The fate of nutrients that enter streams includes in situ uptake and storage, lateral transport to terrestrial systems, denitrification or downstream transport. Current models of nutrient effects suggest that higher loading increases in situ storage of nutrients through increased nutrient content and biomass of algae. However, many stream ecosystems are dominated by detrital, not algal, standing stocks. Nutrient enrichment reduces mass of detritus via increased microbial decomposition, thus lowering total nutrient storage. We measured annual and seasonal storage of carbon (C), nitrogen (N) and phosphorus (P) in detritus-based streams in response to experimental nutrient additions. We tested for net increases or decreases in annual storage of N or P in leaf litter vs algae, and whether nutrient storage responded more to N or P enrichment. Following a pretreatment year, we experimentally enriched five headwater streams for two years with high to low N (472-96 µg/L) and corresponding low to high P (10-85 µg/L) concentrations resulting in dissolved N:P ratios of 128:1 to 2:1. We quantified leaf litter standing stocks and algal biomass and their N and P content monthly. We used annual and seasonal response ratios of retained N and P in leaf litter to test enrichment effects.
Results/Conclusions
Nutrient enrichment reduced mean litter-associated nutrient storage (annual response ratio [RR] of P storage, 0.49-1.1; N storage, 0.34-1.0), and increased algae-associated storage (annual RR of P storage, 3.3-9.1, N storage, 3.4-7.5). The overall effect of nutrient enrichment on storage was negative, however, because leaf litter greatly dominated C stocks in these small forest streams. Phosphorus enrichment drove responses more than did N, due to greater effects of P on reductions in leaf litter storage. Negative effects of P on stream-scale N and P storage occurred despite the fact that P content of resources increased much more than N content due to nutrient enrichment. The effect of nutrients on nutrient storage varied seasonally, such that in the autumn and winter, there was greater storage of both N and P (mean RR P = 1.7, N = 1.6) compared to the pre-treatment year, while in the spring and summer storage was lower than under pretreatment conditions (mean summer RR P = 0.18, N = 0.14). Such amplified temporal variability in 1) resources available to consumers and 2) presumed leakage and loss to downstream ecosystems is important for managing and predicting in situ and downstream effects of nutrient enrichment on small stream networks.