EFFECTS OF NATIVE AND EXOTIC FOREST ON BENTHIC STREAM BIOTA IN NEW-ZEALAND - A COLONIZATION STUDY

Algal biomass, microbial activity and invertebrate colonization were i nvestigated in 20 streams in the South Island, New Zealand. Sixteen st reams drained catchments with native or exotic forest and four were un shaded, non-forested sites. Algal biomass on stones was highest at the unshaded sites and was greater at forested sites east of the Alpine D ivide than at forested sites on the western side. Algal biomass on nut rient-diffusion substrata also showed significant location (east > wes t) and nutrient effects. However, responses to nutrient additions were variable among stream groups, with unshaded and eastern native-forest streams showing the strongest response. Abundances of invertebrates ( mainly Chironomidae) colonizing diffusion substrata were positively co rrelated with algal biomass in eastern native forest streams and unsha ded streams but not the other treatments. Microbial activity, expresse d as loss in weight of cellulose cloth over a three-week period, was u naffected by location/vegetation type but increased significantly in r esponse to nutrient additions.