Transects were cut from alder leaves incubated in a freshwater stream and plated as quadrats so that fungal isolates could be mapped by reconstruction of each transect. Initially there was fewer than one aquatic hyphomycete colonist per quadrat, but the mode increased to 6-7 then progressively decreased to <1. Numbers of species of aquatic hyphomycetes per quadrat rose and fell similarly with a maximum mode of 3-4, as did species per transect with a maximum of 11 and a diversity of 16, comprising 6 'dominant' species and about 10 'occasional' species. The latter showed no pattern of appearance but the dominant group was established early and persisted in a dynamic equilibrium. Aquatic hyphomycetes were initially randomly distributed but developed progressively into clumped consortia which persisted after peak colonization, declining as leaf degradation became total. Colonies of the most persistent aquatic hyphomycete species were initially discrete,developing into a complex network of overlapping colonies and species, no two of which showed positive association. These complexes broke down to large colonies of a few species and finally to 1-2 small colonies. The pattern of isolates of the 18 genera of other fungi was the reverse of that for aquatic hyphomycetes. Only Cladosporium, Epicoccum and Fusarium were important colonizers. The first two appear to be inhibited by aquatic hyphomycetes, but were found to degrade substrates representative of cell-wall polymers vigorously whereas aquatic hyphomycetes showed varied degradative ability. Leaf transects were examined by S.E.M. and epifluorescent microscopy so that hyphal colonization could be followed at progressive stages of leaf degradation. Bacteria on transects were patchily distributed, the temporal pattern indicating inhibition by aquatic hyphomycetes and colonization of senescent hyphae.

All seven species of aquatic hyphomycetes tested produced both polygalacturonases and pectin lyases. The polygalacturonases were constitutive, whereas the pectin lyases were induced on pectic substrates at pH 6.5 and above. Some species could grow on pectic substrates at both pH 5 and pH 7; other species at pH7 only. Tricladium splendens grew as well on a polypectate substrate as it did on glucose. This fungus elaborated three endo-polygalacturonase isoenzymes, as did Articulospora tetracladia. Tetrachaetum elegans secreted a presumed exo-pectin lyase and a pectin esterase, and Mycocentrospora angulata an endo-pectin lyase and a pectin esterase. The four species macerated alder-leaf strips totally within 9 to 12 d at stream pH 7, utilizing predominantly pectin lyases and pectin esterases. These enzymes are stimulated by Ca2+, and this may explain why plant decay proceeds most rapidly in calcium-rich streams of pH 6.5 and above. The frequency with which the four species of aquatic hyphomycetes were observed on experimental leaf packs placed in a stream does not appear to be related to their pectolytic capability. METHODS Fungi and culture media. The following species of aquatic hyphomycetes were isolated from scum samples taken from the River Bourne (Chamier & Dixon, 1982): Articulospora tetracladia Ingold, Lemonniera aquatica de Wild.

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The colonization-pattern of aquatic Hyphomycetes on five-gram leaf packs of oak and alder submerged in a stream was quantified and compared. There were three series of alder leaves, submerged two weeks apart, and one series of oak. Colonization of leaves by pectolytic bacteria was also measured. There were marked similarities in the colonization of all four series. Total spore counts/g dry wt of leaf rose to a peak followed by a decline. The time taken to peak colonization was slower in oak than in alder, and in alder depended on the level of inoculum in the stream, as did the extent of colonization. Pectolytic bacteria counts followed the pattern of total spore counts, suggesting the exploitation of the same substrates by bacteria and fungi. Temperature and micro-environmental factors influence the overall rate of leaf degradation. Alder I was skeletonized in 10 wks, Alders II and III in 12 wks and oak in 25 wks.The resource was shown to have an upper limit of microbial colonization, and within this 'unit-community' of microbes, there was an association of four dominant species of aquatic Hyphomycetes, together with about ten occasional species. The dominant species are subject to selection from the inoculum available in the stream and the formation and maintenance of the association appears to be the result of competitive interactions between species which results in a dynamic equilibrium. There is a low degree of resource specificity. The species equilibrium is 14 for all series, and species numbers are initially low, rise to a peak, then tend to decline. There is a taxonomic similarity of about 60% between successive stands of all series and between matched stands of alder.