BENTHIC ECOLOGY OF A SPRING-FED RIVER OF INTERIOR ALASKA

1. A massive aquifer between the Gerstle, Tanana and Delta rivers in interior Alaska receives water from them and from smaller streams that flow from the Granite Mountains in the Alaska Range. Groundwater from the aquifer intersects the surface in a mid-sized (20 m(3) s(-1) +/- 10%) spring-fed stream, Clearwater Creek. 2. Mean annual air temperature is about -2.6 degrees C. However, even in winter when air temperature often reaches -40 degrees C, the stream does not form a complete ice cover. Water temperature ranges from 0 to 7.8 degrees C. Specific conductance and the concentrations of major ions vary little throughout the year, and summed ionic salinity exceeds 250 mg l(-1) 3. Benthic algal standing crop (as chlorophyll a) was at least an order of magnitude higher than that in a nearby surface-water stream, the upper Chena River. Standing crop peaked in spring and autumn (about 20 mg m(-2)) and averaged about half this value, although biomass of an early spring bloom of Hydrurus foetidus was underestimated. 4. Algal standing crop was inversely related to the concentrations of inorganic nitrogen and orthophosphate-phosphorus in the water column. The ratio of total nitrogen to total phosphorus (as mass concentrations) was always about 30. Measurements of primary production made in Clearwater Creek were among the highest reported for streams in subarctic Alaska. 5. Macroinvertebrate diversity in Clearwater Creek was low. Numbers of 'morpho-species' in monthly Surber samples (0.09 m(2)) averaged nine, and ranged from three to fourteen. However, benthos and drift densities were similar to those reported from other Alaskan streams. In early spring and autumn, drifting macroinvertebrates were primarily Ephemeroptera, Plecoptera and Trichoptera, but in summer, Diptera dominated the drift. The low diversity of macroinvertebrates is hypothesized to be a consequence of the small annual range in water temperature and the relatively constant discharge of Clearwater Creek.