An ecological study of a small New Zealand stream with particular reference to the Oligochaeta

The composition, temporal and spatial distribution, and productivity of profundal benthos were investigated in a Colorado Front Range reservoir which impounds water diverted from the Western Slope of the Rocky Mountains. Horsetooth Reservoir, 10.6 km × 1.0 km, consists of three basins with depths greater than 50 m connected by two equalizing channels ca. 30 m deep. Water quality parameters did not vary significantly between sites, but temperature, pH, and dissolved oxygen varied seasonally. The composition and organic content of sediment exhibited a gradient from inlet to outlet which significantly influenced faunal density and distribution patterns. Although 28 genera of macroinvertebrates were collected, the oligochaetes Tubifex tubifex (Müller) and Limnodrilus hoffmeisteri Claparède comprised 97.6% of the total organisms. Chironomids comprised 2.2%. The relative contribution of chironomids to total biomass decreased with increasing depth; the reverse was true for oligochaetes. Mean annual density ranged from 3,827 to 51,901 total organisms/m² for six sampling sites. Mean annual biomass varied from 0.16 to 2.3 g ash-free dry wt/m². Annual turnover ratios ranged from 3.6 to 4.5. Annual production estimates varied from 7.2 to 82.8 kg/ha ash-free dry weight, averaging 39.3 kg/ha or 26.9 kcal/m².