EFFECTS OF AGRICULTURAL-DEVELOPMENT ON PROCESSING OF TUSSOCK LEAF-LITTER IN HIGH COUNTRY NEW-ZEALAND STREAMS

1. We compare the rates and mechanisms of processing of tussock (Chion ochloa spp.) leaf litter in six New Zealand streams draining grassland catchments that contrast in the extent to which they have been develo ped for pasture. 2. Rates of processing, measured as rate of weight lo ss of leaf packs and rate of leaf softening, were at the slow end of t he spectrum for vascular plant processing. Processing was faster at de veloped sites, mediated mainly through the influence of oxidized nitro gen concentration on microbial activity. 3. Few invertebrate shredders colonized leaf packs and it is unlikely that invertebrates had an app reciable effect on leaf processing in our study streams, which do not effectively retain leaf litter. Very small headwater tributaries appea r to retain leaf litter and possess a more abundant shredder community . 4. Measures of leaf processing in our six streams were significantly correlated with Petersen's (1992) RCE score of stream condition. We d iscuss the potential for using rate of leaf litter processing as a met hod of bioassessment. 5. Even the most degraded stream in our study is classed as 'good' using the RCE inventory system. Human impact in the Taieri River is relatively small compared with the degradation observ ed in some parts of the world.