Abstract  N and P often limit primary and secondary production in ecosystems, but they also can cause eutrophication and negatively influence sensitive species above a certain level or threshold point. Aquatic biodiversity can have negative threshold relationships with water-quality variables at large scales, but the specific mechanism(s) driving these threshold relationships are not well established. We hypothesized that resource quality (i.e., C:P) might partly drive primary consumer (grazer and detritivore) richness thresholds by altering competitive interactions among species with differing resource demands, but might have less influence on predator richness. We estimated total N (TN), total P (TP), and turbidity thresholds for macroinvertebrate richness across trophic levels and feeding groups in Central Plains (USA) streams. We also determined if mean taxon body C:P of groups with diversity losses were negatively related to TP, a pattern that would suggest that eutrophic communities were dominated by a few species with high dietary P demands. Primary consumers were more sensitive to TN and TP (threshold mean = 1.0 mg N/L and 0.06 mg P/L) than secondary consumers (threshold mean = 0.09 mg P/L), a result supporting the resource quality hypothesis. Turbidity reduced richness regardless of feeding mode (threshold mean = 4.7 NTU), a result suggesting that turbidity and nutrient thresholds were driven by different factors. The TP-richness threshold could be driven partially by changes in food quality because the mean body C:P of shredding and collector-gathering taxa declined as TP increased (threshold mean = 0.07 and 0.75 mg P/L, respectively). Mean scraper C:P was not related to TP, a result indicating other factors might be responsible for the scraper richness threshold. Our results suggest that changes in resource quality could contribute to large-scale losses in biodiversity in nutrient-enriched lotic ecosystems. Within shredder and collector-gatherer macroinvertebrate feeding groups, P-rich food might allow faster growing taxa with high body P demands to out-compete slower growing taxa adapted to lower quality food resources. This pattern suggests that biotic integrity is directly linked to nutrients in streams and that toxicity, low dissolved O(2), and increased turbidity might not be the only mechanisms leading to reductions in diversity as nutrient concentrations increase.

Abstract  During August through September of 1993- 95, 83 periphyton samples were collected at 51 stream sites in the Ozark Plateaus. These sites were categorized into six land-use categories (20 forest, 18 agriculture, 10 mining, 1 urban, 1 urban/ mining, and 1 mix), based on land-use percentages in the basin upstream from the site. Results indicate that periphyton communities of riffles of Ozark streams are affected by natural and land-use related factors. These factors include nutrients, dissolved organic carbon, alkalinity, canopy shading, suspended sediment, embeddedness, stream morphometry, and velocity. For several measures of periphyton communities, statistically significant (p<0.05) differences were found among sites assigned to agriculture, forest, and mining categories. Blue-green algae biovolume, relative abundance of blue-green algae, relative biovolume of diatoms, relative abundance of oligotrophic algae, relative abundance of tolerant taxa, and condition index values were among the measures that differed among land-use categories. Although no environmental factors were significantly correlated with total biovolume, several factors were significantly correlated with biovolume of blue-green algae or biovolume of diatoms. Biovolume of blue-green algae was correlated with percent agriculture land use. Biovolume of diatoms was correlated with orthophosphate, total phosphorus, alkalinity, velocity, embeddedness, and dissolved organic carbon. Diatoms often composed the largest percentage of the biovolume (relative biovolume). Diatom relative biovolume was much higher at mining sites (generally 75 to 90 percent of the total biovolume) than at forest or agriculture sites (generally 15 to 80 percent) and was correlated with several factors, including many land-use related factors. The diatoms Cymbella affinis and Cymbella delicatula and the blue-green algae Calothrix often were the most common (relative abundance and relative biovolume) algae in samples. Detrended correspondence analysis (DCA) and hierarchical cluster analyses results indicated differences among land-use category sites. The DCA results were correlated with a number of land-use related factors and channel morphometry. Grazers (specifically, snails and stonerollers) are related to periphyton biovolume and community composition. Total periphyton and diatom biovolume typically were highest at sites where snail density was lowest. Lower relative abundances of diatoms usually occurred at sites with higher snail densities and stoneroller relative abundances.

Abstract  Water resource managers face increasing challenges in identifying what physical and chemical stressors are responsible for the alteration of biological conditions in streams. The objective of this study was to assess the comparative influence of multiple stressors on benthic diatoms at 98 sites that spanned a range of stressors in an agriculturally dominated region in the upper Midwest, USA. The primary stressors of interest included: nutrients, herbicides and fungicides, sediment, and streamflow; although the influence of physical habitat was incorporated in the assessment. Boosted Regression Tree was used to examine both the sensitivity of various diatom metrics and the relative importance of the primary stressors. Percent Sensitive Taxa, percent Highly Motile Taxa, and percent High Phosphorus Taxa had the strongest response to stressors. Habitat and total phosphorous were the most common discriminators of diatom metrics, with herbicides as secondary factors. A Classification and Regression Tree (CART) model was used to examine conditional relations among stressors and indicated that fine-grain streams had a lower percentage of Sensitive Taxa than coarse-grain streams, with Sensitive Taxa decreasing further with increased water temperature (>30 °C) and triazine concentrations (>1500 ng/L). In contrast, streams dominated by coarse-grain substrate contained a higher percentage of Sensitive Taxa, with relative abundance increasing with lower water temperatures (<29 °C) and shallower water depth (<0.3 m). Quantile regression indicated that maximum water temperature appears to be a major limiting factor in Midwest streams; whereas both total phosphorus and percent fines showed a slight subsidy-stress response. While using benthic algae for assessing stream quality can be challenging, field-based studies can elucidate stressor effects and interactions when the response variables are appropriate, sufficient stressor resolution is achieved, and the number and type of sites represent a gradient of stressor conditions and at least a quasi-factorial design. © 2017

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  Nutrients and grazers both can regulate benthic algal structure and function in streams, but the relative strength of each factor depends on stream biotic and abiotic conditions. The abundance of stream organisms and nutrient availability can change rapidly after a flood. Thus, nutrient and grazer influences on algal development and how these drivers interact may vary temporally during recovery. We measured benthic structural and functional development for 35 d after a simulated flood in large outdoor mesocosms under a gradient of 6 nutrient loadings crossed with 6 densities of grazing fish (Southern redbelly dace, Phoxinus erythrogaster). Nutrients influenced algal development more than dace did and were better correlated with algal function (area-specific primary productivity and nutrient uptake) than with structure (biomass). Dace influenced all structural variables and biomass-specific gross primary productivity, but their influence was relatively weak and was observed only early in recovery. Dace influence weakened and nutrient influence strengthened during recovery. Understanding context-dependent relationships in postdisturbance community dynamics is essential for predicting ecosystem responses to future changes in nutrient inputs and biodiversity, particularly in systems, such as headwater streams, with frequent disturbance.