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

Abstract

Summary 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 μg L−1; TN: 127–15 860 μg L−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 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 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 18–48 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, two 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 that a numerical criterion for surface water TP of c. 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. 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.

Diatom Assemblage Changes in Agricultural Alluvial Plain Streams and Application for Nutrient Management

“…The TP concentrations at the time of diatom collection were higher and the range was smaller compared with minimum and maximum values from previous studies. Previous regional studies from North America and Europe reported significant changes in stream algal assemblage structure at TP concentrations ranging between 0.01 and 0.032 mg L −1 (Schneider and Lindstrøm, 2011; Chambers et al, 2012; Smucker et al, 2013, Taylor et al, 2014). It is entirely plausible that if MAP streams representing reference conditions could be located and included in our stressor–response gradient, diatom responses to shifts in TP concentration would occur at much lower levels of enrichment.…”

“…A wide range of studies have established the utility of diatom‐based metrics and species composition as sensitive response measures to nutrient pollution in streams, but few studies have addressed such relationships in habitat‐limited, low‐gradient streams draining large‐river alluvial plains (Kelly and Whitton, 1995; Stevenson et al, 2008; Smucker et al, 2013; Taylor et al, 2014). The current study identified a significant shift in diatom assemblage structure at TP concentrations of ∼0.125 mg L −1 .…”

Long-Term Monitoring Data Provide Evidence of Declining Species Richness in a River Valued for Biodiversity Conservation

“…Stable nitrogen isotope ratios of herbivorous consumers in the Conasauga mainstem downstream from the national forest also show evidence of nitrogen enrichment from agricultural sources (Sharpe and Nichols 2007;Baker 2012). Other studies have found changes in fish community structure associated with increased nutrients (Evans-White et al 2013;Taylor et al 2014), although mechanisms are unclear. However, elevated nutrient loading may be responsible for changes in water clarity and benthic habitats that we have observed in the Conasauga River.…”

Low‐to‐moderate nitrogen and phosphorus concentrations accelerate microbially driven litter breakdown rates

“…In streams, nutrient inputs affect consumers via changes in the quantity, quality, and composition of algal resources (Evans-White et al 2009, Taylor et al 2014. In streams, nutrient inputs affect consumers via changes in the quantity, quality, and composition of algal resources (Evans-White et al 2009, Taylor et al 2014.…”

“…Studies in which nitrogen (N) and phosphorus (P) effects on detrital carbon dynamics (specifically effects on litter mass loss rates) were tested typically used single-concentration combinations of N and P (Gulis and Suberkropp 2003, Greenwood et al 2007, Benstead et al 2009) or landscape or experimental gradients in N or P (P, Rosemond et al [2002]; N, Ferreira et al [2006]; N and P, Pascoal et al [2003], Woodward et al [2012]). However, dissolved N and P concentrations typically covary across land-use gradients (e.g., Taylor et al 2014) and the relative importance and interactive effects of N and P on detrital processing rates are less clear. However, dissolved N and P concentrations typically covary across land-use gradients (e.g., Taylor et al 2014) and the relative importance and interactive effects of N and P on detrital processing rates are less clear.…”