Excretion from the benthic macrofauna covers little of spring nutrient uptake in a small Danish forest stream

Abstract

ABSTRACT Freshwater consumers play a role in ecosystem nutrient cycling and dynamics by excreting metabolic waste products containing ammonium (NH4) and phosphate (PO4). In this study we quantified excretion of a selection of macroinvertebrate taxa and measured nutrient uptake metrics in a small Danish forest stream. Macroinvertebrate excretion was measured in situ from February to April 2019, and uptake metrics were measured using short-term nutrient addition experiments on 2 dates in March and June. To calculate area-specific excretion, quantitative benthic samples were taken on the 3 dominating substratum types: sand, gravel-pebbles, and leaf packs. No significant temperature dependency of invertebrate excretion was found over the measured range (0.3–10.3 °C). Average NH4-N excretion rates ranged from 0.027 (Ptychoptera spp.) to 0.097 µg mg−1 h−1 (Paraleptophlebia submarginata) while PO4-P excretion ranged from 0.0077 (Ptychoptera spp. and Nemoura spp.) to 0.0261 µg mg−1 h−1 (P. submarginata). The uptake length increased from 75 to 117 m for NH4 and from 28 to 71 m for PO4 from March to June. Uptake velocity, by contrast, decreased from March (0.63 m h−1 for NH4 and 1.72 m h−1 for PO4) to June (0.13 m h−1 for NH4 and 0.22 m h−1 for PO4). The estimated contribution from excretion from the entire benthic macrofauna to whole-stream net uptake was 3.4% and 8.9% of NH4 and 0.5% and 2.0% of PO4 in March and June, respectively, suggesting that excretion from benthic macroinvertebrates plays a minor role in meeting whole-stream nutrient demand.