The spatial distribution of fungi on decomposing alder leaves in a freshwater stream.

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.