INCREASING LITTER RETENTION IN MOORLAND STREAMS - ECOLOGICAL AND MANAGEMENT ASPECTS OF A FIELD EXPERIMENT

1. The retention characteristics of two moorland streams in mid-Wales were manipulated for 2 years by the addition of small traps which accu mulated detritus. Leaf litter was also added to these essentially tree less streams at regular intervals to simulate natural inputs to a wood land stream. 2. Leaf traps retained a significantly higher biomass of detritus - both naturally occurring and added - than either the surrou nding stream bed or unmanipulated reference sections. They also suppor ted significantly higher numbers of nemourid, leuctrid, tipulid and el minthid insect larvae. 3. Among functional feeding groups, detritivore s and, to a lesser extent, predators responded to increased detritus a vailability. Numbers of mayfly larvae were low in leaf packs, suggesti ng a negative effect of detritus aggregations on their numbers. 4. The taxa which responded positively to increased detrital biomass, partic ularly stonefly larvae, are known to be tolerant of low pH conditions, whereas those affected detrimentally are generally absent from acid w aters. It is proposed therefore that increasing detrital inputs and li tter retention in culturally acidified upland streams may serve to inc rease their invertebrate productivity.