Introduction

Leaf material is an important primary energy source in many northern temperate streams and rivers (Bärlocher & Kendrick, 1981). It is utilized by various macroinvertebrates which may be classified in terms of their feeding biology. Coarse particulate organic matter (CPOM; particle size > 1 mm) is utilized by a group described as the ‘shredders’. These macroinvertebrates begin the incorporation of leaf material into the food web both by consumption and by producing fine particulate organic matter (FPOM), in the form of leaf fragments and faeces. FPOM (particle size < 1 mm) is utilized by a group of macroinvertebrates described as collectors. Both shredders and collectors are utilized by predators (Cummins, 1973; Anderson & Sedell, 1979).

The utilization of leaf material by shredders is therefore important to the community as a whole. Leaf material that has recently entered freshwaters is of low food quality for macroinvertebrates, as soluble compounds such as sugars and amino acids are rapidly leached into the surrounding waters. What remains after leaching are refractory compounds such as cellulose, pectins and lignin (Nykvist, 1962; Petersen & Cummins, 1974). When leaf material enters freshwater it is rapidly colonized by microorganisms and, especially in the early stages of decomposition, fungi dominate. Although some terrestrial fungi will be present on leaf material as it enters water, they are poorly adapted to conditions attained in temperate freshwaters and therefore do not persist (Bärlocher & Kendrick, 1974). In contrast, aquatic hyphomycetes are well adapted to the freshwater environment.