Effect of pH on microbial degradation of leaf litter in seven streams of the English Lake District.

Rates of degradation of alder, oak and grass leaf packs with associated microbial populations were measured in seven streams pH 6.8-4.9. Streams were chosen from upland and lowland sites of the same river for contrasts in pH, water chemistry and riparian vegetation. The most important factor governing rates of degradation is the physical and chemical nature of the leaf material. At pH 6.8 rates of degradation, 'k', and microbial colonization were higher than at pH [Formula: see text]5.5: 'k' on alder x6; on oak x2; on grass x2. At lowland sites, pH 6.8, higher decay rates were associated with high levels of microbial colonization including c.14 spp of aquatic hyphomycete fungi-regardless of riparian vegetation. Decay rates were similar at upland sites, pH 6.8 and 6.6, involving high levels of colonization by fewer fungal species and fewer bacteria-regardless of riparian vegetation-though grass was barely degraded at upland sites of any pH. At pH[Formula: see text]5.5, slow decay rates were associated with low levels of microbial colonization and few fungal species. Largest microbial populations at low pH associated with riparian trees did not lead to markedly increased decay rates. Factors of water chemistry at low pH appear to inhibit microbial metabolism. The implications of these findings for stream invertebrates active in the winter is discussed.