Leaf litter provides an important nutrient subsidy to headwater streams, but little is known about how tree genetics influence energy pathways from litter to higher trophic levels. Despite the charge to quantify carbon (C) and nitrogen (N) pathways from decomposing litter, the relationship between litter decomposition and aquatic consumers remains unresolved. We measured litter preference (attachments to litter), C and N assimilation rates, and growth rates of a shredding caddisfly (Hesperophylax magnus, Limnephilidae) in response to leaf litter of different chemical and physical phenotypes using Populus cross types (P. fremontii, P. angustifolia, and F hybrids) and genotypes within P. angustifolia. We combined laboratory mesocosm studies using litter from a common garden with a field study using doubly labeled litter ( C and N) grown in a greenhouse and incubated in Oak Creek, Arizona, USA. We found that, in the lab, shredders initially chose relatively labile (low lignin and condensed tannin concentrations, rapidly decomposing) cross type litter, but preference changed within 4 d to relatively recalcitrant (high lignin and condensed tannin concentrations, slowly decomposing) litter types. Additionally, in the lab, shredder growth rates were higher on relatively recalcitrant compared to labile cross type litter. Over the course of a three-week field experiment, shredders also assimilated more C and N from relatively recalcitrant compared to labile cross type litter. Finally, among P. angustifolia genotypes, N assimilation by shredders was positively related to litter lignin and C:N, but negatively related to condensed tannins and decomposition rate. C assimilation was likewise positively related to litter C:N, and also to litter %N. C assimilation was not associated with condensed tannins or lignin. Collectively, these findings suggest that relatively recalcitrant litter of Populus cross types provides more nutritional benefit, in terms of N fluxes and growth, than labile litter, but among P. angustifolia genotypes the specific trait of litter recalcitrance (lignin or tannins) determines effects on C or N assimilation. As shredders provide nutrients and energy to higher trophic levels, the influence of these genetically based plant decomposition pathways on shredder preference and performance may affect community and food web structure.

Summary Whole‐stream metabolism has been estimated by measuring in‐stream oxygen (O2) concentrations since the method was introduced over 50 years ago. However, the influence of measurement location and estimation method on metabolism rates is understudied. We examined how the placement of O2 probes (i.e. depth, separation from the thalweg), differences in methodology (1‐station, 2‐station, area‐weighted) and reach lengths influenced estimated rates of whole‐stream metabolism in a tallgrass prairie watershed. Metabolism estimates made in the thalweg differed from estimates made in backwaters due to disconnection in flow, and estimates made in deep pools differed from surface estimates due to thermal stratification (temporary flow disconnection). The 1‐station respiration estimates differed from short 2‐station reach‐scale estimates (c. 20 m) but were more similar to larger 2‐station reach‐scale estimates (c. 100 m). In contrast, the 1‐station gross primary production was most similar to the short 2‐station reaches occurring immediately upstream and became less similar at longer 2‐station reach lengths. The different estimation methodologies (1‐station, 2‐station, area‐weighted) accounting for the longest reach scale did not result in different metabolism rates. The temporary phenomena of thermal stratification of stream pools during a warm day, which disconnected pool bottoms from the surface waters, likely affected not only the pool estimates but also estimates made in the downstream thalweg (i.e. an O2 deficit accrued from respiration during the day in the bottom of the pool abruptly moved downstream during mixing). Oxygen probe placement mattered and affected rate estimates according to habitat type and reach length (i.e. scale) due to the influence of small‐scale heterogeneity on community respiration. Selection of reach length can be critical for studies depending on whether local heterogeneity is of interest or should be averaged. We conclude that the intuitive use of thalwegs and reaches that are at least 10 times the stream width are likely appropriate for whole‐stream metabolism estimates, although the exact reach length necessary and potential stream‐specific characteristics, such as stratified pools, need to be carefully considered in probe placement. We encourage other studies to report the placement characteristics of O2 probes in streams as well as consider the potential confounding factor of local habitat heterogeneity.

Ecologists often equate litter quality with decomposition rate. In soil and sediments, litter that is rapidly decomposed by microbes often has low concentrations of tannin and lignin and low C:N ratios. Do these same traits also favor element transfer to higher trophic levels in streams, where many insects depend on litter as their primary food source? We test the hypothesis that slow decomposition rates promote element transfer from litter to insects, whereas rapid decomposition favors microbes. We measured carbon and nitrogen fluxes from four plant species to a leaf‐shredding caddisfly using isotopically labeled litter. Caddisflies assimilated a higher percentage of litter carbon and nitrogen lost from slowly decomposing litters (Platanus wrightii and Quercus gambelii). In contrast, rapidly decomposing litters (Fraxinus velutina and Populus fremontii) supported higher microbial biomass. These results challenge the view that rapidly decomposing litter is higher quality by demonstrating that slowly decomposing litters provide a critical resource for insects.

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