Effects of native and exotic forest on benthic stream biota in New Zealand: A colonization study

Algal biomass, microbial activity and invertebrate colonization were investigated in 20 streams in the South Island, New Zealand. Sixteen streams drained catchments with native or exotic forest and four were unshaded, non-forested sites. Algal biomass on stones was highest at the unshaded sites and was greater at forested sites east of the Alpine Divide than at forested sites on the western side. Algal biomass on nutrient-diffusion substrata also showed significant location (east > west) 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 correlated with algal biomass in eastern native forest streams and unshaded streams but not the other treatments. Microbial activity, expressed as loss in weight of cellulose cloth over a three-week period, was unaffected by location/vegetation type but increased significantly in response to nutrient additions.