The effects of land use on the relationships among denitrification, NO₃⁻-N, dissolved organic C (DOC), and other environmental parameters were examined in 9 headwater streams (3 each in forested, agricultural, and urban-dominated subwatersheds) in the Kalamazoo River Watershed, Michigan. Sediment denitrification rates were determined using the chloramphenicol-amended acetylene inhibition technique. Agricultural streams had high concentrations of NO₃⁻-N, DOC, soluble reactive P, and NH₄ -N, whereas forested streams had the lowest concentrations of these nutrients, and urban streams generally exhibited intermediate concentrations. Sediment denitrification rates were highest in agricultural streams and lowest in forested streams throughout the study period. Availability of NO₃⁻-N was the dominant environmental predictor of sediment denitrification rates, limiting denitrification when NO₃⁻-N concentrations were below a calculated threshold of 0.4 mg NO₃⁻-N/L. Other potential controlling variables (e.g., DOC, dissolved O₂, water temperature, and sediment organic matter content) influenced denitrification rates secondarily. Despite higher denitrification rates in agricultural and urban streams compared to forested streams, sediment denitrification removed a smaller proportion of the stream NO₃⁻-N load in agricultural and urban streams relative to forested streams.

We measured the attenuation of ultraviolet B (UVB) and ultraviolet A (UVA) radiation in 32 streams located within the Ontonagon River watershed on the Upper Peninsula of Michigan, USA. Attenuation coefficients (K_d) of UVB and UVA ranged widely among these streams, but generally translated into relatively shallow 1% transmission depths into the water column (2–45 cm for UVB and 6–103 cm for UVA). Both K_{d UVB} and K_{d UVA} were positively correlated with stream dissolved organic C concentration (DOC, range 2–35 mg C/L). Absorbance coefficients of dissolved matter (a_d) of UVB and UVA also were strongly correlated with DOC. K_{d UVA} (but not K_{d UVB}) was weakly related to the concentration of particulate organic C and DOC molar absorptivity. DOC-specific K_{d UVB} was, on average, higher in streams of our study compared to previously published values from lakes and wetlands. We developed a statistical model that predicts UVB flux to benthic organisms. The model incorporates information on water depth, DOC concentration, surface reflectance, and forest canopy cover. This stream-UVB model (SUM) predicts very low UVB flux to the benthic areas of most wetland and forested streams of this region during cloudless, midsummer days. Overall, our results suggest a low likelihood that stream organisms in this region are normally exposed to high levels of ultraviolet radiation because shading is provided by both stream DOC and forest canopy.

Nutritional constraints on herbivores are important factors structuring food webs. Our study presents field data on the nutrient content of benthic invertebrates from Lake Erken (Sweden). The most abundant benthic invertebrates of the littoral community were sampled in 3 seasons at 5 sites to examine the influence of spatial and temporal variation of abiotic conditions on invertebrate C:N:P stoichiometry. The relationships between nutrient ratios and dry mass (DM) of invertebrates were evaluated using regression analysis. Nutrient ratios of periphyton, sampled in an earlier study, were compared to those of benthic invertebrates. C:N, C:P, and N:P ratios differed among invertebrate taxa. Coleoptera had relatively high C:N, C:P, and N:P ratios, whereas Isopoda had low C:N, C:P, and N:P ratios. Nutrient ratios differed with respect to season and location. C:P and N:P ratios were significantly lower and C:N ratios were significantly higher in autumn than in spring and summer. C:P and N:P ratios increased as a functions of invertebrate DM when all taxa were analyzed together. C:N and C:P ratios of periphyton were higher than those of benthic invertebrates, but N:P ratios were similar. Our results suggested that benthic invertebrates have taxon-specific C:N and C:P ratios, but the seasonal variability in ratios indicated that littoral invertebrates were rheostatic rather than homeostatic with respect to nutrient stoichiometry. The temporal and spatial variation in grazer nutrient content must be taken into account when applying stoichiometric theory to benthic food webs.

Important ecological services of low-order streams are greatly affected by urbanization. North Buffalo Creek, in the headwaters of the Cape Fear River basin in Greensboro, North Carolina, receives point- and nonpoint-source pollutants. Natural abundances of the stable isotopes of C (¹³C) and N (¹⁵N) were used to determine the influence of anthropogenic nutrients on seston δ¹⁵N, nutrient concentrations, C/N ratios, and patterns of δ¹³C and δ¹⁵N in foodweb components in North Buffalo Creek during different hydrological conditions. Baseflow seston δ¹⁵N varied significantly among sampling sites. Baseflow seston δ¹⁵N ranged from 3.7‰ to 4.6‰ at forested sites and was slightly enriched at open sites, and probably reflected nonpoint sources of N in North Buffalo Creek. Seston δ¹⁵N also reflected point sources of N in North Buffalo Creek. The most enriched seston δ¹⁵N values (8.4‰) were found directly downstream of the Waste Water Treatment Plant (WWTP). Seston δ¹⁵N values at the Rankin Mill Road (Rankin) site, several km downstream of the WWTP, also were strongly influenced by effluent from the WWTP. The Summit Avenue site (Summit) received textile effluent until June 2001. Before June 2001, seston ¹⁵N at Summit was depleted compared to seston ¹⁵N at sites upstream of the plant, probably because the textile effluent was depleted in ¹⁵N. During storms, seston δ¹⁵N was negatively correlated with nutrient concentrations upstream of the WWTP. However, at Rankin Mill Road, seston δ¹⁵N was positively correlated with NO₃⁻ flux, which explained 54% of the variation in seston δ¹⁵N. δ¹⁵N was not correlated with NH₄ and PO₄⁻³ fluxes at Rankin Mill Road. During storms, seston δ¹⁵N was influenced partially by nonpoint sources of N, a pattern consistent with observed C/N ratios. δ¹³C values for most foodweb components and δ¹⁵N values for all foodweb components varied significantly between sites, suggesting that sources of C and N differed among sites. Foodweb components at Rankin were enriched in ¹⁵N from incorporation of sewage-derived N from the WWTP effluent, whereas, prior to June 2001, foodweb components at Summit appeared to be depleted in ¹⁵N from incorporation of textile effluent. Thus, specific influences of point sources of N could be distinguished in foodweb components. Nonpoint sources and stormwater influenced seston δ¹⁵N during storm events, but these sources could not be distinguished in consumers by using natural abundances of stable isotopes.

Benthic macroinvertebrate families were sampled along 3 rivers in New Brunswick, Canada. Stable isotopes of C and N were compared between body tissue and gut contents of individuals. δ¹³C and δ¹⁵N of body tissue and gut contents were strongly correlated (r = 0.94 and 0.93, respectively) over a wide range of δ values. In nonpredators, only minor fractionation of δ¹³C and δ¹⁵N was observed. In predators, diet–tissue fractionation of ¹³C was minor, but ¹⁵N fractionation that may have been related to diet quality (N content) was observed. The influence of diet quality on N-isotope fractionation was inconsistent in direction and strength among families. Our results suggest that subjecting primary consumers to gut clearance prior to processing for stable-isotope analysis is unnecessary, but the guts of predators should be removed before processing.

Invasion of littoral zones by adventive macrophyte species can facilitate major changes in the ecology of lakes. In Lake Wanaka, a large alpine New Zealand lake, the macrophytes Lagarosiphon major and Elodea canadensis (Hydrocharitaceae) have invaded parts of the lake where they form tall dense plant beds throughout mid-depths (2–7 m) of the littoral zone. We investigated differences in plants, benthic invertebrates, fish, and food webs characterizing native and exotic plant beds in mid-depths of the littoral zone. The 3× higher plant biomass and 2× higher plant surface area in exotic than in native plant beds (quillworts, milfoils, and charophytes) contributed to greater standing stocks and productivity of epiphyton in the exotic plant beds. Invertebrate communities were less dense (1890/m² vs 4030/m²) and less diverse (richness = 9 vs 12) in native than in exotic plant beds because of differences in biomass, productivity, and physical structure of native and exotic plant communities. Invertebrate communities in native beds were dominated by snails, oligochaetes, and nematodes, whereas chironomids, snails, and caddisflies were dominant in exotic beds. Stable isotope signatures (¹³C and ¹⁵N) and dietary analyses indicated that Potamopyrgus antipodarum, the dominant invertebrate taxon in both bed types, consumed mostly epiphyton. In native beds, consumption of sedimentary fine benthic organic matter by oligochaetes and nematodes made significant contributions to C flow, whereas, in exotic beds, consumption of epiphyton by grazers (e.g., snails, caddisflies, and chironomids) was an important pathway for C flow. Macrophytes made only small contributions to C flow in either bed type. The dominant native fish in Lake Wanaka, the bully Gobiomorphus cotidianus, was more abundant in exotic than in native beds, but bully predation rates on snails were significantly lower in exotic than in native beds. Invasion by adventive macrophyte species can cause significant shifts in lake productivity, species composition, and foodweb dynamics.

The relative contribution of microfauna (bacteria and protozoa), and metazoa (meiofauna and macrofauna) to profundal benthic production in Lake Brunnsee was examined in a 1-y study. Abundance of micro-, meio-, and macrofauna was determined and converted to biomass. Annual production was calculated using the [³H]-thymidine incorporation method for bacteria and allometric equations for protozoa and metazoa. Bacteria had the highest biomass and production (1.5 g C/m² and 27 g C m⁻² y⁻¹, respectively). Protozoa had the highest P/B ratio, with an average biomass of 0.1 g C/m² and an annual production of 22 g C m⁻² y⁻¹. Meiobenthos and macrobenthos had biomasses of 0.1 and 1.2 g C/m², respectively, and production estimates of 1.6 g m⁻² y⁻¹ and 3.1 to 6.7 g m⁻² y⁻¹, respectively, depending on the method used for calculation. Although protozoan biomass was small in Lake Brunnsee sediments, production estimates suggest that they may represent an important component of the benthic community.

We determined whether chemosensory selection of detritus by crayfish is affected by past experience and is related to the nutritional quality of the food and subsequent growth on that food. These questions were addressed with a long-term growth assay and with short-term behavioral assays of chemotaxis and chemical preference. A 10-mo growth experiment was conducted in which crayfish (Procambarus clarkii) were reared under different growth conditions, each consisting of a different diet. Crayfish were fed 1 of 3 types of detritus: leaf litter from Populus tremuloides grown under ambient (AMB)- or elevated (ELEV)-CO₂ conditions (which altered the nutritional quality and defensive chemistry of leaf litter) or fish (FISH). Carapace length (CL), mass, molt frequency, and % mortality were recorded. Crayfish reared on the AMB diet had decreased relative growth rate (RGR) and molt frequency and increased % mortality compared to those reared on the ELEV diet, whereas crayfish reared on the FISH diet had higher RGR and molt frequency than those reared on either type of litter diet. C, N, and macromolecular composition of crayfish tissues were determined after all growth assays were complete. Percent C, % N, % protein, and % lipids were lower, and % carbohydrates was higher for crayfish reared on the ELEV diet than for crayfish reared on the AMB or FISH diets, corroborating the results of the growth experiment. Before tissue analysis, crayfish from each growth condition were tested in a Y-maze to determine whether the diet experienced during the growth experiment affected foraging decisions. Crayfish from each growth condition were offered AMB or ELEV detritus prepared as: 1) fresh leaf litter, 2) leaf litter leached in water for 24 h, and 3) leachate from the litter in the form of gelatin cubes. Within each type of detritus preparation, the preferences of crayfish were tested for pairwise combinations of AMB detritus, ELEV detritus, and a no-stimulus control (CON). Preferences were linked to growth variables. Crayfish reared on AMB and ELEV detritus preferred AMB detritus, but those reared on FISH showed no preference for either AMB or ELEV detritus relative to CON. Detrital diets during rearing did not affect food preferences, suggesting that nutritional quality may influence detritus selection and that selective feeding may carry a growth benefit.

We evaluated the sampling efficacy of 1-m² throw traps (active sampler) and baited minnow traps (passive sampler) across an experimental density gradient (1, 2, 4, 6, 8, 10, and 15/m²) of the slough crayfish (Procambarus fallax) in 2 trials with different crayfish populations. In both trials, throw-trap density estimates were highly correlated with actual crayfish density (r² = 0.96). The form of the relationships between density estimates and stocked densities was consistent between trials, and indicated that throw traps captured a similar proportion of the stocked crayfish regardless of the stocked density. When we adjusted the relationships to account for clearing efficiency (proportion of captured animals actually recovered from the trap), the slopes of the regressions were not significantly different from 1 in either trial. Size distributions and sex ratios of crayfish collected by the throw traps accurately reflected those of the stocked populations. Baited minnow traps performed inconsistently between the 2 trials. Catch-per-unit-effort (CPUE) and density were significantly correlated only in Trial 2 (r² = 0.82). The slope of the regression in Trial 2 (0.621) was significantly <1, and the intercept was positive and nearly significant (p = 0.074), indicating that minnow traps captured increasingly smaller proportions of the stocked crayfish as the stocked density increased (i.e., differences between CPUE values underestimated actual differences between stocked densities along the gradient). Minnow traps were biased toward capturing large male crayfish, but the form of the relationships between CPUE and density did not improve when large-male CPUE was used in the regressions. Our results suggest that 1-m² throw traps provide better estimates than baited minnow traps of crayfish densities in shallow vegetated habitats.

Two-phase sampling provides a statistical framework for combining data from qualitative and quantitative sampling methods. It is a useful approach when the survey objective is a drainage-wide estimate of mussel density and the cost of qualitative sampling is small relative to the cost of quantitative sampling. This survey design has several advantages including: 1) no need for a priori classification of stream reaches into sampling strata, 2) allocation of sampling effort so that more time is spent sampling where mussels are at higher density and less time is spent sampling where mussels are not present, 3) the ability to evaluate the relationship between qualitative and quantitative estimates of density, and 4) efficient allocation of effort so that more stream reaches can be surveyed compared to quantitative-only sampling designs with similar effort. The survey design consists of sampling as many sites as possible during a qualitative 1^st phase and sampling far fewer sites during a quantitative 2^nd phase. In 1995, we used a 2-phase sampling design to estimate the distribution and abundance of freshwater mussels in riffle habitat in the Cacapon River, West Virginia. Our estimate of river-wide surface density of freshwater mussels in riffles was 0.59/m² (SE = 0.14). We used resampling simulation based on our data and determined that the most effective 2^nd-phase sampling strategy was to sample a low to moderate proportion of low-density sites and a high proportion of high-density sites.

The Asian clam (Corbicula fluminea) co-occurs with unionid mussels in many riverine ecosystems. Clam populations can reach high densities and may undergo rapid die-offs, particularly during the low flow and warm temperatures of summer drought. Our study objective was to determine whether ammonia produced by decaying clam tissues during die-offs could affect unionid mussels. We induced C. fluminea die-offs in artificial streams to simulate die-off effects in natural habitats, and measured total and unionized ammonia (NH₃-N) and dissolved oxygen (DO) in the overlying water. When water flow was stopped, reductions in DO preceded the onset of mortality in streams having the highest density of clams (∼10,000 clams/m²). NH₃-N concentrations in the overlying water increased in association with clam mortality and reached concentrations of up to 5.04 mg/L at 26 ± 2°C. Temperature significantly influenced the rate of DO reduction and NH₃-N production in the systems, while resumption of water flow led to rapid dissipation of NH₃-N from the water column. We also conducted laboratory experiments to determine median lethal concentrations (LC₅₀s) of total ammonia and NH₃-N for glochidia and juveniles of the unionid mussel Villosa iris, adults of the unionid Pyganodon grandis, and juveniles and adults of C. fluminea. The 96-h LC₅₀s (24-h for V. iris glochidia) for NH₃-N ranged from 0.11 mg/L for V. iris glochidia to 0.8 mg/L for adult C. fluminea, indicating that NH₃-N levels produced by Asian clam die-offs have the potential to exceed acute effects levels for at least some species of unionid mussels.

The Asian clam (Corbicula fluminea) occurs in most of the southeastern US, often sharing habitat with native unionid mussels. Clam populations can reach high densities and, under conditions of low water flow and warm summer temperatures, may experience rapid die-offs. Clams are infaunal, so the interstitial zone may be subject to elevated levels of ammonia and reductions in dissolved oxygen (DO) that could affect organisms such as native mussels that also use this habitat. We conducted laboratory experiments to characterize concentrations of total ammonia and unionized ammonia (NH₃-N) produced in the sediment pore water and in overlying water as a result of clam die-offs. Sediment porewater NH₃-N concentrations ranged between 0.013 and 5.56 mg/L, levels that were consistently higher than NH₃-N concentrations in the overlying water. Levels of NH₃-N in both pore water and overlying water were positively correlated with temperature and density of clams involved in the die-offs. NH₃-N concentrations in chambers maintained at 28°C were 5.56 mg/L, ∼20× levels in chambers maintained at 19°C. Increasing clam density from 200 to 1000 individuals/m² resulted in an increase in porewater NH₃-N from 0.17 to 0.55 mg/L. NH₃-N concentrations in some tests exceeded acutely toxic levels for some species of unionid mussels (0.022 to 5.56 mg/L). DO was always lower in pore water (2.01 to 6.74 mg/L) than in overlying water (5.02 to 8.67 mg/L) in chambers containing dead Asian clams, and low DO could have further exacerbated stress associated with exposure to NH₃-N. Overall, our results indicate that NH₃-N production and DO reductions associated with Asian clam die-offs could pose a risk to unionid mussels, particularly during warm low-flow summer months.

Adult aquatic insects are important energy subsidies for terrestrial predators, but the effects of terrestrial predators on emerged aquatic insects have been widely neglected. We compared emergence of aquatic insects from predator-free exclosures and open cages to test the hypothesis that riparian arthropod predators can reduce the abundances of emerged aquatic insects. We used emergence traps over the aquatic and terrestrial sides of the shoreline to collect insects that emerged from the water or crawled onto land to emerge. The abundances and taxonomic composition of emerged aquatic insects and riparian arthropod predators changed seasonally. Riparian arthropods consumed 45% of emerged aquatic insect biomass from terrestrial traps in spring and 45% from aquatic traps in summer. The dominant riparian predator at the time of emergence determined the specific predation effect. Stoneflies that emerged into terrestrial traps were significantly reduced when ground beetles were the most abundant predators; caddisflies that emerged into aquatic traps were significantly reduced when spiders were the most abundant predators. Thus, taxon-specific predation by riparian arthropods can affect the taxonomic composition of emerged aquatic insects.

Forested depressional wetlands are an important seasonal wetland type across eastern and central North America. Macroinvertebrates are crucial ecosystem components of most forested depressional wetlands, but community compositions can vary widely across the region. We evaluated variation in macroinvertebrate faunas across eastern and central North America using 5 published taxa lists from forested depressional wetlands in Michigan, Ontario, Wisconsin, Florida, and Georgia. We supplemented those data with quantitative community descriptions generated from 17 forested depressional wetlands in South Carolina and 74 of these wetlands in Minnesota. Cluster analysis of presence/absence data from these 7 locations indicated that distinct macroinvertebrate communities existed in northern and southern areas. Taxa characteristic of northern forested depressional wetlands included Sphaeriidae, Lumbriculidae, Lymnaeidae, Physidae, Limnephilidae, Chirocephalidae, and Hirudinea (Glossophoniidae and/or Erpodbellidae) and taxa characteristic of southern sites included Asellidae, Crangonyctidae, Noteridae, and Cambaridae. Quantitative sampling in South Carolina and Minnesota indicated that regionally characteristic taxa included some of the most abundant organisms, with Sphaeriidae being the 2^nd most abundant macroinvertebrate in Minnesota wetlands and Asellidae being the 2^nd most abundant macroinvertebrate in South Carolina wetlands. Mollusks, in general, were restricted to forested depressional wetlands of northern latitudes, a pattern that may reflect a lack of Ca needed for shell formation in acidic southern sites. Differences in community composition probably translate into region-specific differences in the ecological functions performed by macroinvertebrates in forested depressional wetlands.

Our study investigated whether algae-based water-quality assessments are affected by differences between algal assemblages on hard substrates (rocks, wood) and soft substrates (fine-grained sediments). We analyzed a US Geological Survey National Water-Quality Assessment (NAWQA) program data set that consisted of 1048 pairs of samples collected from hard and soft substrates at 551 river sampling locations throughout the US. Biovolume and diversity of algal assemblages, biovolume of major taxonomic groups, and abundance of motile diatoms differed significantly between samples collected from hard and soft substrates at the same sites. Ordinations of assemblages from hard and soft substrates were highly concordant and provided similar information on environmental gradients underlying species patterns. The strengths of relationships between composition of algal assemblages and water chemistry parameters (conductivity, pH, total P, and total N) did not differ consistently between substrate types. Performance of weighted averaging (WA) inference models did not differ between models based on assemblages from hard and soft substrates. Moreover, the predictive power of inference models developed from single-substrate data sets was not reduced when these models were applied to samples collected from other substrates. We concluded that the choice of substrate to sample should depend on the assessment indicators to be used. If indicators based on the autecologies of many algal taxa (e.g., inference models or autecological indices) are used, restricting samples to a single type of substrate is unnecessary. If algal diversity, total algal biovolume, or abundance of specific algal taxa is used, samples should be collected from a single type of substrate.

We demonstrated the use of statistical tolerance intervals as a method for deriving acceptable thresholds for benthic macroinvertebrate community metrics. Tolerance intervals are simply confidence intervals based on percentiles, and they allow selection of acceptable limits (referred to as tolerance limits) and a desired level of statistical confidence for a metric distribution (e.g., of a reference population). We used benthic macroinvertebrate community data from several long-term monitoring projects for streams on the US Department of Energy's Oak Ridge Reservation in eastern Tennessee, USA, for the demonstration. We focused on 3 benthic macroinvertebrate community metrics: density, total taxonomic richness, and taxonomic richness of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa. Tolerance intervals yielded less restrictive thresholds than those produced by simple percentiles because the former approach includes variation of the reference data, whereas percentiles are distribution-free. The less restrictive thresholds produced by tolerance intervals will decrease the frequency with which metric values from test sites will be classified as unacceptable because data variation is included, but thresholds calculated using the tolerance interval approach may be better suited for studies that require a greater level of statistical rigor than routine monitoring or general screening surveys (e.g., biocriteria, environmental impact assessments). Conversely, approaches that use simple percentiles may be more appropriate for screening studies. Greater accuracy of tolerance limits can be achieved by increasing sample size, reducing variation (e.g., removing outliers, data transformation), and including data that incorporate both spatial and temporal variation. However, alternative approaches should be used if the data are not normally distributed. Tolerance limits can be adjusted easily to achieve the level of environmental protection desired or required, while providing a level of statistical confidence in derived thresholds. For this reason, a tolerance interval approach should be considered seriously, particularly if the study objectives require a known level of statistical certainty.