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

Authors: Taylor, Jason; KING, RYAN - Baylor University; PEASE, ALLISON - Texas Tech University; WINEMILLER, KIRK - Texas A&M University

Submitted to: Freshwater Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2013
Publication Date: 2/6/2014
Publication URL: http://handle.nal.usda.gov/10113/58916
Citation: Taylor, J.M., King, R.S., Pease, A.A., Winemiller, K.O. 2014. Nonlinear response of stream ecosystem structure to low-level phosphorus enrichment. Freshwater Biology. 59(5)969-984.

Interpretive Summary: Water resource managers seek quantitative nutrient criteria that are protective of freshwater environments. We sampled algae and fish from 38 central Texas streams representing a wide range of nutrient pollution with the goal of identifying minimum amounts of nutrient enrichment that result in big changes in freshwater communities. Our results indicate that water concentrations for total phosphorus should not exceed 21 µg L-1 in order to prevent negative shifts in the nutrient chemistry and species composition of algae, as well as significant declines in two sensitive fish species. Proliferation of weedy alga species and increased abundance of invasive fishes are likely when surface-water concentrations exceed this phosphorus limit. These changes can be interpreted as threshold responses to nutrient enrichment and provide useful management criteria for preventing degradation of high quality streams associated with phosphorus enrichment.

Technical Abstract: 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.