Drift distance of macroinvertebrates throughout summer in headwater tributaries of the Calapooia River

Abstract

Drifting invertebrates from small headwater streams are a food subsidy for fishes and other downstream consumers in larger streams. This subsidy can become especially important for stream fishes during summer due to high metabolic demand caused by elevated water temperatures. The source length in small streams contributing these drifting invertebrates to fish bearing habitat has not been established. We conducted an experiment to determine if summer drift delivered by headwater streams to larger channels is influenced by contributions originating more than 100 m upstream. We conducted this experiment on tributaries of the Calapooia River, Oregon, USA. We blocked drift with 250 μm mesh nets in three streams 100 m upstream of our sample nets continuously for 48 d. We also sampled five streams without blocking nets. Four 24 h samples of drift were collected prior to the placement of blocking nets and every two weeks afterwards at each stream. Overall mean abundance and mean biomass of drift did not differ between blocked and unblocked streams for any sampling period. There was a temporal trend at all sites of declining invertebrate abundance and biomass. We found no treatment effect for any of the most common insect orders or for the 16 genera that comprised at least 1% of the total organisms captured. The majority of invertebrate drift is of local origin in headwater streams during summer baseflow.