Invertebrate drift is a key process in riverine ecosystems controlling aquatic invertebrate movement, distribution, and availability to fish as prey. However, accurately sampling drift across a wide range of invertebrate sizes is difficult because small invertebrates slip through coarse-mesh drift nets, and fine mesh clogs more easily, which reduces filtration efficiency and measurement accuracy. To avoid this limiting tradeoff, we developed a gas-powered drift pump which pours 20 m3/hour of river water through nested 80- and 750-m nets suspended in the air, and we tested it against a conventional 250-m drift net during low and high flows in a clearwater Alaskan river. The drift pump detected a geometric mean drift concentration of 467 invertebrates m-3 and maximum of 5637 m-3, eleven times the mean concentration of 42 m-3 from the drift net. Invertebrates 3 mm length, primarily chironomids, comprised the entire difference. Studies in which the drift of 0.5 – 3 mm invertebrates might be relevant, such as foraging models investigating the growth of juvenile drift-feeding fishes, should consider using similar methods to quantify small invertebrate drift, lest they underestimate it by an order of magnitude.
