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

Jason M. Taylor, Ryan S. King, Allison A. Pease, Kirk O. Winemiller
Nonlinear response of stream ecosystem structure to low-level phosphorus enrichment 2014 v.59 no.5 pp. 969-984
invasive species, nutrient content, nitrogen content, surface water, nitrogen, nutrient availability, carp, pastures, wastewater treatment, Cyprinus carpio, environmental factors, carbon, Etheostoma, minnows, nonpoint source pollution, Cyprinella lutrensis, periphyton, ecosystems, streams, phosphorus
Anthropogenic inputs of nitrogen (N) and phosphorus (P) create novel environmental conditions that alter biological organization and ecosystem functioning in freshwaters. We studied 38 wadeable streams spanning an N and P gradient to contrast responses of algal and fish assemblages to nutrient enrichment. Surface-water total P (TP) and total N (TN) were correlated across our study sites, but TP 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 µg L-1 TP and 500 - 1000 µg L-1 TN; beyond these values, ratios exhibited minimal additional decline. Algae assemblage structure was strongly linked to surface water TP, TN, periphyton nutrient ratios, 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 µg L-1 (90% CI of 18 to 48 µg L-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, 2 benthic fish species, Etheostoma spectabile and Campostoma anomalum, significantly declined with TP > 28 µg L-1 (90% CI, 24 - 56 µg L-1) and 34 µg L-1 (90% CI, 21 - 56 µg L-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 a numerical criterion for surface-water TP of approximately 20 µg L-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. Managers should consider potential low-level enrichment effects when developing criteria for ecosystems that have evolved under low levels of nutrient availability.