BENTHIC COMMUNITIES OF FOREST STREAMS IN THE SOUTH ISLAND, NEW-ZEALAND - EFFECTS OF FOREST TYPE AND LOCATION

Algal biomass, standing stocks of particulate organic matter (POM) and benthic invertebrate communities of 20 streams draining native forest , exotic conifer forest and tussock grassland catchments in the South Island, New Zealand, were examined in spring/summer 1994-95. Sixteen w ere forest streams, 8 in native and 8 in exotic forest, with 4 of each kind located on each side of the Main Alpine Divide and therefore in different climatic (including rainfall) zones. Chemistry of stream wat er reflected geographic location more strongly than forest type with w estern streams being slightly more acid and having lower ionic concent rations than those in the east. Algal biomass on stones in forest stre ams ranged from 7-51 mg chi . m(-2), and regardless of forest type, ap peared to be limited primarily by light and substrate stability. POM s tanding stocks were low (22-254 g AFDW m(-2)) and dominated by coarse materials (>13 mm) in both native and exotic forest streams. Benthic f aunas of all stream groups were dominated numerically by Chironomidae and Ephemeroptera and contained similar numbers of species (35-40). Ho wever, differences in community composition were found between eastern and western streams, and these were greater than between forest types . Total invertebrate abundances were moderately low (2008-6083 m(-2)) and were significantly correlated with epilithic algal biomass. In con trast, correlations between invertebrate numbers and detrital biomass were weak, perhaps reflecting poor retention of riparian inputs. Our r esults support the view that forest type has little effect on stream b enthic communities in the South Island, New Zealand, and that geograph ical factors (climate, location) are of greater importance.