LEAF-LITTER PROCESSING OF ALDER AND EUCALYPTUS IN THE AGUERA STREAM SYSTEM (NORTHERN SPAIN) .2. MACROINVERTEBRATES ASSOCIATED

The macroinvertebrate fauna associated with leaf breakdown of Alnus gl utinosa and Eucalyptus globulus was studied at four sires of the Aguer a stream basin (Northern Spain). Experiments were carried out during a utumn 1988-winter 1989, by using litter bags. Differences in density o f colonizing macroinvertebrates were observed depending on site and li tter type. Alder leaves registered higher densities of invertebrates t han eucalyptus leaves at headwater sites, where eucalyptus leaves do n or enter the stream naturally (plantations of this species are absent in these reaches). There were no differences in density between both t ypes of litter in downstream reaches, where nutrient enrichment of lea ves occurred and the bags were colonized more abundantly than in headw aters. Some of the main taxa such as Echinogammarus sp., Leuctra spp. and Nemouridae colonized more abundantly the alder litter than that of eucalyptus. Both shredders and collectors showed higher densities in alder than eucalyptus leaves upstream, but these differences between l itter type disappeared downstream. As a response to the environmental changes imposed by the stream gradient, functional groups of macroinve rtebrates associated with litter bags changed. Shredders were practica lly absent, from the lowest site, while collectors significantly incre ased downstream. Shredders feeding on litter of high quality (alder) i n headwaters, and enhancement of microbial activity in downstream reac hes help to explain decomposition processes of the studied materials.