Well‐documented in terrestrial settings, priming effects describe stimulated heterotrophic microbial activity and decomposition of recalcitrant carbon by additions of labile carbon. In aquatic settings, algae produce labile exudates which may elicit priming during organic matter decomposition, yet the directions and mechanisms of aquatic priming effects remain poorly tested. We tested algal‐induced priming during decomposition of two leaf species of contrasting recalcitrance, Liriodendron tulipifera and Quercus nigra, in experimental streams under light or dark conditions. We measured litter‐associated algal, bacterial, and fungal biomass and activity, stoichiometry, and litter decomposition rates over 43 days. Light increased algal biomass and production rates, in turn increasing bacterial abundance 141%–733% and fungal production rates 20%–157%. Incubations with a photosynthesis inhibitor established that algal activity directly stimulated fungal production rates in the short term. Algal‐stimulated fungal production rates on both leaf species were not coupled to long‐term increases in fungal biomass accrual or litter decomposition rates, which were 154%–157% and 164%–455% greater in the dark, respectively. The similar patterns on fast‐ vs. slow‐decomposing L. tulipifera and Q. nigra, respectively, indicated that substrate recalcitrance may not mediate priming strength or direction. In this example of negative priming, periphytic algae decoupled fungal activity from decomposition, likely by providing labile carbon invested towards greater fungal growth and reproduction instead of recalcitrant carbon degradation. If common, algal‐induced negative priming could stimulate heterotrophy reliant on labile carbon yet suppress decomposition of recalcitrant carbon, modifying energy and nutrients available to upper trophic levels and enhancing organic carbon storage or export in well‐lit aquatic habitats. plain language summary is available for this article.

Resource quantity and quality are fundamental bottom‐up constraints on consumers. Best understood in autotroph‐based systems, co‐occurrence of these constraints may be common but remains poorly studied in detrital‐based systems. Here, we used a laboratory growth experiment to test limitation of the detritivorous caddisfly larvae Pycnopsyche lepida across a concurrent gradient of oak litter quantity (food supply) and quality (phosphorus : carbon [P:C ratios]). Growth increased simultaneously with quantity and quality, indicating co‐limitation across the resource gradients. We merged approaches of ecological stoichiometry and co‐limitation theory, showing how co‐limitation reflected shifts in C and P acquisition throughout homeostatic regulation. Increased growth was best explained by elevated consumption rates and improved P assimilation, which both increased with elevated quantity and quality. Notably, C assimilation efficiencies remained unchanged and achieved maximum 18% at low quantity despite pronounced C limitation. Detrital C recalcitrance and substantive post‐assimilatory C losses probably set a minimum quantity threshold to achieve positive C balance. Above this threshold, greater quality enhanced larval growth probably by improving P assimilation toward P‐intensive growth. We suggest this interplay of C and P acquisition contributes to detritivore co‐limitation, highlighting quantity and quality as potential simultaneous bottom‐up controls in detrital‐based ecosystems, including under anthropogenic change like nutrient enrichment.

Stoichiometric homeostasis is a key concept of ecological stoichiometry theory that describes organisms’ ability to maintain internal elemental composition given variable resource elemental composition. Observational field studies are used frequently to examine homeostasis, and >½ of studies assessing homeostasis in the last 15 y were based on field data. We explored whether field surveys are appropriate to assess consumer homeostasis. We used examples from benthic macroinvertebrates to argue that most field data are inappropriate to assess homeostasis because investigators using field studies: 1) face difficulty accurately characterizing consumer and resource stoichiometry, 2) rarely use sufficient range and replication across discrete levels of resource stoichiometry, and 3) are challenged by great variation of consumer stoichiometry resulting from confounding factors (body size, ontogeny, sex, and others) across study sites. We used a simple modeling approach to show how ontogeny and tissue-spe...