streams. The occurrence of different species of fungi in aquatic systems is dependent upon season, however. Both Gupta and Mehrota (1989) and Thomas et al. (1989) identified seasonal changes in fungal species composition of the community. Thomas et al. (1989) reported the greatest conidial abundance in early autumn (500-600 conidial"1) and the lowest during the winter (300500 conidial"1). Using stepwise multiple regression analysis of the conidial abundance and a variety of environmental measures, they showed that spore abundance was significantly related to temperature (R2 = 0.62) and somewhat related to a combination of both temperature and rainfall. Species composition, however, was not related to temperature, but to rainfall and conductivity.

These studies, however, do not relate to the impact of seasonal variations in plant litter composition and changes in the physicochemical composition of that litter that occur during succession. In a study of the comparative decomposition of yellow poplar (Liriodendron tulipifera), red maple (Acerrubrum), and white oak (Quercus alba) leaf material in streams, Griffith et al. (1995) demonstrated differences in decomposition constants between leaf species and between sites, with pH and temperature being major variables between sites. In general leaf litters decomposed more readily in streams of higher pH and higher temperature (Table 2.22), but some of the between-site differences were less obvious if the data were temperature-corrected, suggesting that this was a primary driving variable. The temporal pattern of production of pectinase enzyme was also different among leaf litter types and may be related to differences in chemical composition, as observed by the physical changes among leaf litters during decomposition. Both white oak and red maple exhibited skeletonization, whereas yellow poplar leaves just became increasingly softer. Similar observations have led Gessner et al. (1993) to demonstrate temporal successions of aquatic fungi as leaf litters decompose. Their findings are analogous to the successions observed on terrestrial leaf litter. Fungal communities on alder leaf litter were dominated by five to six species during early colonization at 2 weeks of incubation (Flagellospora curvula, Tetracahetum elegans, Lemonniera centrospharea, L. aquatica, and L. terrestris ). At 4 weeks, the species composition was more

Table 2.22 Decomposition constants (-k) of Leaf Litters in Three Streams of Differing pH and Temperature



Cumulative degree days

White oak -k


Yellow poplar -k (day"1)

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