system (Shearer & Webster,

C. A. Shearer and J. Webster

419 Mycocentrospora acerina 6

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5

I .MD •• Ol.o=":::::;2~C;3='C:4~.c:5~

OU=='£2~~3=-.c:4~~5:::l

u

~ 6

u

Alder (6 month )

5

.34

4

3 2

3

~

o

Oak (6 months)

6

E 5

I

III 2

4

3

o

2

Sites

3

4

Fig. 9. Mean number of conidia of Mycocentrospora acerina produced per twig (n = 5) on corticated and decorticated twigs of alder and oak submerged for 2 and 6 months at each sampling site. " no conidia; MO, lost sample; TNTC, conidia too numerous to count; @j, corticated; . , decorticated. Table 3. Relative mean frequencies (n = 5) of conidia and number of species per twig' and H' diversity at each sampling site on decorticated twigs of alder and oak submerged for 6 months Sampling sites I

II

IV

III

Species

ALD

OAK

ALD

OAK

ALD

OAK

ALD

Tricladium splendens Mycocentrospora acerina Heliscus lugdunensis

0'64 0'30 0'03 0'03

0'69 0'11 0'05 0'14

0'1I 0'80 0'07 0'02

0'20 0'50 0'28 0'02

0'03 0'40

0'57

..

Sigmoid, Form 3

Alalospora acuminala Tric/adium chaeloc/adium Dimorphospora foliicola Flagellospora cumula

OAK

0'01 0'55

0'28 0'14

Sigmoid, Form 4

lNTC' lNTC lNTC lNTC

lNTC lNTC lNTC

Clavariopsis aqualica Tumularia aqualica Anguillospora longissima lNTC

Sigmoid, Form 5

Anguillospora crassa No. of species H' Diversity

4 0'86

5 0'97

4 0'07

4 1'10

3 0'80

3 0'95

8

8

, Only species with relative frequencies of 0'01 and greater are included. • Conidia with mean frequencies greater than 350 per twig. C Conidia too numerous to count.

1985 a). The three species which occurred on wood throughout the river system, H. lugdunensis, T. splendens and M. acerina, occurred abundantly and could be considered dominant species. In a parallel study of leaves (Shearer & Webster, 1985 a) only one dominant species, Clavatospora longibrachiata (Ingold) Nils. ex Marv. & Nils. occurred throughout the river. Apparently all four of these species are able to grow and sporulate throughout a wide range of stream conditions. Distribution patterns differed temporally among species. Lunulospora curvula, considered a summer species in this area, and Goniopila rnonticola, produced considerable numbers of conidia in the first sample, and none to a very few conidia in the second sample, while Mycocentrospora acerina, Dirnorphospora foliicola and Flagellospora curvula produced none to a very few conidia in the first sample but abundant conidia in the

second. Such distinct species replacements are not usually observed on leaves; rather the initial colonizers persist as new species are added and then members of the community disappear as the substrate is degraded (Chamier & Dixon, 1982; Biirlocher & Schweizer, 1983; Shearer, unpub!. obs). It is likely that chemical and structural differences between wood and leaves account for differences in colonization patterns. Submerged wood is decomposed more slowly than leaves (Shearer & von Bodman, 1983) and decomposition is limited to surface areas (Aumen et al. 1983), most likely because of poor diffusion of oxygen into compact waterlogged tissue. If environmental conditions become unfavourable to a species and its growth rate decreases, the fungus could easily be scoured from the wood surface along with rotted wood particles by the abrasion of running water and/or grazing and 27-2

Aquatic hyphomycetes on twigs gouging by invertebrates. Since microbial colonization of wood in water is likely limited to surfaces, this would provide uncolonized substrata available to a different species favoured by the new environmental conditions. In leaves, which are thin, colonized areas are usually grazed or disintegrate owing to fungal enzymatic activity and no new substratum is made available. It is possible, however, that on leaves, a newly arrived species could overgrow and outcompete an existing one or could grow in between the hyphae of existing species, but the amount of unused substratum would be small in comparison to that available for wood. Leaves, with a high surface to volume ratio, present surfaces that can trap and be colonized by a variety of species initially. In contrast, woody debris, with a lower surface to volume ratio, may trap fewer species initially, but by revealing successive layers of substrate over time, may be colonized by a number of species successively. The 'time-release' of substratum provides an interesting system in which to study successional and seasonal replacements in aquatic hyphomycete communities and merits further study. Appreciation is expressed to Tony Davey for preparing and submerging baits and assistance with field work.

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(Received for publication 4 January 1990 and in revised form 31 May 1990)

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