Abstract  Many streams within the United States are impaired due to nutrient enrichment, particularly in agricultural settings. The present study examines the response of benthic algal communities in agricultural and minimally disturbed sites from across the western United States to a suite of environmental factors, including nutrients, collected at multiple scales. The first objective was to identify the relative importance of nutrients, habitat and watershed features, and macroinvertebrate trophic structure to explain algal metrics derived from deposition and erosion habitats. The second objective was to determine if thresholds in total nitrogen (TN) and total phosphorus (TP) related to algal metrics could be identified and how these thresholds varied across metrics and habitats. Nutrient concentrations within the agricultural areas were elevated and greater than published threshold values. All algal metrics examined responded to nutrients as hypothesized. Although nutrients typically were the most important variables in explaining the variation in each of the algal metrics, environmental factors operating at multiple scales also were important. Calculated thresholds for TN or TP based on the algal metrics generated from samples collected from erosion and deposition habitats were not significantly different. Little variability in threshold values for each metric for TN and TP was observed. The consistency of the threshold values measured across multiple metrics and habitats suggest that the thresholds identified in this study are ecologically relevant. Additional work to characterize the relationship between algal metrics, physical and chemical features, and nuisance algal growth would be of benefit to the development of nutrient thresholds and criteria.

Abstract  The Red River is a transboundary, multijurisdictional basin where water-quality standards are often different across state lines. The state agencies with USEPA Region VI focused resources to organize water-quality data from within this basin and have it statistically analyzed to evaluate the relationships between nutrients and sestonic chlorophyll-a (chl-a). There were 152 sites within the Red River basin that had nutrient and sestonic chl-a measurements; these sites were narrowed down to 132 when a minimum number of observations was required. Sestonic chl-a levels increased with increasing nutrient concentrations; these regressions were used to predict nutrient concentrations at 10 µg chl-a L-1. Total nitrogen (TN) and phosphorus (TP) concentrations (at 10 µg chl-a L-1) varied across the Red River basin and its ecoregions from 0.10 to 0.22 mg TP L-1 and 0.75 to 2.11 mg TN L-1. Nutrient thresholds were also observed with sestonic chl-a at 0.14 mg TP L-1 and 0.74 mg TN L-1 using categorical and regression tree analysis (CART). The CART analysis also revealed that hierarchical structure was important when attempting to predict sestonic chl-a from TN, TP, and conductivity. The ranges of TN and TP concentrations that resulted in chl-a concentrations that exceeded 10 µg chl-a L-1 were similar in magnitude to the threshold in TN and TP that resulted in increased sestonic chl-a. This corroborating evidence provides useful guidance to the states with jurisdiction within the Red River basin for establishing nutrient criteria, which may be similar when the Red River and its tributaries cross political boundaries.

Abstract  1. Algal-community metrics were calculated for periphyton samples collected from 976 streams and rivers by the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Programme during 1993-2001 to evaluate national and regional relations with water chemistry and to compare whether algal-metric values differ significantly among undeveloped and developed land-use classifications.

2. Algal metrics with significant positive correlations with nutrient concentrations included indicators of trophic condition, organic enrichment, salinity, motility and taxa richness. The relative abundance of nitrogen-fixing algae was negatively correlated with nitrogen concentrations, and the abundance of diatom species associated with high dissolved oxygen concentrations was negatively correlated with both nitrogen and phosphorus concentrations. Median algal-metric values and nutrient concentrations were significantly lower at undeveloped sites than those draining agricultural or urban catchments.

3. Total algal biovolume did not differ significantly among major river catchments or land-use classifications, and was only weakly correlated with nitrate (positive) and suspended-sediment (negative) concentrations. Estimates of periphyton chlorophyll a indicated an oligotrophic-mesotrophic boundary of about 21 mg m(-2) and a mesotrophic-eutrophic boundary of about 55 mg m(-2) based on upper and lower quartiles of the biovolume data distribution.

4. Although algal species tolerance to nutrient and organic enrichment is well documented, additional taxonomic and autecological research on sensitive, endemic algal species would further enhance water-quality assessments.

Abstract  We used in situ nutrient-enrichment experiments at sites in 4 Texas streams spanning a steep nutrient gradient to test the hypothesis that periphyton photoautotrophs and heterotrophs are differentially limited by N and P, respectively. We contrasted the responses of total periphyton biomass (ash-free dry mass, [AFDM]), photoautotrophic biomass (chlorophyll a [CHLA], % CHLA of AFDM [% CHLA]), alkaline phosphatase activity (APA), and nitrogenase activity (NA) to ambient N and P concentrations among sites and among nutrient-diffusing substrata (NDS) (control, N, P, and N+P) within sites. Periphyton biomass (AFDM and CHLA) increased steadily with increasing ambient N and P among sites. Both APA and NA were relatively high on the control treatments at the lowest nutrient site and decreased with increasing ambient N and P until falling to negligible levels at the highest nutrient site. Within sites, P enrichment decreased APA but did not increase % CHLA or CHLA/cm(2). P enrichment did increase AFDM/cm(2) at one of the low nutrient sites. N enrichment reduced NA and increased % CHLA and CHLA/cm(2) without increasing AFDM/cm(2). N+P enrichment resulted in the highest % CHLA and CHLA/cm(2) at the 3 lowest nutrient sites but did not increase AFDM/cm(2). Combined, these findings suggest that photoautotrophs in periphyton communities may be more limited than heterotrophs by N or become more competitive for P under elevated N. The high levels of APA and NA on the controls coupled with minimal difference in AFDM among nutrient treatments at most sites despite increases in % CHLA on N and N+P treatments provide experimental evidence that differential nutrient limitation probably occurs in periphyton communities. Our results support the view that the single-limiting-nutrient paradigm may lead to overly simplistic interpretations of periphyton responses to nutrient enrichment.

Abstract  During the summers of 2005 and 2006 the U.S. Geological Survey, in cooperation with the Texas Commission on Environmental Quality, evaluated nutrient and biological conditions in small streams in parts of the Edwards Plateau of Central Texas. Land-cover analysis was used to select 15 small streams that represented a gradient of conditions with the potential to affect nutrient concentrations across the study area, which comprises two of four subregions of the Edwards Plateau ecoregion. All 15 streams were sampled for water properties, nutrients, algae, benthic invertebrates, and fish in summer 2005, and eight streams were resampled in summer 2006. Streams that did not receive wastewater effluent had relatively low nutrient concentrations and were classified as oligotrophic; streams receiving wastewater effluent had relatively high nutrient concentrations and were classified as eutrophic. Nutrient concentrations measured in the least-disturbed streams closely matched the U.S. Environmental Protection Agency nutrient criteria recommendations based on estimated reference concentrations. Nitrogen/phosphorus ratios indicated streams not affected by wastewater effluent might be limited by phosphorus concentrations. Algal indicators of nutrient condition were closely related to dissolved nitrogen concentrations and streamflow conditions. Ambient dissolved nitrogen concentrations (nitrite plus nitrate) were positively correlated with benthic algal chlorophyll-a concentrations. The correlation of benthic algal chlorophyll-a with instantaneous nitrite plus nitrate load was stronger than correlations with ambient nutrients. Increased nutrient concentrations were associated with increased macroalgae cover, wider diel dissolved oxygen ranges, and reduced diel dissolved oxygen minimums. Benthic invertebrate aquatic life use scores generally were classified as High to Exceptional in study streams despite the influence of urbanization or wastewater effluent. Reductions in aquatic life use scores appeared to be related to extremely low flow conditions and the loss of riffle habitats. Benthic invertebrate aquatic life use scores and several of the metrics used to compute composite aquatic life use scores tended to increase with increasing total nitrogen concentrations. Fish community aquatic life use scores generally were classified as High or Exceptional with the exception of a few samples collected from streams receiving wastewater effluent that were classified as Intermediate. Fish community aquatic life use scores and several fish community metrics were positively correlated with nutrient concentrations and macroalgae cover. The majority of the positive correlations among nutrient concentrations, macroalgae cover, and fish metrics were strongly influenced by relatively high nutrient concentrations. Both benthic and planktonic chlorophyll-a measures were related to nutrients, but this study indicates that benthic chlorophyll-a was the better choice for monitoring nutrient enrichment because (1) the relation between benthic chlorophyll-a and nutrients was stronger, and (2) a strong relation between benthic chlorophyll-a and nutrients persisted after removal of the sites influenced by wastewater effluent, which indicates superior ability of benthic chlorophyll-a to discriminate between conditions at lower nutrient concentrations. The transect-based algal abundance estimate technique is a useful tool for identifying eutrophic conditions, assessing nuisance algal growth, and making broad comparisons among sites, but it appears to lack the fine resolution to identify lesser degrees of nutrient enrichment. Several individual benthic invertebrate and fish metrics were correlated with nutrient conditions, but correlations were generally positive and the reverse of what would be expected when nutrient enrichment causes a proliferation of algal growth and stream degradation. However, the benthic invertebrate functional feeding group metrics showed some