Nonlinear response of stream ecosystem structure to low-level phosphorus enrichment

Anthropogenic inputs of nitrogen (N) and phosphorus (P) create environmental conditions that alter biological organisation and ecosystem functioning in fresh waters. We studied 38 wadeable streams spanning a N and P gradient to contrast responses of algal and fish assemblages to nutrient enrichment. Surface water total P (TP) and total N (TN) concentrations represented a wide range (TP: 7-2380 mu gL(-1); TN: 127-15 860 mu gL(-1)) and were correlated across our study sites. Total P explained significantly more variance in periphyton carbon (C)-to-nutrient (C:P, C:N) and N:P ratios than TN. Abrupt, nonlinear declines in these ratios were observed between 20 and 50 mu gL(-1) TP and 500-1000 mu gL(-1) TN; beyond these values, ratios exhibited minimal additional decline. Algae assemblage structure was strongly linked to surface water TP, TN and catchment-scale nutrient sources (wastewater treatment plant (WWTP) discharges and % pasture cover). In particular, there were synchronous declines in frequency and cell densities of many alga species associated with TP concentrations >21 mu gL(-1) (90% CI of 18-48 mu gL(-1)) as well as simultaneous increases in tolerant species associated with increasing enrichment. Fish assemblage structure was most strongly associated with % pasture, WWTP discharges and fine sediment cover, yet also showed significant but weaker correlations with surface water and periphyton nutrient variables. However, two benthic fish species, Etheostoma spectabile and Campostoma anomalum, significantly declined with TP >28 mu gL(-1) (90% CI, 24-56 mu gL(-1)) and 34 mu gL(-1) (90% CI, 21-56 mu gL(-1)), respectively. Conversely, the tolerant minnow Cyprinella lutrensis and invasive carp Cyprinus carpio increased nonlinearly with increasing surface water TP. Our results provide new insights into interpretation and analysis of assemblage-level responses to nutrient enrichment. Our findings indicate that a numerical criterion for surface water TP of c. 20 mu gL(-1) would be needed to maintain natural algae assemblages and at least two specialist fishes within our study region. Proliferation of weedy alga species and increased abundance of invasive fishes are also likely when surface water concentrations exceed these thresholds. While many streams likely exceed these thresholds, managers should consider potential low-level enrichment effects when developing criteria for ecosystems to protect existing nutrient-limited streams.