Leaf-litter decomposition and associated macroinvertebrate communities were compared in standardized leaf packs across forest streams in recently clearcut (n = 9) and reference (n = 12) low-order catchments on the Boreal Shield in northeastern Ontario, Canada. Logging was conducted under best management practices that included application of 30- to 100-m-wide no-harvest buffer zones on both sides of each stream. No significant differences were detected between sites in logged and reference streams for any reach- or catchment-level characteristics (except % area logged) or water-quality variables. Coarse-mesh leaf-pack mass loss was significantly lower (t-test, p = 0.003), and the ratio of fine-mesh to coarse-mesh leaf-pack mass loss was significantly higher (t-test, p = 0.008) in logged than in reference streams, but no difference in fine-mesh leaf-pack mass loss was detected between logged and reference streams. A stepwise multiple regression model of coarse-mesh leaf-pack mass loss on 15 reach- and catchment-level characteristics indicated that only logging presence/absence (r = −0.524) and average reach velocity (r = 0.397) were significantly and independently associated with leaf-litter decomposition. Macroinvertebrate communities on leaf packs in logged streams were different from those in reference streams. Taxonomic richness was significantly lower in logged than in reference streams. A multivariate ordination and analysis of similarity separated logged from reference streams, and abundances of the 3 most discriminating taxa were significantly lower in logged than in reference streams. A multivariate BVSTEP routine indicated that macroinvertebrate community structure was most strongly associated with logging presence/absence among the suite of site characteristics. Leaf-litter decomposition and aquatic macroinvertebrate community structure were successful bioindicators of catchment logging impacts, even when logging was conducted under best management practices. Effects on litter decomposition and leaf-pack macroinvertebrate communities seem to have been caused by upland logging disturbances because riparian areas were undisturbed in logged catchments.

We developed empirical models for fish, macroinvertebrate, and diatom assemblages to assess the biological condition of 268 streams sampled from 1993 to 2002 in 7 major river basins in the Appalachian region of the USA. These models estimate the expected taxonomic composition at each site based on observed variation in taxonomic composition at reference sites. The index, O/E, is the ratio of the number of predicted taxa that were observed (O) to that expected (E) to occur at a site and is a measure of taxonomic completeness. We compared how O/E for each assemblage varied among major landuse settings and whether impaired assemblages were associated with particular physicochemical conditions. We also examined concordance among assemblages in their response to stress. Biological, chemical, and physical data were collected following consistent protocols. We used land-cover criteria, published data, and topographic maps to classify sites by major landuse setting. Fish, macroinvertebrate, and diatom assemblages had been sampled at 73, 108, and 52, respectively, of the least disturbed sites used to establish reference conditions. The models accounted for a substantial portion of the natural variation in taxonomic composition across sites that was associated with biogeographic, climatic, and basin-scale factors and generally were unbiased across the range of environmental gradients observed in the region. Assessments at nonreference sites showed that impairment of fish and macroinvertebrate assemblages was most strongly associated with agriculture and urban land uses, whereas impairment of diatom assemblages was most strongly associated with mining in the basin. Concordance in assessments among assemblages was not strong. Assessments based on 2 assemblages differed in 28 to 57% of cases, and assessments were never concordant for cases where all 3 assemblages were sampled. Furthermore, only ½ of these cases would have been assessed as ecologically impaired had only 1 assemblage been sampled. Differences between observed and predicted frequencies of occurrence for individual taxa were generally consistent with known tolerances to environmental stressors and might aid in identifying causes of biological impairment.

The effects of nutrient enrichment of stream water on epilithic chironomid larval assemblages were examined during spring in a Mediterranean catchment. Three reaches that differed in degree of enrichment were selected for comparison. Water chemistry, epilithon biomass (as ash-free dry mass [AFDM]), epilithic algal biomass (as chlorophyll a [chl a]), algal biovolumes, and algal composition were analyzed to determine their association with density and taxonomic composition of epilithic chironomids. Kendall's τ coefficient of rank correlations and canonical correlation analysis were used to evaluate correlations between the chironomid and algal assemblages. Significant correlations were found between chironomid larval densities and total algal cell densities and biovolumes. Chironomid larval densities were not significantly correlated with chl a or AFDM. Several contrasting trends were identified in the relationships between chironomid and algal assemblages. Densities of fixed-tube scrapers, such as Eukiefferiella claripennis and Cricotopus bicinctus, were strongly correlated with biovolumes of Rhodophyceae and Diatomophyceae, whereas densities of free-living scrapers, such as Thienemaniella, were slightly correlated with Chlorophyceae biovolume. Therefore, nutrient enrichment strongly influenced epilithic chironomid assemblage structure through its effects on biovolumes and cell densities of the epilithic algal assemblages.

Nutrient diffusing substrates (NDS) are an important tool for evaluating periphyton nutrient limitation. The rate at which nutrients are released from NDS depends on both the initial nutrient concentration and the length of time that NDS are in place. Whether temperature also affects nutrient release rates from NDS is unclear. However, this information is important because temperature effects on release rates could confound experimental results for NDS-based experiments testing rates of accumulation of periphyton biomass when stream water temperature is variable. We measured N and P release rates from NDS vials with 3 initial concentrations (0.05, 0.1, and 0.5 mol/L) of nutrients at 3 temperatures (4, 15, and 21°C) for 21 d. Release rates of both nutrients were greater for vials with higher nutrient concentrations and for vials at warmer temperatures. For all concentrations, release rates decreased log linearly with time, a result that might have important implications for patterns of colonization and subsequent interspecific interactions within the periphyton community. In our opinion, temperature-caused differences in release rates are not biologically important because the differences were much smaller (3%) than expected changes in periphyton maximum growth rates over similar temperature ranges (∼300%). Our results suggest that seasonal and site-related differences in temperature will not significantly affect nutrient release rates within the range of temperatures we tested, but researchers should consider nutrient concentration carefully when planning studies using NDS.

The Montana Department of Environmental Quality (MDEQ) uses 2 forms of benthic macroinvertebrate indicators for detection of stream impairment, a multimetric index (MMI) and a predictive model of observed to expected taxa (O/E), each of which is calibrated to streams across the state. As part of the routine monitoring program, some sample locations were subjected to repeated sampling, i.e., multiple samples were collected from stream reaches in spatial and temporal proximity. Results from repeated sampling allow calculation of precision estimates, which are important for describing a portion of the uncertainty (systematic error) associated with field sampling and site assessments. In this project, we evaluated 131 and 77 repeated-sample pairs for the MMI and O/E, respectively, using 4 different measures of precision: coefficient of variability (CV), 90% confidence intervals, relative % difference (RPD), and % difference for the final assessments. MMI and O/E had similar consistency and repeatability. Segregating the data set and calculations by region or field method yielded generally similar precision estimates for the indicators, although precision was slightly better in the mountains using the Hess field-sampling method than in other regions or with other field methods. Evaluation of RPD showed that assessments (impaired/nonimpaired) on the basis of the MMI differed between samples in 18.3% of repeated-sample pairs and assessments on the basis of O/E differed between samples for 19.5% of repeated-sample pairs. Recommended measurement quality objectives were 10 to 15% for CV and 15 to 20% for RPD for both indicators. Field-sampling precision was the focus of our paper, but we emphasize that detecting the presence of stressors or degraded conditions is the primary objective of the MDEQ stream condition indicators.

The evolutionary relationships of the surface genera of shrimps of the family Atyidae from the Caribbean were inferred using mitochondrial 16S ribosomal DNA and cytochrome oxidase I gene sequences. The genetic divergence among the 4 Caribbean genera (Atya, Jonga, Micratya, Potimirim) is extensive and dates from between the Eocene and Miocene. This result suggests a vicariant origin or the ancient dispersal of some taxa. Most intrageneric divergences date to the late Miocene–Pliocene and, thus, are probably the result of dispersal. Some species show low levels of intraspecific genetic divergence between distant islands, and thus, present-day or geologically recent gene flow is likely. This gene flow is probably a consequence of the amphidromous life histories of most Caribbean freshwater shrimps. Despite the ancient divergences between the genera, the Caribbean surface atyids form a single evolutionary lineage when compared with atyid shrimp from throughout the world, and this result implies an ancient evolutionary radiation in the Caribbean. The sister group to the Caribbean atyids are the large-bodied and robust Atya-like shrimps of the Indo-Pacific, which share a similar size and shape with Caribbean Atya. Thus, the common ancestor probably was also large and robust. In contrast, the other Caribbean atyids are much smaller, and Jonga has a distinct morphology that is associated with a switch from lotic to lentic environments. This radiation may have been the result of the absence from the Caribbean of other small shrimps that are common in the Indo-Pacific.

The shells of most North American freshwater mussel species are not sexually dimorphic. During the brooding period, gravid females can be identified by inspection of marsupial gills; however, it is difficult to separate nongravid females from males in species lacking sexual dimorphism. The ability to differentiate males from females throughout the year would assist mussel conservation and research. Our objective was to test the accuracy and safety of a method to determine the sex of live mussels. We used a syringe to extract ∼0.2 mL of gonadal fluid from 67 Elliptio dilatata and 65 Actinonaias ligamentina. The fluid was stained and examined microscopically for developing gametes. This method was safe and effective for determining the sex of mussels. After 1 y, survival was indistinguishable between test and control groups for both species. We sacrificed 4 to 7 E. dilatata and A. ligamentina at 3-mo intervals and examined histological sections of gonads. Sex assigned from examination of gonadal fluid and histological sections agreed in most cases (E. dilatata: 100%, A. ligamentina: 89%).

N and C cycles in headwater streams are coupled, and land use can modify these cycles by increasing N availability and removing riparian vegetation. To increase our understanding of how land use modifies the controls on N cycling, we quantified rates of 2 microbial N transformations in a total of 18 agricultural and urban streams (with and without riparian buffers) for 3 y to examine how riparian vegetation and land use influence sediment nitrification and denitrification. Nitrification rates were highest in agricultural streams in late spring. Nitrification was not related to streamwater NH₄ concentrations but was positively related to sediment C content (linear regression, r² = 0.72, p < 0.001). This result suggests that benthic decomposition provided NH₄ (via mineralization) to increase sediment nitrification. Denitrification rates did not differ among landuse types but were positively related to sediment C content and streamwater NO₃^– concentration (multiple linear regression, R² = 0.78, p < 0.001). Sediment C content, the primary predictor of denitrification rates, did not differ among land uses, but streamwater NO₃^– concentration, the secondary predictor of denitrification rates, was highest in winter and in agricultural streams, indicating that land use and season were more important determinants of denitrification than coupled nitrification. Substrate availability (N and C) for N transformations generally did not differ between buffered and unbuffered streams within a similar landuse type, probably because of the confounding influence of tile drainage systems, which effectively decouple stream channels from their riparian zones. Land use influenced the delivery of the necessary substrates for N transformations but decreased the role of riparian zones in stream N cycling by simplifying the drainage network of headwater streams.

Benthic secondary production plays an important role in the population, community, and ecosystem dynamics of lakes. However, whole-lake estimates of benthic secondary production are rare, and very little is known about intralake patterns in production. We measured benthic secondary production for the dominant macroinvertebrate taxa in Crampton Lake, a 26-ha north-temperate lake at the border of Wisconsin and Michigan, USA. Production was estimated by the size–frequency method for each taxon at each depth where it occurred. Confidence intervals for production estimates were determined by bootstrapping. A small number of taxa were responsible for most benthic secondary production, and most taxa were not highly productive. Thus, we observed a log-normal rank–production relationship. This relationship might be common in other lakes. However, production was not strongly related to abundance, a result that suggests that even rare taxa might contribute substantially to secondary production. Whole-lake benthic secondary production averaged 4.4 g dry mass m⁻² y⁻¹. Across depths, confidence intervals for area-specific rates of production generally overlapped, although point estimates were somewhat higher in the littoral zone (4.8–6.5) than in the profundal zone (3.2–4.5). Despite this similarity in rates, >65% of whole-lake benthic secondary production occurred in the littoral zone because of lake morphometry. A synthesis of limited published data revealed multiple patterns in the relationship between depth and benthic secondary production in other lakes. In most lakes, area-specific rates of production either declined or remained constant as depth increased. This observation, combined with the fact that most lakes are small and predominantly littoral, suggests that littoral dominance of whole-lake benthic secondary production might be widespread, and might help explain recent findings that lake fishes rely heavily on littoral production.

Interest in understanding the influence of ultraviolet-B (UVB; 280–320 nm) radiation in aquatic ecosystems has increased since the early 1990s. Pollution from historic mining operations coupled with physicochemical characteristics of Rocky Mountain streams that increase exposure of benthic communities to UVB provided an opportunity to examine how UVB interacted with heavy metal contamination to structure stream communities. We integrated a series of UVB addition experiments done in stream microcosms with a large-scale UVB shading experiment to test the hypothesis that effects of UVB were greater on benthic communities from metal-polluted streams than from reference streams. Microcosm experiments involved short-term exposure (7–10 d) of natural benthic macroinvertebrate communities collected from reference and metal-contaminated sites to lamp-generated UVB. In all cases, abundance decreased in UVB-treated streams compared to controls. Moreover, effects of UVB addition were significantly greater on communities from metal-polluted sites than from reference sites. The field experiment involved shading portions of the streambed from UVB for 60 d at 12 streams along a Zn gradient. Median Zn concentration at these sites ranged between 5 and 530 μg/L, and mean UVB reaching the streambed varied from 6.5 to 29.0 J/cm². Results of the field experiment indicated that removal of UVB significantly increased total macroinvertebrate abundance and abundance of grazers, mayflies, caddisflies, Orthocladiinae midges, and the mayfly Baetis bicaudatus compared to controls. Grazer abundance was significantly greater in UVB removal treatments compared to controls, but UVB removal had no effect on algal biomass. As with the microcosm experiments, the effects of UVB removal on benthic communities were generally greater at metal-polluted sites than at reference sites. We speculate that the energetic cost of regulating metals might inhibit the ability of some organisms to repair efficiently DNA damaged by UVB exposure. Our results demonstrate that benthic communities in Colorado Rocky Mountain streams are negatively influenced by UVB radiation and that communities subjected to long-term metal exposure are more sensitive to UVB than are reference communities. As a consequence, the effects of increased UVB radiation reaching the earth's surface might be more severe than previously considered in systems receiving multiple stressors.

The absence of quantified relationships among sediment supply, stream channel conditions, and biological responses limits our ability to predict the cumulative watershed effects of management activities in forested mountainous watersheds. We addressed this uncertainty by testing whether increased sediment supply resulted in elevated levels of streambed fine sediment stored in pools and riffles and whether fine bed material was correlated with spawning-gravel quality and altered benthic macroinvertebrate assemblages in 6 streams of the Klamath Mountains, northern California. Sediment supply was estimated using 2 models: 1) an empirical model of landslide volumes based on terrain types present in a basin, and 2) a surface erosion model using a locally calibrated version of the universal soil loss equation. Riffle-surface fine sediment and the fractional volume of pools filled with fine sediment (V*) were both positively correlated with estimated sediment supply, whereas subsurface spawning-gravel permeability was inversely correlated with estimated sediment supply. Fine sediment levels were relatively low compared to published values. Reach-average values of riffle-surface fine sediment ranged from 4 to 16% and V* values ranged from 0.05 to 0.20. Based on established relationships between subsurface flow rates and salmonid egg survival, median predicted egg survival was quite low and ranged from 15 to 38%. Riffle-surface fine sediment and common benthic macroinvertebrate biological metrics were not correlated, but several taxa showed responses to riffle-surface fine sediment. Taxa that showed negative responses to fine sediment are hypothesized to be more available as prey for salmonids than taxa that showed positive responses. This relationship suggests that fine sediments might cause an overall reduction in prey availability for salmonids. Monitoring the effects of increased sediment supply in steep, forested streams should focus on fine sediment in pools and riffles, salmonid spawning-gravel quality, and specific macroinvertebrate taxa that are especially responsive to fine sediment. The linkages described in our paper can be used to make quantitative predictions of the cumulative watershed effects of management activities on stream conditions, salmonid habitat, and benthic macroinvertebrates.

Naturally parasitized fish were collected and examined for glochidial infestations to evaluate host use by 2 mussel species, the yellow lampmussel (Lampsilis cariosa) and tidewater mucket (Leptodea ochracea), that are listed as threatened in Maine. Fish were captured at 13 sites in 3 river watersheds throughout the range of L. cariosa and L. ochracea in Maine to determine if host fish identified in the laboratory are used in nature and to assess additional species as potential hosts. A species-specific molecular identification key using restriction fragment length polymorphism patterns of the mitochondrial NADH dehydrogenase subunit 1 gene was used to identify 687 glochidia from 230 fish. White perch (Morone americana) and yellow perch (Perca flavescens) were previously identified as host fish for L. cariosa in laboratory trials and were confirmed as host fish in wild populations. Five additional species were identified as potential hosts for L. cariosa. Three of these potential hosts are native (banded killifish [Fundulus diaphanus], chain pickerel [Esox niger], and white sucker [Catostomus commersoni]), and 2 are introduced (smallmouth bass [Micropterus dolomieu] and largemouth bass [Micropterus salmoides]). For L. ochracea, the host status of white perch was confirmed in wild populations, and 1 additional species, the banded killifish, was identified as a potential host. White perch was the most frequently and heavily parasitized host; several fish in multiple sites were heavily infested with L. cariosa or L. ochracea glochidia. In contrast, only 1 individual of each of the other fish species, including yellow perch, was infested with L. cariosa or L. ochracea glochidia. We present data collected incidentally for confirmed or potential host fish for other mussel species in the study area. These species include Elliptio complanata, Lampsilis radiata, Anodonta implicata, and Alasmidonta undulata.

Watershed urbanization decreases diversity and taxonomic richness of aquatic insect communities, and headwater streams are particularly susceptible to degradation from urbanization. Patterns of taxon loss between urban headwater communities and communities in adjacent downstream, higher-order reaches might indicate which processes are controlling taxon loss and the extent to which unique headwater taxa are lost after urbanization. We compared insect communities in urban and rural watersheds and investigated if community similarity between headwater streams and adjacent higher-order main-stem reaches was greater in urban than in rural watersheds. We sampled insect communities in 3 urban and 3 rural watersheds in Maryland's Piedmont region during 3 seasons. Mean taxonomic richness was 4.3× greater and the Shannon diversity index was 1.8× greater in rural than in urban headwater streams. Simpson's index was 1.9× greater in urban than in rural headwater streams. The Jaccard similarity index calculated between headwater and main-stem communities was 1.6× greater for urban sites than rural sites during autumn, and the proportion of headwater taxa shared with the main-stem community was 1.8× greater for urban than rural sites. Redundancy analysis also indicated significantly greater similarity between urban headwater and main-stem communities than between rural headwater and main-stem communities. As expected, urbanization decreased diversity, and the communities remaining in urban headwaters were mostly subsets of the communities in the main-stem streams. This result suggested that taxa unique to headwaters were at the greatest risk of local extirpation after watershed urbanization. A significant interactive effect of landuse type and the longitudinal position of a reach along the headwater on taxonomic richness and the Jaccard index suggested that patterns of taxon loss partially depended on the proximity of a headwater reach to the main-stem stream. Overall, the results suggested that water- and habitat-quality degradation were not the only effects of watershed urbanization that determined the composition of insect communities in urban headwaters.

Spatial heterogeneity among microhabitat patches in aquatic ecosystems creates refuges (e.g., substrate interstices) that protect organisms against a variety of environmental constraints. Aquatic insects have evolved the ability to alter their life-history traits in response to environmental variability. Our objective was to determine whether differences in hydraulic and substrate conditions cause phenotypic plasticity in nymphs of the stonefly Nemoura cinerea Retzius. We experimentally manipulated near-bed current velocities (3 and 9 cm/s) and substrate types (medium sand and coarse gravel) in indoor artificial channels. Changes in the head capsule width, mesothoracic wing pad length, total body length, body mass, and behavior were studied over a 4-wk period. Morphometric and behavioral differences were primarily associated with the mechanism of refugium use (although confounded by substrate size), which reduced energy expenditure. Growth (body size and mass) was reduced in sand (absence of refugia) relative to growth in gravel, regardless of current velocity, and nymphs made almost no investment in body length (variation in allometry). The effect of current velocity on behavior was less marked in sand than in gravel because of a possible confounding effect of direct exposure of nymphs to flow on sand (i.e., no available refugia). In sand, individuals were located mainly on the food source (erect leaf discs) at current velocities of 3 cm/s, but feeding was depressed. Therefore, nymphal development was slow on sand. In gravel, nymphs followed different developmental trajectories depending on current velocity because hydraulic stress seemed to trigger accelerated development. Our study shows that physical factors associated with habitat structure can result in adaptive phenotypic plasticity among aquatic insect populations, and that a lack of shelter might have a strong impact on stonefly fitness.

We evaluate the current state of knowledge concerning the ecosystem- and community-level importance of N₂ fixation in streams. We reviewed the literature reporting N₂-fixation contributions to stream N budgets and compared in-stream N₂-fixation rates to denitrification and dissolved inorganic N (DIN)-uptake rates. In-stream N₂ fixation rarely contributed >5% of the annual N input in N budgets that explicitly measured N₂ fixation, but could contribute higher proportions when considered over daily or seasonal time scales. N₂-fixation rates were statistically indistinguishable from denitrification and DIN-uptake rates from the same stream reach. However, published N₂-fixation rates compiled from a wide variety of streams were significantly lower than denitrification or DIN-uptake rates, which were indistinguishable from one another. The data set we compiled might be biased because the number of published N₂-fixation measurements is small (9 studies reporting rates in 22 streams), the range of stream conditions (NO₃^–-N concentration, discharge, season) under which N₂-fixation and other N-processing rates have been measured is limited, and all of the rate estimates have associated methodological artifacts. To broaden our understanding of how N₂ fixation contributes to stream ecosystems, studies must measure all rates concurrently across a broad range of stream conditions. In addition, focusing on how N₂ fixation supports food webs and contributes to benthic community dynamics will help us understand the full ecological ramifications of N₂ fixation in streams, regardless of the magnitude of the N flux into streams from N₂ fixation.

Factors affecting the reproductive success of freshwater mussels in lotic systems are poorly understood. Gravidity, fecundity, and fertilization success of Actinonaias ligamentina were examined at 4 sites along a 63-km reach of the Green River immediately below the Green River Dam, Kentucky. No gravid females were collected at the site closest to the dam, and the percentage of gravid females at downstream sites ranged from 20 to 36%. Not all females became gravid, despite the presence of early stages of ova in the gonadal fluid. This observation suggests that female A. ligamentina undergo a resting stage and, therefore, might not become gravid every year. Fecundity differed among sites and increased with distance from the dam. Fertilization rates ranged from 32 to 97% among sites and increased with distance from the dam. Fertilization rate was independent of local mussel density and position in the mussel bed. The high fertilization rates observed in the upstream portions of mussel beds indicate that freshwater mussel sperm have the ability to travel to distant females in lotic systems. Therefore, females are not necessarily dependent upon nearby males for fertilization. Successful fertilization of A. ligamentina at low mussel densities in the Green River suggests that natural recovery of rare endangered species might be possible if host fish and suitable conditions for juvenile survival and growth are present.