The Relationship between Nutrient Concentration and Stream Insect Abundance
Darcie L. Quamme and Patrick A Slaney
Abstract.—The relationship between added soluble nutrient concentration and the abundance and taxonomic composition of stream insects was determined in an experiment using streamside troughs. Target phosphorus (P) concentrations were 0, 0.5, 2.5, 5, 10, and 50 µg/L at a N:P ratio of 1:1 (wt.:wt.). All treatments were replicated three times except 50 µg/ L, which was unreplicated. Peak algal biomass (PB) increased with nutrient concentration linearly to 7.4 mg/m2 at 2.5 mg P/L and reached an asymptote at 9.2 mg/m2 (2.7× the controls) at 10 µg P/L. Adult baetid mayflies increased 2- and 4-fold when caught in drift nets and emergent insect traps, respectively, at a phosphorus concentration of 10 µg/L compared with controls. Numbers of benthic baetids, nemourid, and perlodid stoneflies and hydroptilid and polycentripodid trichopterans increased 1.6, 2.3, 2.9, 2.8, and 1.2-fold, respectively, at 10 µg P/L compared with controls. Adult and nymphal baetids and benthic nemourids, perlodids, and hydroptilids initially increased rapidly at nutrient concentrations of 0–2.5 µg P/L and reached asymptotes at concentrations of 2.5–10 µg P/L. Exclusion of insects from a single unfertilized trough suggested that grazing limited peak biomass of periphyton to low levels. Increased abundances of aquatic insects resulted from greater periphyton availability at relatively low dissolved-nutrient additions ranging from 0.5 to 10 µg P/L.