Responses of detrital pathways to nutrients may differ fundamentally from pathways involving living plants: basal carbon resources can potentially decrease rather than increase with nutrient enrichment. Despite the potential for nutrients to accelerate heterotrophic processes and fluxes of detritus, few studies have examined detritus-nutrient dynamics at whole-ecosystem scales. We quantified organic matter (OM) budgets over three consecutive years in two detritus-based Appalachian (U.S.A.) streams. After the first year, we began enriching one stream with low-level nitrogen and phosphorus inputs. Subsequent effects of nutrients on outputs of different OM compartments were determined using randomized intervention analysis. Nutrient addition did not affect dissolved or coarse particulate OM export but had dramatic effects on fine particulate OM (FPOM) export at all discharges relative to the reference stream. After two years of enrichment, FPOM export was 340% higher in the treatment stream but had decreased by 36% in the reference stream relative to pretreatment export. Ecosystem respiration, the dominant carbon output in these systems, also increased in the treatment stream relative to the reference, but these changes were smaller in magnitude than those in FPOM export. Nutrient enrichment accelerated rates of OM processing, transformation, and export, potentially altering food-web dynamics and ecosystem stability in the long term. The results of our large-scale manipulation of a detrital ecosystem parallel those from analogous studies of soils, in which net loss of organic carbon has often been shown to result from experimental nutrient addition at the plot scale. Streams are useful model systems in which to test the effects of nutrients on ecosystem-scale detrital dynamics because they allow both the tracking of OM conversion along longitudinal continua and the integrated measurement of fluxes of transformed material through downstream sites.

Abstract. Nutrient enrichment affects bacteria and fungi associated with detritus, but little is known about how biota associated with different size fractions of organic matter respond to nutrients. Bacteria dominate on fine (,1 mm) and fungi dominate on coarse (.1 mm) fractions, which are used by different groups of detritivores. We measured the effect of experimental nutrient enrichment on fungal and bacterial biomass, microbial respiration, and detrital nutrient content on benthic fine particulate organic matter (FPOM) and coarse particulate organic matter (CPOM). We collected FPOM and CPOM from 1 reference and 1 enriched stream. CPOM substrates consisted of 2 litter types with differing initial C:nutrient ratios (Acer rubrum L. and Rhododendron maximum L.). Fungal and bacterial biomass, respiration, and detrital nutrient content changed with nutrient enrichment, and effects were greater on CPOM than on FPOM. Fungal biomass dominated on CPOM (,99% total microbial biomass), whereas bacterial biomass dominated on FPOM (,95% total microbial biomass). These contributions were unchanged by nutrient enrichment. Bacterial and fungal biomass increased more on CPOM than FPOM. Respiration increased more on CPOM (up to 300% increase) than FPOM (,50% increase), indicating important C-loss pathways from these resources. Microbial biomass and detrital nutrient content were positively related. Greater changes in nutrient content were observed on CPOM than on FPOM, and changes in detrital C:P were greater than changes in detrital C:N. Threshold elemental ratios analyses indicated that enrichment may reduce P limitation for shredders and exacerbate C limitation for collector-gatherers. Changes in CPOMdominated pathways are critical in predicting shifts in detrital resource quality and C flow that may result from nutrient enrichment of detritus-based systems.

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Summary We assessed the key role of aquatic fungi in modifying coarse particulate organic matter (CPOM) by affecting its breakdown rate, nutrient concentration and conversion to fine particulate organic matter (FPOM). Overall, we hypothesised that fungal‐mediated conditioning and breakdown of CPOM would be accelerated when nutrient concentrations are increased and tested the degree to which fungi were critical to CPOM processing and to FPOM production by an invertebrate consumer. We manipulated the presence and absence of fungi, exogenous nutrients [nitrogen (N) and phosphorus (P)] and an invertebrate consumer in a full‐factorial laboratory experiment and quantified their effects on CPOM mass loss and nutrient concentration, and the quantity and nutrient concentration of FPOM produced during leaf breakdown. Mass of CPOM lost and the quantity of FPOM produced were highest in nutrient‐enriched treatments containing fungal decomposers. Across all treatments, FPOM produced was a constant proportion of CPOM lost (67%). Loss of CPOM due to shredders was highest at fungal biomass values >16 mg g dry mass−1, which occurred with nutrient enrichment. Nitrogen concentration in CPOM increased in treatments with nutrients and fungi, while CPOM P concentration was primarily affected by nutrients. In contrast, FPOM P concentration declined in treatments with fungi and nutrients, suggesting either sequestration via CPOM‐associated fungi or preferential assimilation by shredders. Nitrogen in FPOM increased with nutrients, but was unaffected by fungi. Our results indicate that aquatic fungi play a critical role in facilitating energy and nutrient flow through detrital pathways and that their ability to mediate organic matter transformations is significantly influenced by nutrient enrichment.

Many North American blackwater rivers exhibit low dissolved O 2 (DO) that may be the result of benthic respiration. We examined how tree species affected O 2 demand via the quantity and quality of litter produced. In addition, we compared areal estimates of surface leaf-litter microbial respiration to sediment O 2 demand (SOD) and ecosystem respiration (ER) in stream and swamp reaches of a blackwater river to quantify contributions of surface litter decomposition to O 2 demand. Litter inputs averaged 917 and 678 g m −2 y −1 in the swamp and stream, respectively. Tree species differentially affected O 2 demand via the quantity and quality of litter produced. Bald cypress (Taxodium distichum) contributed most litter inputs because of its dominance and because it produced more litter per tree, thereby making greater relative contributions to O 2 demand in the swamp. In the stream, water oak (Quercus nigra) produced litter supporting lower fungal biomass and O 2 uptake rates, but produced more litter than red maple (Acer rubrum). Breakdown rates in the swamp were faster, whereas standing stock decreases were lower than in the stream, indicating greater organic matter retention. Surface litter microbial respiration accounted for 89% of SOD (6.37 g O 2 m −2 d −1 ), and 57 to 89% of ER in the swamp. Our findings suggest that surface litter drives the majority of O 2 demand in some blackwater swamps, and tree species with higher rates of litterfall may make larger contributions to ER. Forested swamps may be hotspots of O 2 demand in blackwater rivers because low water velocities enhance retention.