Homeostatic organisms regulate their elemental composition by retaining nutrients that are limiting and eliminating those in excess. We argue that this type of homeostatic regulation by consumers might decouple the downstream movement of limiting and nonlimiting nutrients in streams. To illustrate the influence of consumers on nutrient spiraling, we developed a longitudinal model of stream nutrient dynamics that explicitly incorporates stoichiometry and recycling. First, we simulated N- and P-tracer addition experiments in a P-limited stream with a particle-tracking algorithm that allowed us to follow the pathways of N and P atoms in the model ecosystem. Then, we varied the biomass, N:P ratio, and strength of stoichiometric regulation of consumers to quantify how these parameters affected modeled spiraling metrics. The particle-tracking simulation showed that the average time for a nutrient atom to complete a spiral increased with the fraction of atoms that entered the consumer compartment in each spiral, which in turn, increased with increasing consumer biomass. Increasing the consumer N:P ratio to exacerbate consumer stoichiometric imbalance with food resources changed the residence times of nutrients in the food web by increasing the downstream velocity of the nonlimiting nutrient and delaying downstream transport of the limiting nutrient. Decreasing the strength of stoichiometric regulation of consumers dampened the observed effects of increased consumer biomass and N:P ratios on nutrient spiraling. Our model results illustrate that consumers have the potential to influence stream nutrient dynamics through differential excretion of limiting and nonlimiting nutrients, but only at relatively high biomass. Stream biogeochemistry has largely focused on factors controlling the uptake of dissolved nutrients, but understanding how these nutrients are retained and recycled once they enter the stream food web will lead to a more complete understanding of nutrient dynamics in streams.

Wetland or stream drying can be a severe disturbance for aquatic animals, but the severity of a drying disturbance is context dependent. We used a controlled drying experiment to investigate the resistance of populations of 2 crayfish species (Procambarus fallax and Procambarus alleni) found in south Florida wetlands in 3 sediment types (sand, marl, and peat). The species had different putative burrowing abilities, and we tested the hypothesis that the severity of drying disturbance for populations of burrowing crayfish would be species and substrate dependent. Observations of initial burrowing success indicated that the species could burrow equally well in peat, whereas P. alleni burrowed more effectively than P. fallax in marl and sand. No P. fallax burrowed in the dense sand substrate. The number of crayfish surviving 2 mo underground was a function of particular combinations of species and substrate. Survival of both species was high in peat (∼66%) and similarly high for P. alleni in marl. Survival of P. fallax in marl was considerably lower (31%), and no P. fallax survived in sand. Despite their initial burrowing success, only a few P. alleni survived in the sand in shallow burrows. Survivors of both species showed some loss of body mass from initial conditions, but P. fallax lost relatively more mass. The substrate-specific survival of P. fallax helps explain their persistence at relatively high densities in peat-dominated wetlands that dry every 1 to 3 y and their small populations in marl-dominated marshes with similar drying frequencies. Attempts to explain or forecast the net effects of wetland drying for crayfish populations and other burrowing animals should consider that resistance to drying disturbances will depend on characteristics of both species and substrate types. These considerations have important implications for hydrological restoration of aquatic ecosystems that have pronounced gradients of hydrology and substrates.

Many running waters are polluted by organic effluents resulting from agricultural, industrial, and urban activities. Organic pollution consists of the release of high levels of labile C, which usually is rapidly consumed by heterotrophic microbes, and large quantities of N and P, which influence algal productivity and elemental composition (nutrient:C ratio). The nutrient:C ratio of periphyton is somewhat flexible and can be linked to nutrient level in the water. In contrast, elemental stoichiometry of stream macroinvertebrates is constrained within narrower ranges. This observation leads us to the question whether macroinvertebrates with higher nutrient:C ratios, and presumably high nutrient requirements, are more likely to be found in more nutrient-rich (i.e., polluted) sites, containing food (e.g., periphyton) with higher nutrient:C ratios. The capability of macroinvertebrate taxa to thrive in polluted streams often is referred to as tolerance. Empirical data show that taxa have different tolerances to organic pollution. However, mechanisms that explain their tolerance are largely unexplored. We used published data to test the degree to which tolerance to organic pollution is linked to the elemental composition (nutrient:C ratios) of freshwater invertebrates and their functional feeding group. We showed that the nutrient:C ratios of macroinvertebrate taxa are statistically linked to taxon-specific tolerance to organic pollution. This relationship is strongest among scrapers. The observed relationship between pollution tolerance and macroinvertebrate stoichiometry indicates that macroinvertebrate elemental stoichiometry can determine species distribution along an organic pollution gradient.

We developed a set of great-river macroinvertebrate indices of condition (GRMINs) for the littoral benthos of mid-continent great rivers in the US (Missouri, Upper Mississippi, and Ohio). We used a multiscale (site, reach, landscape), multimetric abiotic stressor gradient to select macroinvertebrate assemblage metrics sensitive to human disturbance. We used the same stressor gradient to derive reference expectations by empirical modeling. We developed GRMINs for each of 5 reaches: the upper Missouri River, the lower Missouri River, the unimpounded Upper Mississippi River, the impounded Upper Mississippi River, and the Ohio River. We created 2 versions of each index. One version was based on the absolute value (AV) of the correlation between invertebrate metrics and the stressor gradient and potentially included all types of metrics (e.g., diversity, composition, tolerance to pollution, feeding guild). The second version included no tolerance-value-based metrics (NTV). Each index included up to 10 metrics. The indices were reliable for general assessment of great-river aquatic conditions except in the unimpounded Upper Mississippi River. The AV and NTV versions of the GRMIN were highly correlated (r  =  0.94). The indices revealed longitudinal variation in condition on great-river reaches and among-river variation in relative condition. No sites near or <100 km downriver of Kansas City, Missouri, on the Missouri River or Minneapolis–St. Paul, Minnesota, on the Upper Mississippi River were in least-disturbed condition. GRMIN indices are an indicator of biotic response to general human disturbance. They are appropriate for local and regional-scale ecological assessments and long-term monitoring.

We reviewed global P export and its controlling factors from 685 world rivers. We used available continuous (runoff, rainfall, catchment area, % land use, and population density) and discrete (runoff type, soil type, biome, dominant land use, dominant type of forest, occurrence of stagnant water bodies in catchment, and Gross Product per Capita [GPC]) variables to predict export of P fractions. P export (kg P km⁻² y⁻¹) spanned 6 orders of magnitude worldwide. The distribution of all fractions of P export (total P [TP], soluble reactive P [SRP], and nonSRP [dissolved organic and particle-bound P]) was right skewed. Export of nonSRP had the highest coefficient of variability, and nonSRP was the dominant part of export. The available environmental variables predicted global P export fairly well (R²  =  0.73) if total N export was included in calculations. The unexplained variance in P export might be attributed to noise in the data set, inaccuracy of measurements of environmental variables at fine scales, lack of quantitative data on anthropogenic P sources, insufficient knowledge of P behavior in catchment soils, and nonlinearity of controlling processes. P exports were highly variable among catchment types, and runoff and population density were the predictors shared by most models. P export appeared to be controlled by different sets of environmental variables in different types of catchments. Quasi-empirical, mechanistic models of P export performed better than did empirical models. Our mechanistic understanding of P export could be improved by refining current analytical methods to obtain fast and reliable values of all P fractions in aquatic ecosystems and by incorporating better and more detailed data on catchment features, anthropogenic sources of P, and instream variables in a mechanistic modelling framework.

We tested 2 sources of growth rate variability in a lotic species of unionoid mussel, Pronodularia japanensis, in an agricultural drainage channel in Japan. First, we examined the effects of handling (excavation with short-term aerial exposure). Mussels were tagged, released, and recaptured every 0, 1, 2, or 3 mo to measure growth rate. Growth rates of individuals were not affected by the frequency of excavation. Second, we examined the effect of P. japanensis density on the growth rates of juveniles. Juveniles (shell length <30 mm) were exposed to varying densities of adults in experimental cages in 3 different seasons. In early summer in habitats with slow currents, growth rate of juveniles was significantly and positively related to density of P. japanensis. Our results suggest that excavating unionoid mussels at intervals as short as 1 mo is unlikely to cause physiological stresses on individuals and that density-dependent effects on growth rates might be important only at certain times of the year.

Many streams and rivers in the Pacific Northwest of the US are inherently oligotrophic, and primary production in these ecosystems is assumed to be nutrient-limited. In many of these streams, reductions in the amount of marine-derived nutrients delivered by spawning salmonids could be exacerbating the degree of oligotrophication. To test whether primary producers are nutrient-limited, nutrient diffusing substrate (NDS) experiments were used to measure algal responses to amendments of N, P, and a combination of N and P (N P) in 13 Salmon River basin streams in central Idaho, USA. Thirty-eight experiments were conducted between 2003 and 2006 to determine whether nutrient limitation varied among streams and over time within individual streams. Primary producers in most streams showed some form of nutrient limitation. Thirty-nine percent of our experiments suggested N and P colimitation, 18% suggested N limitation, 11% suggested primary N and secondary P limitation, and 32% did not indicate limitation by either N or P. The type of nutrient limitation within individual streams varied with time, and the relative importance of N or P changed seasonally or annually in 7 of the 13 streams. Algal accrual rates on control and treatment substrates were most strongly predicted by water temperature, light, and ambient concentrations of N and P. Among all of the experiments, algal accrual rates were greater on N substrates in streams with lower ambient N concentrations and greater ambient P concentrations. Our results suggest that a combination of N and P typically limits primary producers in these streams. Our efforts to characterize current nutrient limitation in these streams will be of value to managers considering nutrient additions as a tool to improve stream productivity to benefit threatened and endangered salmonids.

The use of subsurface sediments by benthic prey as spatial refugia from predators can potentially influence predator consumption, and hence density-dependent relationships. We studied how predatory fish affect subsurface sediment use by spiny- and smooth-shelled morphs of the mudsnail, Potamopyrgus antipodarum, on New Zealand's South Island. Spines reduce consumption risk by small benthic fishes (e.g., bullies [Gobiomorphus spp.]). Thus, we predicted that spiny morphs would be more likely to use sediment surfaces with relatively high algal food supplies than the more vulnerable smooth morphs when exposed to fish predators. Epi- and endobenthic samples from 5 streams with fish showed that ∼80% of snails typically were retrieved from subsurface sediments, whereas only 33% of snails from fishless Jackson's Creek occurred in subsurface sediments. A laboratory experiment revealed that significantly more snails used subsurface sediments when with common bullies (G. cotidianus) than when without. Subsurface sediment use in the presence of bullies was similar between shell morphs, a result suggesting that use of spatial refugia might be a more important predator defense than shell armature for Potamopyrgus. Regardless of fish presence, more spiny than smooth morphs burrowed into sand, but subsurface use did not differ between shell morphs in cobbles and gravel. Fewer snails were active on rock surfaces at night than during the day in a stream run, apparently to reduce encounters with nocturnally hunting bullies and shortfinned eels (Anguilla australis). Our results suggest that if fish predation pressure were reduced, then surface densities of Potamopyrgus would increase significantly. Given the snail's superior competitive abilities, this increase could alter top-down and bottom-up effects in food chains and threaten local biodiversity.

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 O₂, and increased turbidity might not be the only mechanisms leading to reductions in diversity as nutrient concentrations increase.

The ability to classify the biological condition of unsurveyed streams accurately would be an asset to the conservation and management of streams. We compared the ability of 5 modeling methods (classification and regression trees, conditional inference trees, random forests [RF], conditional random forests [cRF], and ordinal logistic regression) to predict stream biological condition (very poor, poor, fair, or good) based on benthic macroinvertebrate Index of Biotic Integrity data taken from the Maryland Biological Stream Survey. Predictor variables included land use and land cover (e.g., impervious surface, row-crop agriculture, and population density) and landscape measures (annual precipitation and watershed area). We included 1561 sites on small nontidal streams in the Maryland portion of the Chesapeake Bay watershed. We used 1248 sites (80%) as a training data set to build models and 313 sites (20%) as an independent evaluation data set. RF and cRF models most accurately predicted observed integrity scores in the evaluation data set, but we selected the cRF as the best model because of weaknesses in the RF model (e.g., biased variable selection). Percent impervious surface was the most important variable in the cRF model, and the probability that a site was in very poor or poor biological condition increased rapidly as % impervious cover increased up to 20%. When applied to predict stream biological conditions in all 7908 small nontidal stream reaches in the study area, the cRF model predicted that 33.8% were in fair, 29.9% in good, 22.7% in poor, and 13.6% in very poor biological condition. Our analyses can be used to manage and conserve freshwater and estuarine resources of Maryland and the Chesapeake Bay watershed. Model predictions for unsurveyed streams can help target field studies to identify high-quality streams deserving of conservation and impaired streams in need of restoration.

Stable H isotope ratios (D/H, expressed as δD) hold promise as an additional tool for elucidating food sources for consumers in aquatic ecosystems. We tested the applicability of δD as a food source tracer in streams of New Brunswick, Canada. First, we analyzed δD and δ¹³C in biofilm and terrestrial leaves and compared signal-to-noise ratios (variability within sources vs variability between sources) between the 2 elements. Signal-to-noise ratios were roughly similar, and 23 of the 31 sites had isotopically distinct food sources based on δD compared to 20 of 31 based on δ¹³C. Second, we used mixing models to estimate % aquatic H and % aquatic C for benthic invertebrates at a subset of sites. Of 16 samples, only 1 had δ¹³C that was outside the range of the 2 food sources (yielding % aquatic C > 100%), but 12 of the 16 samples had δD outside the range of the food sources, a result suggesting confounding effects of water and lipids on total body H content. Last, we analyzed δD in laboratory-reared consumers (brook trout and water striders) and in their diet before and after lipid extraction to estimate diet–tissue fractionation. Large differences between consumer and diet were apparent before lipid extraction, but no significant differences were found after lipid extraction. All of these measures indicate that δD could serve as a complementary, but not alternative, isotopic method for estimating food sources for consumers in streams. Further laboratory trials are needed to explore the influence of lipids on δD values.

Streams are potential hotspots for retention and removal of NO₃⁻, and understanding the mechanisms that enhance NO₃⁻ reactivity in stream systems is critical for predicting and preventing eutrophication. Both dissolved organic C (DOC) and dissolved O₂ (DO) influence NO₃⁻ removal processes. Assessing the individual impacts of NO₃⁻, DO, and DOC concentrations on stream NO₃⁻ removal is difficult because these factors covary and are coupled through the C and N cycles. We used an experimental approach to quantify the influences of NO₃⁻, DOC, and DO on NO₃⁻ transport in headwater streams of the Ipswich and Parker River watersheds (Massachusetts, USA) with contrasting levels of DOC and DO. In a 1^st set of experiments, we added NO₃⁻ to address how uptake kinetics differed between a low-DO/high-DOC stream (Cedar Swamp Creek) and a high-DO/low-DOC stream (Cart Creek). In a 2^nd set of experiments, we manipulated, for the first time at the reach scale, both DO and DOC in a factorial experiment. DO was added to the low-DO stream by injecting O₂ and was removed from the high-DO stream by adding sodium sulfite. DOC was added both alone and in combination with the DO manipulations. NO₃⁻ concentration was an important control of NO₃⁻ uptake velocity in our study streams, consistent with previous findings. The results of the DOC and DO manipulations suggested that DO determines whether a stream has net NO₃⁻ uptake or production and that the presence of DOC magnifies the DO response processes. Addition of DOC by itself did not lead to increased NO₃⁻ uptake. In addition, we observed organic matter priming effects, wherein the addition of labile organic matter resulted in accelerated metabolism of naturally occurring DOC in the water column. Priming effects have not been reported previously in stream systems. Results from our experiments suggest that NO₃⁻ uptake in streams might arise from complex interactions among DOC, DO, and NO₃⁻, and ultimately, from the influence of DO on dominant stream processes.

Headwater streams dominate natural landscapes and provide essential functions for downstream waters. However, because of minimal legal protection, they often are piped or buried to accommodate urban growth. Urbanization also alters stream base flows. The combined impact of these factors on channel location is unknown. We assessed the effects of urbanization on the location and length of ephemeral, intermittent, and perennial streams. We randomly selected 150 of 6686 potential channel origins in Hamilton County (Cincinnati), Ohio, USA, for field assessments, and mapped 122 ephemeral, 74 intermittent, and 45 perennial flow origins in these channels. On average, 1:100,000- and 1:24,000-scale US Geological Survey maps underestimated channel length by 85% and 78%, respectively. Mean catchment areas for ephemeral and intermittent flow origins were smaller in forested (0.66 ha and 3.60 ha, respectively) than in urban areas (5.13 ha and 6.79 ha, respectively). These values indicate 93% and 46% county-wide losses of ephemeral and intermittent channel length, respectively, with urbanization. In contrast, the mean catchment area for perennial flow origins was larger in forested (48.12 ha) than in urban (31.22 ha) areas, resulting in a 22% gain in perennial channel length with urban development. Increased perennial channel length was partially explained by reduced forest cover, a result suggesting that reduced evapotranspiration can significantly increase stream base flows. Most variation (59–74%) in catchment area of ephemeral, intermittent, and perennial flow origins was explained by catchment relief, with higher relief corresponding to smaller catchments. Urbanization can decrease (e.g., via reduced infiltration) or increase (e.g., via lawn irrigation and septic tanks) the permanence of flows, thus confounding any overall effect of urban land cover on hydrologic permanence. Site-specific differences in physiography (e.g., bedrock, springs) and landscape management (e.g., stream impoundments) suggest that field surveys are necessary for accurate stream delineation. These results highlight the extensive effects of urbanization on the presence and hydrologic permanence of headwater streams, raise issues with current jurisdictional policy in the US, and emphasize the need to examine the cumulative effects of headwater stream loss on downstream ecosystems.

Since the 1940s, use of road salt as a deicing agent has increased substantially in regions of the US with cold winters. Despite its ubiquitous application and known negative consequences for aquatic and human health, little research has documented the effects of road salt on the water quality of either streams or groundwater in regions, such as New Hampshire (NH), with harsh northern climates. We measured stream Na and Cl⁻ concentrations in 44 basins spanning a gradient of urbanization in southeastern and central NH. Among all sampled basins, stream Na and Cl⁻ concentrations were highly correlated with basin % road pavement (r²  =  0.75 for Na , and 0.78 for Cl⁻). In southeastern NH, concentrations also were correlated strongly with % impervious surface (r²  =  0.86 for Na , and 0.92 for Cl⁻). Groundwater salt concentrations in 143 private wells were significantly correlated with % impervious surface within a 500-m radius of each well, but the proportion of explained variance was small (r²  =  0.07 for Na , and 0.10 for Cl⁻). Concentrations of salt in streams and groundwater were surprisingly high. Mean concentrations of Na ranged from <1 to 298 mg/L and of Cl⁻ ranged from <1 to 573 mg/L. Mean Cl⁻ concentration in 1 small stream exceeded the US Environmental Protection Agency (EPA) chronic toxicity standard of 230 mg/L, and 9% of groundwater samples exceeded the secondary EPA maximum contamination levels for drinking water (250 mg/L of either Na or Cl⁻). In our long-term study basin, the Lamprey River, specific conductance increased over the period from 1978 to 2008, a result that indicated a corresponding increase in Na and Cl⁻ concentrations. Both Na and Cl⁻ concentrations in the Lamprey River were negatively correlated with flow, but the slope of the relationship decreased after a significant flood in 2006. Our data suggest that road-salting practices are contributing to the salinization of stream water and groundwater in NH, and that hydrologic variability, which is predicted to increase with climate change, could strongly affect the degree of salinization observed in surface waters.

Terrestrial allochthonous inputs represent the primary energy source for small stream ecosystems, and activities that alter those inputs produce cascading effects through multiple trophic levels. Urban development affects riparian forests through direct removal or through modifications to species composition and structure. Our study evaluated stand composition and litterfall inputs to small streams across a range of urbanization. Heterogeneity was high, but a consistent pattern of deciduous dominance was observed in more-disturbed areas and a mosaic of conifer, mixed, and deciduous forest patches in less-disturbed areas. Such mosaics are a region-dependent pattern that results from Pacific Northwest forest succession. Riparian vegetation disturbance level increased as total impervious area in the watershed increased. Annual mean daily litterfall rates ranged from 0.0 to 2.5 g m⁻² d⁻¹ in 128 trap locations distributed over 13 stream reaches that represent the range of riparian vegetation conditions currently present in the Puget Lowland. Conifer, mixed, and deciduous plots in watersheds with disturbance produced 12.5 to 19.3 mg N m⁻² d⁻¹ and 0.9 to 1.3 mg P m⁻² d⁻¹. Deciduous riparian forests associated with moderate urban development produced organic matter loads similar to those of conifer forests, but the greater prevalence of leaf material produced 54% higher N loads and 40% higher P loads from litter inputs compared to conifer-dominated riparian areas. Conversion from coniferous to deciduous vegetation also produced subtle shifts in timing of inputs. Anthropogenic activities shape and maintain the current vegetation regime in the Puget Lowland, and this regime has profound implications for nutrient processes and aquatic productivity.

The relationships among urbanization, stream hydraulics, and aquatic biology were investigated across a gradient of urbanization in 30 small basins in eastern Wisconsin, USA. Simulation of hydraulic metrics with 1-dimensional unsteady flow models was an effective means for mechanistically coupling the effects of urbanization with stream ecological conditions (i.e., algae, invertebrates, and fish). Urbanization, characterized by household, road, and urban land density, was positively correlated with the lowest shear stress for 2 adjacent transects in a reach for the low-flow summer (p < 0.001) and autumn (p < 0.01) periods. Urbanization also was positively correlated with Reynolds number and % exposed stream bed during months with moderate to low flows. Our study demonstrated the value of temporally and spatially explicit hydraulic models for providing mechanistic insight into the relationships between hydraulic variables and biological responses. For example, the positive correlation between filter-feeding invertebrate richness and minimum 2-transect shear stress observed in our study is consistent with a higher concentration of water-column particulates available for filtration. The strength of correlations between hydraulic and biological metrics is related to the time period (annual, seasonal, or monthly) considered. The hydraulic modeling approach, whether based on hourly or daily flow data, allowed documentation of the effects of a spatially variable response within a reach, and the results suggest that stream response to urbanization varies with hydraulic habitat type.

Mitigation of urban effects on streams requires an understanding of the paths by which urban effects are transmitted from catchments to streams and how those effects are attenuated with distance. We assessed whether modeling attenuation from impervious surfaces and septic tanks along drainage lines improved prediction of 3 instream ecological indicators. Eleven regression models were calculated for each indicator (Escherichia coli, NO₃/NO₂, and Stream Invertebrate Grade Number Average Level [SIGNAL; a macroinvertebrate assemblage composition index]). Predictor variables included imperviousness or septic-tank density with no attenuation (i.e., total imperviousness and tank density), with overland attenuation (exponential decay with distance along topographic flow paths to stormwater drain or to stream), and with overland and instream attenuation (exponential decay with distance travelled within the stream). Escherichia coli was best predicted by the weighted density of septic tanks, with their influence attenuated to near 0 within tens of meters of the stream and within thousands of meters along streams. These results suggest strong overland attenuation of bacterial contamination and indicate that stormwater drains are not important pathways for septic leakage in this area. NO₃/NO₂, which is mobile through soils, was best predicted by nonattenuated septic-tank density within catchments. SIGNAL was best predicted by either impervious area or septic tanks within tens of meters of stormwater drains, a result suggesting that SIGNAL is most strongly affected by stormwater runoff routed through stormwater pipes to streams. Management of dry-weather fecal and N contamination in the study streams should focus on septic tank management (stricter maintenance regimes or replacement with other sewage management systems). Near-stream septic tanks are probably most important for fecal contamination, whereas catchment-wide management probably will be required for reduction of dry-weather N concentrations. Management of broader instream ecological condition, as indicated by macroinvertebrate assemblage composition, probably will require catchment-wide retention of stormwater runoff (using tanks or infiltration systems) to mimic natural flow-paths between catchments and streams.

We examined the influence of land use/land cover (LULC) on macroinvertebrate assemblages and environmental conditions in streams draining 18 small watersheds in the Southern Outer Piedmont ecoregion in Georgia, USA, over a 24-mo period of record. Specifically, we analyzed relationships among LULC categories (i.e., % impervious surface [IS], pasture, silviculture, and deciduous/evergreen forest) and hydrological, physicochemical, and benthic habitat variables, and macroinvertebrate metrics. Macroinvertebrate metrics were related primarily to % IS and % deciduous forest cover, with lowest biotic integrity (species diversity, taxon richness, biological stream condition index) found in high % IS watersheds. Biotic integrity declined with decreasing % forest cover throughout the seasons, and multiple regression models and partial correlation analysis revealed that physicochemical and benthic habitat variables explained more variation in macroinvertebrate metrics throughout the seasons than did hydrological variables at most sites. Based on nonmetric multidimensional scaling, heavily urbanized sites were strongly separated from all other sites in terms of assemblage structure. Total ordination distance among seasonal samples from the same sites increased as % forest cover increased. This pattern might have arisen because urbanized streams had high abundances of a few tolerant, persistent taxa and lacked many ephemeral taxa found in less disturbed systems. The influence of anthropogenic LULC on macroinvertebrate assemblages appears to be consistent throughout the year and reduces seasonal changes in assemblages. LULC-associated differences among assemblages are strongly associated with changes in physicochemistry and benthic habitat conditions that probably are mediated by hydrological alterations associated with altered LULC in the watersheds of this region.

Research that explores the biological response to urbanization on a site-specific scale is necessary for management of urban basins. Recent studies have proposed a method to characterize the biological response of benthic macroinvertebrates along an urban gradient for several climatic regions in the USA. Our study demonstrates how this general framework can be refined and applied on a smaller scale to an urbanized basin, the Santa Clara Basin (surrounding San Jose, California, USA). Eighty-four sampling sites on 14 streams in the Santa Clara Basin were used for assessing local stream conditions. First, an urban index composed of human population density, road density, and % urban land cover was used to determine the extent of urbanization upstream from each sampling site. Second, a multimetric biological index was developed to characterize the response of macroinvertebrate assemblages along the urban gradient. The resulting biological index included metrics from 3 ecological categories: taxonomic composition (% Ephemeroptera, Plecoptera, and Trichoptera), functional feeding group (shredder richness), and habit (% clingers). The 90^th-quantile regression line was used to define the best available biological conditions along the urban gradient, which we define as the predicted biological potential. This descriptor was then used to determine the relative condition of sites throughout the basin. Hierarchical partitioning of variance revealed that several site-specific variables (dissolved O₂ and temperature) were significantly related to a site's deviation from its predicted biological potential. Spatial analysis of each site's deviation from its biological potential indicated geographic heterogeneity in the distribution of impaired sites. The presence and operation of local dams optimize water use, but modify natural flow regimes, which in turn influence stream habitat, dissolved O₂, and temperature. Current dissolved O₂ and temperature regimes deviate from natural conditions and appear to affect benthic macroinvertebrate assemblages. The assessment methods presented in our study provide finer-scale assessment tools for managers in urban basins.

Adult dispersal and completion of life cycles by aquatic insects are essential for the persistence of populations, colonization of new habitats, and maintenance of genetic diversity. However, life-cycle stages and processes associated with the terrestrial environment often are overlooked when the effect of watershed urbanization on the persistence of insects associated with streams is examined. We reviewed and synthesized current literature on the known effects of watershed urbanization on the terrestrial stage of stream insects. Some research has directly examined the effects of watershed urbanization on dispersal, but much of the evidence we present is indirect and from related studies on aquatic insect life-history traits and dispersal abilities in nonurban watersheds. Our goal is to provide examples of potential impacts that warrant further study, rather than to provide a comprehensive review of all life-history studies. We discuss how watershed land use, riparian condition, and habitat quality affect: 1) adult fitness, 2) adult dispersal, and 3) habitat fragmentation, and 4) how these factors interact with species traits. In general, we found that the local- and landscape-scale changes to stream, riparian, and upland habitats that typically result from anthropogenic activities have the potential to prevent the completion of aquatic insect life cycles and to limit adult dispersal, and therefore, can affect population persistence. When considered within the spatial context of dendritic stream networks, these effects, particularly those on adult dispersal, might have important implications for design and assessment of restoration projects. We discuss a framework for how to determine the relative importance of effects on specific life-cycle stages and processes for the absence of larval populations from urban streams. Overall, more research on terrestrial life-cycle stages and processes and on adult dispersal is required to understand how urbanization might affect population persistence of insects in urban streams.

Many studies have shown that streams degrade in response to urbanization in the watershed. These studies often are based on use of biotic and abiotic variables to measure stream health across a gradient of land cover/land use. The results of these studies can be applied to other urban systems, but often fail to provide a mechanistic understanding of the urban impact, in part, because of the nature of the experimental design. We analyzed the advantages and disadvantages of using environmental gradient studies to further understanding of urban stream systems. We also evaluated alternative experimental design approaches, including best management practice monitoring, long-term watershed studies, paired-watershed studies, and before–after control–impact studies, which could be used to complement the gradient approach. We illustrate these theoretical discussions with an urban paired-watershed case study in the Etowah watershed in northern Georgia. Our goal is to move experimental designs in a direction that will further our mechanistic understanding of the effects of existing urbanization on aquatic ecosystems and will provide opportunities to evaluate stream responses to environmentally sensitive urban land cover.

Studies of the effects of urbanization on stream ecosystems have usually focused on single metropolitan areas. Synthesis of the results of such studies have been useful in developing general conceptual models of the effects of urbanization, but the strength of such generalizations is enhanced by applying consistent study designs and methods to multiple metropolitan areas across large geographic scales. We summarized the results from studies of the effects of urbanization on stream ecosystems in 9 metropolitan areas across the US (Boston, Massachusetts; Raleigh, North Carolina; Atlanta, Georgia; Birmingham, Alabama; Milwaukee-Green Bay, Wisconsin; Denver, Colorado; Dallas-Fort Worth, Texas; Salt Lake City, Utah; and Portland, Oregon). These studies were conducted as part of the US Geological Survey's National Water-Quality Assessment Program and were based on a common study design and used standard sample-collection and processing methods to facilitate comparisons among study areas. All studies included evaluations of hydrology, physical habitat, water quality, and biota (algae, macroinvertebrates, fish). Four major conclusions emerged from the studies. First, responses of hydrologic, physical-habitat, water-quality, and biotic variables to urbanization varied among metropolitan areas, except that insecticide inputs consistently increased with urbanization. Second, prior land use, primarily forest and agriculture, appeared to be the most important determinant of the response of biota to urbanization in the areas we studied. Third, little evidence was found for resistance to the effects of urbanization by macroinvertebrate assemblages, even at low levels of urbanization. Fourth, benthic macroinvertebrates have important advantages for assessing the effects of urbanization on stream ecosystems relative to algae and fishes. Overall, our results demonstrate regional differences in the effects of urbanization on stream biota and suggest additional studies to elucidate the causes of these underlying differences.

Urbanization is increasing rapidly in tropical regions. Information on the response of tropical streams to urbanization is critically needed, but this information is limited. Here we summarize our current understanding of urban stream ecosystems in Puerto Rico, a tropical island with a high level of industrialization and large urban areas. We focused on 16 tributaries of the Rio Piedras watershed, which drains the San Juan metropolitan area, and 16 tributaries of the Turabo watershed, a rapidly developing suburban area. Urban effects on nutrients and aquatic macroinvertebrates were similar to those described for other geographic regions. PO₄, K, and Mg concentrations increased considerably with urbanization, PO₄-P ranged from <5 µg/L in streams draining forested watersheds to >500 µg/L in heavily urbanized streams. Macroinvertebrate assemblage composition changed to dominance by tolerant taxa (e.g., snails, Chironomidae) as the proportion of urban land increased in the subwatershed. In contrast, other factors did not follow expected responses to urbanization. Stream hydrology was equally flashy in streams draining forested and urbanized watersheds. Urban streams were commonly channelized, but in contrast to studies done elsewhere, channel incision was not greater in urban than in forested streams. Degree of urbanization was not related to the integrity of the fish assemblages. River connectivity seems to be more important than urbanization in determining fish assemblage composition in Puerto Rican streams because all native species are migratory (i.e., diadromous). Urbanization increased water temperature and microbial activity. Overall, urban streams in Puerto Rico presented some unexpected responses to urbanization that seem to be the result of its island or coastal characteristics. Examples from Puerto Rico provide information on how tropical streams might respond to the impacts of urbanization.

Urban streams have been the focus of much research in recent years, but many questions about the mechanisms driving the urban stream syndrome remain unanswered. Identification of key research questions is an important step toward effective, efficient management of urban streams to meet societal goals. We developed a list of priority research questions by: 1) soliciting input from interested scientists via a listserv and online survey, 2) holding an open discussion on the questions at the Second Symposium on Urbanization and Stream Ecology, and 3) reviewing the literature in the preparation of this paper. We present the resulting list of 26 questions in the context of a review and summary of the present understanding of urban effects on streams. The key questions address major gaps in our understanding of ecosystem structure and function responses (e.g., what are the sublethal impacts of urbanization on biota?), characteristics of urban stream stressors (e.g., can we identify clusters of covarying stressors?), and management strategies (e.g., what are appropriate indicators of ecosystem structure and function to use as management targets?). The identified research needs highlight our limited understanding of mechanisms driving the urban stream syndrome and the variability in characteristics of the effects of urbanization across different biogeoclimatic conditions, stages of development, government policies, and cultural norms. We discuss how to proceed with appropriate management activities given our current incomplete understanding of the urban stream syndrome.