Land-management agencies need quantitative, statistically rigorous monitoring data, often at large spatial and temporal scales, to support resource-management decisions. Monitoring designs typically must accommodate multiple ecological, logistical, political, and economic objectives and constraints. We present a long-term bioregional monitoring program to assess the status and change in populations of the federally listed candidate frog species, Yosemite toad (Anaxyrus [Bufo] canorus) and mountain yellow-legged frog (Rana muscosa/sierrae complex), on USDA Forest Service lands in the Sierra Nevada, California. The program takes advantage of advances in survey design and analysis to: 1) collect data at a metapopulation scale (i.e., small basins), 2) provide occupancy data on ≥2 species with overlapping ranges with the same field-monitoring protocols, 3) provide occupancy estimates applicable to the entire range of each species in the study region, 4) incorporate information from historical occupancy records, and 5) link the survey design to an existing survey design. We estimated occupancy assuming imperfect detection by extending existing procedures for maximum likelihood estimation to incorporate the unequal probability of selection used in the survey design. From 2002 to 2009, we estimate that the Yosemite toad used 0.25 ± 0.01 (SE), 0.86 ± 0.04, and 0.86 ± 0.03 of basins over its range, with historical presence, and with presence since 1990, respectively, and the mountain yellow-legged frog used 0.04 ± 0.01, 0.43 ± 0.04, and 0.47 ± 0.04 of basins over its range, with historical presence, and with presence since 1990, respectively. Survey date and snow pack affected detection of the Yosemite toad but not of the mountain yellow-legged frog. Monitoring costs were reduced by using a complex survey design with panels that required generalizing existing methods for estimating occupancy under imperfect detection.

Native species in lakes commonly are affected by cultural eutrophication and introductions of nonnative species. The effects of these disturbances on benthic communities in large lakes have been understudied, despite the integral role that benthos play in maintaining habitat complexity and ecosystem processes. Lake Tahoe has experienced progressive eutrophication and introductions of nonnative species over the past several decades, but how its unique benthic communities have been affected is unclear. The density of Lake Tahoe's benthic fauna was compared between 1960s surveys and our recent (2008–2009) survey, and the association of zoobenthos with macrophytes was examined for contemporary and historical samples. The density of benthic invertebrates and the occurrence of macrophytes in benthic samples have declined dramatically since collections made in the 1960s. Lakewide densities of benthic invertebrate taxa endemic to Lake Tahoe have declined by 80 to 100%, and the community structure of benthic invertebrate assemblages has changed considerably. Several native benthic invertebrate taxa were closely associated with deepwater macrophytes in the 1960s, but contemporary invertebrate association with macrophytes could not be evaluated reliably because of the scarcity of macrophytes in contemporary samples. Declines in native benthic invertebrate density could be related to the loss of habitat and food resources previously provided by abundant deepwater macrophyte assemblages. In addition, establishment and increases in density of nonnative species that occurred after the benthic surveys of the 1960s probably have affected native benthic invertebrate communities. The observed declines in Lake Tahoe's native benthic invertebrate and macrophyte communities suggest that they are severely threatened.

We compared biomass and community structure of macroinvertebrates among 3 flow zones (deep, rapid, flat) of riffles at 3 sites in a gravel-bed river. We evaluated bed stability in these zones with a 2-dimensional hydrodynamic simulation over a range of discharge levels. Deep zones had higher flow velocity and coarser bed materials than other zones. Rapid zones were shallower with higher flow velocity than flat zones. The probability of bed movement was greatest in rapid zones and was lowest in deep zones based on bed shear stress and the size of bed materials. Total macroinvertebrate biomass was dominated by filterer insects and was highest in deep zones and lowest in rapid zones across the sites. This trend was most conspicuous for taxa that build retreats on stones, such as net-spinning caddisflies, which have a sessile life form and prefer stable environments. The trend was less apparent for taxa that move freely on the bed, such as baetid and heptageniid mayflies. The macroinvertebrate community differed between the middle and peripheral areas at deep zones. Peripheral areas were dominated more by taxa that stay under stones. The channel bed topography in deep zones of riffles is likely to support high macroinvertebrate biomass by providing greater bed stability and higher water flow, the combination of which is relatively uncommon in gravel-bed rivers.

Ecologically meaningful and scientifically defensible nutrient criteria are needed to protect the water quality of USA streams. Criteria based on our best understanding of naturally occurring nutrient concentrations would protect both water quality and aquatic biota. Previous approaches to predicting natural background nutrient concentrations have relied on some form of landscape categorization (e.g., nutrient ecoregions) to account for natural variability among water bodies. However, natural variation within these regions is so high that use of a single criterion can underprotect naturally occurring low-nutrient streams and overprotect naturally occurring high-nutrient steams. We developed Random Forest models to predict how baseflow concentrations of total P (TP) and total N (TN) vary among western USA streams in response to continuous spatial variation in nutrient sources, sinks, or other processes affecting nutrient concentrations. Both models were relatively accurate (root mean square errors <12% of the range of observations for independent validation sites) and made better predictions than previous models of natural nutrient concentrations. However, the models were not very precise (TP model: r²  =  0.46, TN model: r²  =  0.23). An analysis of the sources of variation showed that our models accounted for most of the spatial variation in nutrient concentrations, and much of the imprecision was caused by temporal or measurement variation. We applied 2 methods to determine upper prediction limits that incorporated model error and could be used as site-specific nutrient criteria. These site-specific candidate nutrient criteria better accounted for natural variation among sites than did criteria based on regional average conditions, would increase protection for streams with naturally low nutrient concentrations, and specified more attainable conditions for those streams with naturally higher nutrient concentrations.

Water clarity is a strong indicator of regional water quality. Unlike other common water-quality metrics, such as chlorophyll a, total P, or trophic status, clarity can be accurately and efficiently estimated remotely on a regional scale. Satellite-based remote sensing is useful in regions with many lakes where traditional field-sampling techniques may be prohibitively expensive. Repeated sampling of easily accessed lakes can lead to spatially irregular, nonrandom samples of a region. Remote sensing remedies this problem. We applied a remote monitoring protocol we had previously developed for Maine lakes >8 ha based on Landsat satellite data recorded during 1995–2010 to identify spatial and temporal patterns in Maine lake clarity. We focused on the overlapping region of Landsat paths 11 and 12 to increase availability of cloud-free images in August and early September, a period of relative lake stability and seasonal poor-clarity conditions well suited for annual monitoring. We divided Maine into 3 regions (northeastern, south-central, western) based on morphometric and chemical lake features. We found a general decrease in average statewide lake clarity from 4.94 to 4.38 m during 1995–2010. Water clarity ranged from 4 to 6 m during 1995–2010, but it decreased consistently during 2005–2010. Clarity in both the northeastern and western lake regions has decreased from 5.22 m in 1995 to 4.36 and 4.21 m, respectively, in 2010, whereas lake clarity in the south-central lake region (4.50 m) has not changed since 1995. Climate change, timber harvesting, or watershed morphometry may be responsible for regional water-clarity decline. Remote sensing of regional water clarity provides a more complete spatial perspective of lake water quality than existing, interest-based sampling. However, field sampling done under existing monitoring programs can be used to calibrate accurate models designed to estimate water clarity remotely.

The stoichiometric ratios of organisms and their food resources can influence C and nutrient dynamics in aquatic ecosystems. Several investigators have quantified linkages between nutrient enrichment and consumer stoichiometry for stream detritivores, but very few have systematically quantified the effect of P enrichment on leaf-litter stoichiometry. Here, we examine the potential stoichiometric changes of 2 species of leaf litter subjected to varying levels of P enrichment in laboratory microcosms and mixed species across a natural P gradient of streams in the Ozark Highlands Region, Arkansas, USA. Leaf-litter %P content increased and C∶P ratios decreased with increasing levels of P enrichment and with increasing lability of the leaf species. In the laboratory study, C∶P of maple and oak leaves in the control treatment was ∼2500, whereas this ratio decreased to 500 and 1000 in the high-P treatments, respectively. Total P (TP) was inversely related to leaf-litter C∶P along the natural P gradient of streams in the Ozarks. Our results add to the growing body of information on the potential bottom-up effects of anthropogenic nutrient loading in streams and the influence of water-column nutrients and leaf quality on this response.

Taxonomic sufficiency (TS) has been proposed for assessing community composition and environmental impacts as a way to balance the need to indicate the biology of the organisms present with time and effort needed for species identification. TS has been applied most often to marine and freshwater macroinvertebrates, but tests of its usefulness are lacking for other freshwater groups. We analyzed the effects of taxonomic resolution, functional groupings, and data transformation on multivariate community patterns in periphyton, macrophytes, macroinvertebrates, and fishes, and on the quantification of biodiversity and environmental gradients. The applicability of TS differed strongly among taxonomic groups, depending on the average taxonomic breadth of the species sets. Numerical data resolution had more pronounced effects on community patterns than taxonomic resolution. Richness was strongly affected by data aggregation, but diversity indices were statistically reliable up to order level. Taxonomic aggregation had no significant influence on ability to detect environmental gradients. Functional surrogates based on biological traits, such as feeding type, reproductive strategy, and trophic state, were strongly correlated (ρ  =  0.64–0.85) with taxonomic community composition. However, environmental correlations were generally lower with data aggregated to functional traits rather than to species. TS was universally applicable within taxonomic groups for different habitats in one biogeographic region. Aggregation to family or order was suitable for quantifying biodiversity and environmental gradients, but multivariate community analyses required finer resolution in fishes and macrophytes than in periphyton and macroinvertebrates. Sampling effort in environmental-impact studies and monitoring programs would be better invested in quantitative data and number of spatial and temporal replicates than in taxonomic detail.

Fire and grazers (such as Bison bison) were historically among the most important agents for maintaining and managing tallgrass prairie, but we know little about their influences on water-quality dynamics in streams. We analyzed 2 y of data on total suspended solids (TSS), total N (TN), and total P (TP) (3 samples per week per stream during flow) in 3 prairie streams with fire and bison grazing treatments at Konza Prairie Biological Station, Kansas (USA), to assess whether fire and bison increase the concentrations of these water-quality variables. We quantified the spatial and temporal locations of bison (∼0.21 animal units/ha) with Global Positioning System collars and documented bison trails, paw patches, wallows, and naturally exposed sediment patches within riparian buffers. Three weeks post-fire, TN and TP decreased (t-test, p < 0.001), but TSS did not change. Bison spent <6% of their time within 10 m of the streams, increased the amount of exposed sediment in the riparian areas, and avoided wooded mainstem branches of stream (χ² test, p < 0.001). Temporal trends suggest that low discharge or increased bison density in the stream may increase TSS and TP during the summer months. Our results indicate a weak connection between TSS and nutrients with bison access to streams over our 2-y study and indicate that low TSS and nutrients characterize tallgrass prairie streams with fire and moderate bison densities relative to surrounding land uses.

Early detection of aquatic invasive species is a critical task for management of aquatic ecosystems. This task is hindered by the difficulty and cost of surveying aquatic systems thoroughly. The New Zealand mudsnail (Potamopyrgus antipodarum) is a small, invasive parthenogenic mollusk that can reach very high population densities and severely affects ecosystem functioning. To assist in the early detection of this invasive species, we developed and validated a highly sensitive environmental deoxyribonucleic acid (eDNA) assay. We used a dose–response laboratory experiment to investigate the relationship between New Zealand mudsnail density and eDNA detected through time. We documented that as few as 1 individual in 1.5 L of water for 2 d could be detected with this method, and that eDNA from this species may remain detectable for 21 to 44 d after mudsnail removal. We used the eDNA method to confirm the presence of New Zealand mudsnail eDNA at densities as low as 11 to 144 snails/m² in a eutrophic 5^th-order river. Combined, these results demonstrate the high potential for eDNA surveys to assist with early detection of a widely distributed invasive aquatic invertebrate.

We used field-derived data from streams in Nevada, USA, to quantify relationships between stream biological condition, in-stream stressors, and potential sources of stress (land use). We used 2 freshwater macroinvertebrate-based indices to measure biological condition: a multimetric index (MMI) and an observed to expected (O/E) index of taxonomic completeness. We considered 4 categories of potential stressors: dissolved metals, total dissolved solids, nutrients, and flow alteration. For physicochemical factors that varied predictably across natural environmental gradients, we quantified potential stress as the site-specific difference between observed (O) and expected (E) levels of each factor (O–E_stress). We then used 2 sets of Random Forest models to quantify relationships between: 1) biological condition and potential stressors, and 2) stressor values and land uses. The 2 indices of biological condition were differentially responsive to stressors, indicating that no single measure of biological condition could fully characterize assemblage response to stress. Total dissolved solids (as measured by electrical conductivity [EC]) and metal contamination were the stressors most strongly associated with biological degradation. The most likely sources of these stressors were agriculture, urban development, and mining. Our findings highlight the need to develop EC criteria for streams. Measures of biological condition and stress that account for natural variability should reduce errors of inference and increase confidence in causal analyses. This approach will require development of robust models capable of predicting physical and chemical reference conditions. Causal analyses for individual sites require appropriate hypotheses about which stressors and what levels of stress can cause biological degradation. Our study demonstrates the usefulness of field data collected from multiple sites within a region for developing these hypotheses.

We investigated how the source and composition of stream dissolved organic matter (DOM) influenced rates of benthic bacterial C production (BCP) in 20 forested, headwater streams in southern Tasmania. We also assessed whether the source and composition of stream DOM was influenced by clearfell forest harvesting (1–19 y after harvest). Stream DOM was dominated by humic- and fulvic-like fluorescence (86.3–95.5%) as measured by parallel factor (PARAFAC) analysis of DOM fluorescence. Several reach-scale environmental variables showed significant positive (leaf-area index, sediment total N, organic C) or negative (stream temperature) linear relationships with BCP. However, an increasing contribution of terrestrial DOM, as measured by a decreasing fluorescence index (FI), was the strongest variable driving in situ benthic BCP (R²  =  0.38, p  =  0.004, n  =  20). Forest harvesting did not significantly affect DOM source despite the major reach-scale disturbance that clearfell forestry represents. Nevertheless, conflicting evidence was found for changes in DOM composition after harvesting. Catchment-scale processes probably are more important than reach-scale processes in determining stream DOM biogeochemistry because clearfelled areas are small relative to the total catchment area. Our results demonstrate that freshly leached, terrestrial DOM can influence stream ecosystem processes through the tight biogeochemical linkage that exists between forested, headwater streams and their surrounding terrestrial environment.

C, N, and P content were measured across the ontogeny of lotic aquatic insects representing a diversity of life-history characteristics. The relationship between individual mass and nutrient content was used to show ontogenetic patterns of nutrient content by species. Species analyzed for C and N content exhibited a quasihomeostatic pattern across ontogeny. Percent C and %N varied among taxa irrespective of ontogeny, with %C ranging from 47.4 to 56.2% and %N ranging from 9.6 to 11.6%. P content also varied by species but declined nonlinearly across ontogeny and was best represented by a power function. Percent P varied from >7% in 1^st-instar Tabanus larvae to only 0.34% in adult male Ambrysus circumcinctus. Females had more P per unit mass than males in 6 of the 10 species that could be sexed. In the leptophlebiid mayflies, %P increased in mature female nymphs relative to the penultimate developmental class, whereas %P content of males continued to decline to eclosion. Maximum terminal mass by species was the main factor driving the magnitude of change in %P through their ontogeny. Small-bodied, rapidly growing species exhibited the sharpest decline in P content. Nonhomeostatic patterns in %P across ontogeny and between sexes has important implications for population- and community-level dynamics and ecosystem processes. First, small-bodied, high-%P taxa have faster growth rates than larger individuals, which supports one of the predictions of the growth-rate hypothesis (GRH). Second, elemental imbalance between consumers and their food changes across ontogeny, and therefore, nutrient recycling rate by a species changes with population age structure. Last, community structure may reflect nutrient availability in food such that enriched environments are more likely to be dominated by taxa with high growth rates and, thus, relatively high P demand.

Biological N₂ fixation hypothetically could balance the N∶P supply ratio in lakes and reservoirs, causing P to be the ultimate limiting element. However, the efficiency and time scale of this mechanism has been poorly studied and remains largely unknown. We compared the relative rates of N supply to the epilimnion of 3 reservoirs of similar age and morphology but with differing external nutrient inputs. Our objective was to determine if N₂-fixation rates in these reservoirs exceeded other internal N supply pathways during the growing season, a situation that would suggest that these systems are still evolving in terms of nutrient-limitation status. Phytoplankton N₂-fixation rates ranged from 3.4 to 16 g N m⁻² y⁻¹ in the reservoirs. These rates are among the highest N₂-fixation rates reported in the scientific literature. Measureable N₂ fixation also occurred in sediments in contact with reservoir epilimnia (0.85–1.5 g N m⁻² y⁻¹). Sediment N₂-fixation rates exceeded rates of N regeneration from epilimnetic sediments (0.10–0.24 g N m⁻² y⁻¹), a result suggesting that much of this fixed N either accumulates seasonally in sediments or is lost via coupled nitrification–denitrification. NH₄ flux from hypolimnia (0.13–1.8 g N m⁻² y⁻¹) and lower metalimnia (∼0.00–1.8 g N m⁻² y⁻¹) was a substantial internal N supply to epilimnia during summer stratification. However, phytoplankton N₂-fixation rates always exceeded rates of N input from these internal sources. These findings suggest that, even though the annual cycle of ecosystem N inputs via N₂ fixation has occurred over many years or decades in these reservoirs, reactive N accumulation has not been sufficient to alleviate seasonal N deficiency and lead to perpetual P limitation of phytoplankton.

We quantified microscale flow forces and their ability to entrain the freshwater polychaete, Manayunkia speciosa, the intermediate host for 2 myxozoan parasites (Ceratomyxa shasta and Parvicapsula minibicornis) that cause substantial mortalities in salmonid fishes in the Pacific Northwest. In a laboratory flume, we measured the shear stress associated with 2 mean flow velocities and 3 substrates and quantified associated dislodgement of polychaetes, evaluated survivorship of dislodged polychaetes, and observed behavioral responses of the polychaetes in response to increased flow. We used a generalized linear mixed model to estimate the probability of polychaete dislodgement for treatment combinations of velocity (mean flow velocity  =  55 cm/s with a shear velocity  =  3 cm/s, mean flow velocity  =  140 cm/s with a shear velocity  =  5 cm/s) and substrate type (depositional sediments and analogs of rock faces and the filamentous alga, Cladophora). Few polychaetes were dislodged at shear velocities <3 cm/s on any substrate. Above this level of shear, probability of dislodgement was strongly affected by both substrate type and velocity. After accounting for substrate, odds of dislodgement were 8× greater at the higher flow. After accounting for velocity, probability of dislodgement was greatest from fine sediments, intermediate from rock faces, and negligible from Cladophora. Survivorship of dislodged polychaetes was high. Polychaetes exhibited a variety of behaviors for avoiding increases in flow, including extrusion of mucus, burrowing into sediments, and movement to lower-flow microhabitats. Our findings suggest that polychaete populations probably exhibit high resilience to flow-mediated disturbances.

Despite improved understanding of how aquatic organisms are influenced by environmental conditions at multiple scales, we lack a coherent multiscale approach for establishing stream conservation priorities in active coal-mining regions. We classified watershed conditions at 3 hierarchical spatial scales, following a house–neighborhood–community approach, where houses (stream segments) are embedded within neighborhoods (Hydrologic Unit Code [HUC]-12 watersheds) embedded within communities (HUC-10 watersheds). We used this information to develop a framework to prioritize restoration and protection in two HUC-8 watersheds in an intensively mined region of the central Appalachians. We used landscape data to predict current conditions (water chemistry and macroinvertebrate biotic integrity) for all stream segments with boosted regression tree (BRT) analysis. Mining intensity, distance to mining, and coal type were the dominant predictors of water quality and biological integrity. A hardness–salinity dimension of the water-chemistry data was explained by land-cover features and stream elevation. We compiled segment-level conditions to the HUC-12 and HUC-10 watershed scales to represent aquatic resource conditions hierarchically across 3 watershed-management scales. This process enabled us to relate stream-segment watershed conditions to watershed conditions in the broader context, and ultimately to identify key protection and restoration priorities in a metacommunity context. Our hierarchical classification system explicitly identifies stream restoration and protection priorities within a HUC-12 watershed context, which ensures that the benefits of restoration will extend beyond the stream reach. Highest protection priorities are high-quality HUC-12 watersheds adjacent to low-quality HUC-12 watersheds. Highest restoration priorities are HUC-12 watersheds in poor–fair condition within HUC-10 watersheds of good–excellent condition, whereas lowest restoration priorities are isolated HUC-12 watersheds. In high-priority HUC-12 watersheds, stream segments with the highest restoration priority are those that maximize watershed-scale restorability. A similar process for classifying conditions and restoration priorities may be valuable in other heavily impacted regions where strategic approaches are needed to maximize watershed-scale recovery.

We used an integrative-taxonomy approach to help resolve taxonomic issues within the genus Micropsectra (Diptera:Chironomidae). We used partial cytochrome c oxidase subunit I (COI) and nuclear carbamylphosphate synthetase (CAD) sequences and morphological data to provide a framework for better understanding of North American species in this group of nonbiting midges. As part of our results, we describe 3 new species: Micropsectra neoappendica, n. sp., Micropsectra penicillata, n. sp., and Micropsectra subletteorum, n. sp., and 1 species new to the north-central USA, Micropsectra xantha (Roback). Two of the species, M. neoappendica n. sp. and M. subletteorum n. sp., initially appeared to be morphologically identical to species known from the Palearctic. However, molecular data indicated that they were genetically distinct, and reexamination of adult and pupal morphology revealed slight but consistent diagnostic differences. The implications of using species-level identifications for cryptic-species complexes in biological monitoring and conservation management are briefly discussed with reference to our findings. Our results emphasize the importance of using molecular tools in conjunction with traditional morphological techniques when studying Chironomidae diversity, especially when relying on diagnoses from other geographic regions.

Chironomid larvae possess giant polytene chromosomes. When genes on these chromosomes undergo transcription, they are visible as puffs. The nucleolar organizer region (NOR), visible as an especially large puff, shrinks when a larva is at particular developmental stages or is subjected to chemical stress. However, whether reduced NOR size is indicative of reduced growth is unknown. Therefore, we conducted 2 experiments to examine the relationship between NOR size and chironomid growth under controlled laboratory conditions. In the 1^st experiment, we quantified the effect of ration quality on larval growth and NOR size. In the 2^nd, we determined whether NOR size varied as a function of recent growth, independently of larval size. The combined results demonstrated that NOR size varied as a function of prepupal development and was positively correlated with a chironomid's most recent growth rate, independently of its biomass. The finding that NOR size is related to growth validates its use as a biomarker of sublethal stress. NOR size also has potential value as a measure of instantaneous growth state of field-collected larvae and, thus, may provide a surrogate measure useful for estimating secondary production in natural populations.

The objective of our study was to compare leaf breakdown rates (k) and the influence of microorganisms and aquatic invertebrates on mass loss of Eichhornia crassipes (Mart.) Solms and Typha domingensis Pers in 2 reservoirs (eutrophic and oligotrophic). We hypothesized that k would be higher in eutrophic than in oligotrophic conditions because of increased microbial activity in eutrophic conditions. We collected green leaves, which we air-dried, weighed, placed in litter bags, and incubated in each reservoir. We calculated k (negative exponential model) for each species in each reservoir. We characterized initial leaf chemistry, estimated total microbial biomass (as adenosine triphosphate [ATP]) and fungal biomass (as ergosterol), and evaluated invertebrate community composition and structure. Both species decomposed faster in the eutrophic reservoir. During leaf breakdown, bacteria were more important in the eutrophic reservoir, whereas fungi were more important in the oligotrophic reservoir. Invertebrate communities differed between reservoirs, but invertebrates did not affect k in either reservoir. Our results indicate that leaf breakdown may have been accelerated by greater nutrient availability and variations in O₂ concentration and water temperature that increased microbial community metabolism in the eutrophic reservoir. Typha domingensis held nutrients in its tissues for longer than E. crassipes, and might be useful for management of nutrients in reservoirs, whereas E. crassipes decomposed rapidly and would not be useful for controlling eutrophication.

Zoological remains were examined from the sediments of the Pingualuit Crater Lake, Nunavik, Canada. Our objective was to describe past climate events in the area of delayed deglaciation in northernmost Ungava Peninsula. Our record covers 3 separate sections of deglacial and postglacial invertebrate dynamics interrupted by laminated proglacial sediments and a basin-scale erosive slumping event. The abundance of animal remains in the ultra-oligotrophic and extremely deep arctic lake was low, but distinct faunal assemblages were found among the intervals, results implying that they were environmentally heterogeneous. The lowermost fine-grained interval (before 6850 calibrated years before present [cal BP]), revealed that Cladocera Chydorus sphaericus-type and Bosmina (Eubosmina) longispina-type were common in the lake, whereas Chironomidae were relatively rare. The dominance of B. longispina-type showed that planktonic communities were successful at the time, probably indicating more favorable climatic conditions than today soon after the last deglaciation (∼7000 cal BP). In the middle interval (between ∼6850 and 5750 cal BP), chironomids became more common and were dominated by Heterotrissocladius subpilosus-type and Protanypus, taxa that are characteristic of oligotrophic lakes. The extirpation of B. longispina-type suggests that planktonic invertebrate communities were not successful, probably because of predation by Arctic char. The presence of the chironomid Oliveridia tricornis-type during the late Holocene (between ∼4200 and 600 cal BP) suggested general climate cooling. Our paleoclimatic conclusions on the regional environmental history suggest a stationary ice front in the initial stages of the Holocene, favorable climatic conditions in the mid-Holocene and a general late-Holocene cooling. Our records also indicate a subtle increase in nutrient availability throughout the Holocene. The paleoecological record from Pingualuit Crater Lake is valuable in describing the faunal history and biotic resilience in this environmentally extreme lake, which presently contains one of the world's softest and most transparent waters.

Identifying the ecological mechanisms that determine foodweb structure is critical for understanding the causes and consequences of diversity. Food-chain length (FCL) is a product of the biotic interactions within a community and the environment, but how environmental variation affects FCL is not well understood. We examined how gradients of ecosystem size and environmental variation in hydroperiod affected the FCL of ponds. Using C and N stable isotopes, we found that average FCL was 3.3 and varied by 3 trophic levels across ponds and years. We showed that ponds with shorter hydroperiods have shorter food chains, and that FCL is not strongly influenced by ecosystem size. These results demonstrate support for the dynamic constraints hypothesis, which states that less-predictable environments should have shorter food chains. Our data are not consistent with the prediction of the ecosystem size hypothesis that larger ecosystems have longer food chains. Insect and amphibian richness increased with increasing pond size and hydroperiod, results indicating that insertion of new species into the pond communities is a driving mechanism causing variation in FCL. Omnivory could explain variation in FCL, but our results show that the incidence of omnivory was similar across the environmental gradients. Ours is one of the few empirical studies to link structural changes in the food web with variability in FCL. We showed that temporal variation and species composition shape pond communities and influence foodweb structure.

We hypothesized that C. fluminea could remove anthropogenic N at a rate sufficient to affect particulate N in the water column of North Buffalo Creek, North Carolina, USA, a 4^th-order stream that receives treated urban wastewater. We used stable-isotope analysis to evaluate trophic relationships between seston and C. fluminea and conducted field sampling and laboratory experiments to evaluate the potential qualitative and quantitative effects of C. fluminea on seston. Corbicula fluminea δ¹⁵N was 3 to 5‰ enriched compared to seston along a longitudinal transect downstream of the wastewater treatment plant (WWTP), a result consistent with use of seston N. However, seston δ¹³C declined and C. fluminea δ¹³C showed no pattern with distance downstream, a result that was inconsistent with a trophic relationship between seston and C. fluminea. In 2 laboratory experiments designed to measure filter-feeding rate and qualitative effects on seston, seston ash-free dry mass and chlorophyll a data indicated that C. fluminea either was not filtering or was filtering at a rate insufficient to affect seston concentration over the course of the experiments. δ¹⁵N data showed that the sediment was an N source for C. fluminea, but δ¹³C and C∶N data from the same experiments indicated that C. fluminea probably affected seston quality by suspending benthic algae and returning settled algae to the water column. These results illustrate that the food sources for C. fluminea and implications of C. fluminea activity in stream ecosystems should be evaluated more fully.

N₂-fixing bacteria convert atmospheric N₂ gas to biologically available forms, yet N limits primary production in many ecosystems. Despite an abundant source of N, other elements may limit N input via N₂-fixation in benthic environments. We examined how N, P, and the micronutrients Fe and Mo (cofactors of nitrogenase) might affect N₂-fixation. We used short-term nutrient additions to assess macronutrient effects on N₂-fixation of intact epilithic biofilm in Ditch Creek, Wyoming (USA). Additions of NH₄ , NO₃⁻, and PO₄³⁻ at varying concentrations did not alter N₂-fixation over 4 h. We assessed macro- and micronutrient effects on N₂-fixation, biofilm biomass, and composition of the biofilm algal assemblage at Ditch, Spread, and Kimball Creeks, Wyoming, via 6-wk nutrient amendments with nutrient-diffusing substrata. Fe and Mo additions did not affect N₂-fixation or biofilm biomass, but NO₃⁻ and PO₄³⁻ altered the biofilm biomass, algal assemblage structure, and N₂-fixation. N and P colimited biofilm biomass in Ditch Creek, and P limited biofilm biomass in Spread Creek. N₂-fixation was inhibited by NO₃⁻ and stimulated by PO₄³⁻. Inhibition by NO₃⁻ was stronger than stimulation by PO₄³⁻. Trends in accumulation of N₂-fixers, mostly Epithemia sorex with N₂-fixing endosymbionts, corresponded to effects of nutrient additions on N₂-fixation. The algal assemblage shifted from N₂-fixer dominance in control and P treatments to fewer N₂-fixers and increased biovolume of nonN₂-fixing diatoms in the N and N P treatments. N₂-fixation/unit E. sorex biovolume was inhibited by NO₃⁻ and stimulated slightly by PO₄³⁻. A combination of demographic and mutualistic responses to nutrients altered N input via N₂-fixation.

We developed and evaluated a multimetric index of lake diatom condition (LDCI) based on surface-sediment samples for the National Lake Assessment (NLA) by the US Environmental Protection Agency. We selected final metrics in each of 5 categories for use in composite metrics of biological condition, which we combined in a hierarchical multimetric index. The final metrics selected for the LDCI had responses as predicted based on ecological principles, wide ranges, high signal-to-noise ratios, the ability to distinguish reference from disturbed lakes, and low intercorrelation. The final metrics were: Shannon diversity, taxon richness, % reference taxa, % tolerant taxa, % epiphytic individuals, % chain-forming individuals, % low-P taxa, % low-N taxa, % high-P taxa, % high-N taxa, % Achnanthidium individuals, % Cocconeis individuals, and % Cyclotella Stephanodiscus individuals. We developed and tested models for adjusting the expected reference LDCI at a site for natural variation among sites. The best model used lake morphology, watershed attributes, and climate variables to predict expected reference LDCI for each lake. The adjusted LDCI was the observed LDCI at a lake minus the expected reference LDCI modeled for that lake. The adjusted LDCI varied less among reference sites than the LDCI, indicating the adjusted LDCI better accounted for natural variability among sites than the LDCI. However, it had less power than the LDCI to separate reference and disturbed lakes, a result attributed to lower values of expected reference LDCI in landscapes with high agricultural land use. The adjusted LDCI is the first multimetric diatom index developed for lakes, applied at a scale as large and diverse as the USA, and corrected for natural variation among lakes. According to the adjusted LDCI, 47.1% of USA lakes were in reference condition, 27.0% were in fair condition, and 23.2% were in poor condition.

Decomposition of senesced primary production starts processing chains in aquatic systems. Shredding macroinvertebrates convert coarse particulate organic matter (CPOM) to fine particulate organic matter (FPOM) that supports 2 other feeding groups, collecting and filtering macroinvertebrates. This linkage is often invoked by aquatic ecologists, but the effect of detritivore assemblage composition on production of FPOM is relatively understudied. I manipulated detritivore assemblage composition (Limnephilus sp., Caecidotea sp., and Hyalella azteca) in aquatic mesocosms stocked with green speckled alder leaves (Alnus incana rugosa). I measured production rate, size distribution, and stoichiometry of FPOM produced through time. Detritivore species richness had a positive effect on FPOM production resulting from inclusion of the functionally dominant shredder, Limnephilus sp., in mixed-species treatments (e.g., sampling effect). Mixed-species treatments had significantly faster particle production than predicted from single-species treatments. The significant increases in particle production in mixed-species treatments could have resulted from release of Limnephilus sp. from intraspecific competition, facilitation between shredders, or both processes. FPOM size distribution and C∶N varied significantly among treatments and was affected by species interactions in mixed-species treatments. The presence of Limnephilus sp. significantly skewed the FPOM size distribution and increased the mass of particles >250 µm by ∼60%. These results suggest that the specific shredding insects in an assemblage could strongly affect production of FPOM and the size distribution and stoichiometry of FPOM produced by the benthos of a stream.

We examined how invasion of tropical riparian forests by an exotic N-fixing tree (Falcataria moluccana) affects organic-matter dynamics in a Hawaiian river by comparing early stages of leaf-litter breakdown between the exotic F. moluccana and native Metrosideros polymorpha trees. We examined early decomposition stages because of low leaf-litter retention rates (<20 d) that result from the flashy nature of tropical Pacific Island streams. Leaf breakdown rates, fungal biomass, and invertebrate abundances were 40, 120, and 30% greater, respectively, for F. moluccana than M. polymorpha leaves. Leaf-litter breakdown was largely a result of stream flow and to a lesser extent fungal colonization. Invertebrates were not an important factor in leaf-litter breakdown. Initial tannin content, leaf C∶N, and toughness were important intrinsic factors inhibiting leaf breakdown and fungal colonization. Regression analyses between remaining N content (%) and ash-free dry mass of leaf litter revealed that the early stages of F. moluccana leaf-litter breakdown are a source of N to streams invaded by F. moluccana and contribute a conservatively estimated 2.1 to 5.7% to the available total dissolved N pool. Direct input of F. moluccana leaf litter influences early stages of leaf-litter breakdown in tropical streams with low leaf-litter retention rates. Direct input of leaf litter also contributes somewhat to N inputs, but subsurface flows through N-rich soils of F. moluccana-invaded riparian forests probably are a greater source.