Litter P content drives consumer production in detritus‐based streams spanning an experimental N:P gradient

Abstract

Ecological stoichiometry theory (EST) is a key framework for predicting how variation in N:P supply ratios influences biological processes, at molecular to ecosystem scales, by altering the availability of C, N, and P relative to organismal requirements. We tested EST predictions by fertilizing five forest streams at different dissolved molar N:P ratios (2, 8, 16, 32, 128) for two years and tracking responses of macroinvertebrate consumers to the resulting steep experimental gradient in basal resource stoichiometry (leaf litter %N, %P, and N:P). Nitrogen and P content of leaf litter, the dominant basal resource, increased in all five streams following enrichment, with steepest responses in litter %P and N:P ratio. Additionally, increases in primary consumer biomass and production occurred in all five streams following N and P enrichment (averages across all streams: biomass by 1.2×, production by 1.6×). Patterns of both biomass and production were best predicted by leaf litter N:P and %P and were unrelated to leaf litter %N. Primary consumer production increased most in streams where decreases in leaf litter N:P were largest. Macroinvertebrate predator biomass and production were also strongly positively related to litter %P, providing robust experimental evidence for the primacy of P limitation at multiple trophic levels in these ecosystems. However, production of predatory macroinvertebrates was not related directly to primary consumer production, suggesting the importance of additional controls for macroinvertebrates at upper trophic positions. Our results reveal potential drivers of animal production in detritus-based ecosystems, including the relative importance of resource quality vs. quantity. Our study also sheds light on the more general impacts of variation in N:P supply ratio on nutrient-poor ecosystems, providing strong empirical support for predictions that nutrient enrichment increases food web productivity whenever large elemental imbalances between basal resources and consumer demand are reduced.

Experimental N and P additions alter stream macroinvertebrate community composition via taxon‐level responses to shifts in detrital resource stoichiometry

“…We chose to present biomass rather than abundance because accumulation of biomass represents an important ecosystem function and is therefore a more functionally explicit metric than abundance alone. Furthermore, changes in biomass observed in this study are largely a function of altered abundance rather than the effect of nutrient enrichment on consumer body size (Demi et al, ). As such, analyses performed with biomass and abundance data produced similar results and interpretation.…”

“…We predicted that N and P fertilization, and subsequent shifts in basal resource stoichiometry, would alter the taxonomic structure of macroinvertebrate communities in the study streams by differentially affecting individual consumer populations based upon principles of EST. Overall, we expected a greater response to reduced resource C:P, resulting from increased stream water P concentrations (Demi, Benstead, Rosemond, & Maerz, ) than to reduced C:N, due to the greater relative imbalances between body and resource C:P for most consumers in detritus‐based headwater streams (Cross, Benstead, Rosemond, & Wallace, ).…”

“…As such, we hypothesized that leaf litter C:P would be a significant driver of overall patterns of community composition, given its characteristically high C:P (Cross et al, ; Martinson et al, ), generally greater response to elevated nutrient concentrations than other common resource pools (Tant et al, ), and large role in driving patterns of invertebrate production in forested streams (Cross et al, ; Demi et al, ; Wallace, Eggert, Meyer, & Webster, ). Furthermore, we predicted that leaf litter C:P, but not C:N, would be a significant driver of shredder biomass, both at the assemblage‐level and among common shredder taxa, and that among those taxa, those with low body C:P would exhibit greater responses to nutrient enrichment than those with high body C:P (due to their larger elemental imbalances under ambient conditions).…”

Detrital nutrient content and leaf species differentially affect growth and nutritional regulation of detritivores

“…Nutrient enrichment increases detrital microbial biomass and nutrient contents, improving detritivore consumption rates, assimilation efficiencies, and growth rates (Danger et al 2013, Jochum et al 2017, Halvorson et al 2017b). Recent work further suggests detritivore responses to detrital P content are stronger than those to N content, suggesting animal growth and regulation may be more sensitive to resource P than N enrichment (Demi et al 2018).…”

Simulating rewetting events in intermittent rivers and ephemeral streams: A global analysis of leached nutrients and organic matter

“…Our data serve also as a basis for further upscaling and modeling of the processes observed in the laboratory to address ecological implications of rewetting events at catchment scales. Potential implications for the functioning of rivers could be determined by the effect of leached substances on the degree of nutrient limitation of microorganisms downstream, and therefore community composition (Demi, Benstead, Rosemond, & Maerz, ) as well as on the fate of refractory substances and intensification of their decomposition through the so‐called “priming effect” (Guenet, Danger, Abbadie, & Lacroix, ). The results of our study support the recent call for developing effective strategies for the management of IRES to avoid negative consequences for downstream ecosystems caused by excessive nutrient and OM load.…”