Tadpoles are often abundant and diverse consumers in headwater streams in the Neotropics. However, their populations are declining catastrophically in many regions, in part because of a chytrid fungal pathogen. These declines are occurring along a moving disease front in Central America and offer the rare opportunity to quantify the consequences of a sudden, dramatic decline in consumer diversity in a natural system. As part of the Tropical Amphibian Declines in Streams (TADS) project, we examined stream macroinvertebrate assemblage structure and production for 2 y in 4 stream reaches at 2 sites in Panama. One site initially had healthy amphibians but declined during our study (El Copé), and 1 site already had experienced a decline in 1996 (Fortuna). During the 1^st y, total macroinvertebrate abundance, biomass, and production were generally similar among sites and showed no consistent patterns between pre- and post-decline streams. However, during the 2^nd y, tadpole densities declined precipitously at El Copé, and total macroinvertebrate production was significantly lower in the El Copé streams than in Fortuna streams. Functional structure differed between sites. Abundance, biomass, and production of filterers generally were higher at Fortuna, and shredders generally were higher at El Copé. However, shredder production declined significantly in both El Copé reaches in the 2^nd y as tadpoles declined. Nonmetric dimensional scaling (NMDS) based on abundance and production indicated that assemblages differed between sites, and patterns were linked to variations in relative availability of basal resources. Our results indicate that responses of remaining consumers to amphibian declines might not be evident in coarse metrics (e.g., total abundance and biomass), but functional and assemblage structure responses did occur. Ongoing, long-term studies at these sites might reveal further ecological consequences of the functional and taxonomic shifts we observed.

N is a critical element for all living things, including the flora and fauna in aquatic ecosystems. However, N exists in many forms, and some organisms might preferentially use one compound over another. We used ¹⁵N tracers in laboratory microcosm experiments to measure preferential uptake of 4 different N compounds (NH₄-N, NO₃-N, glycine, methionine) by filamentous green algae (Cladophora glomerata; autotrophs), microbes in stream sediment (heterotrophs), and freshwater snails (Physa acuta; consumers) in response to varying trophic complexity. Seven trophic complexity treatments were applied: 3 types of single-organism treatments (filamentous algae, sediment microbes, or snails), three types of 2-organism treatments (filamentous algae sediment microbes, sediment microbes snails, filamentous algae snails), and 1 treatment consisting of all organisms (filamentous algae sediment microbes snails). We combined trophic complexity treatments (n  =  7) with 1 of 5 ¹⁵N tracers: control (no ¹⁵N), ¹⁵NH₄, ¹⁵NO₃, ¹⁵N-glycine, and ¹⁵N-methionine (enriched to ~3000‰). Each trophic complexity treatment was replicated 4 times for each ¹⁵N tracer treatment to yield a total of 140 microcosms. We incubated microcosms for 5 d after which organisms were rinsed, dried, ground, and analyzed for ¹⁵N content. Overall, uptake of ¹⁵NH₄ was greater than uptake of any other N form for all organisms. However, trophic complexity changed the relative uptake rates of different N forms. Cladophora preferentially used ¹⁵NH₄ when alone, but when in the presence of heterotrophic competitors or consumers, Cladophora ¹⁵NO₃ uptake doubled. Cladophora uptake of ¹⁵N-glycine and ¹⁵N-methionine decreased by ½ in the presence of other organisms. In contrast, when sediment microbes were exposed to Cladophora or Physa, ¹⁵NH₄ and ¹⁵N-methionine uptake rates increased. The N form incorporated by Physa was dependent on the food source present (Cladophora vs sediment microbes). The highest N uptake rates for all N forms were associated with the presence of Cladophora. These data indicate that N uptake is a plastic trait, and organisms can alter use of N forms in response to the presence of competitors or consumers.

Fish density models are essential tools for fish ecologists and fisheries managers. However, applying these models can be difficult because of high levels of model complexity and the large number of parameters that must be estimated. We designed a simple fish density model and tested whether it could predict fish densities in lotic systems with meaningful levels of accuracy and precision. We built our 6-parameter model on 2 key assumptions: 1) fish population density is a power function of mean body mass (i.e., the self-thinning relationship), and 2) energetic resources are transferred from lower to higher trophic levels at a nearly constant rate (i.e., trophic transfer efficiency). We estimated the self-thinning and trophic transfer efficiency parameters by randomly sampling from values reported in the primary literature. Remaining parameters were net primary production, trophic level, the production∶biomass ratio, and mean body mass. We used empirical parameter estimates and fish density estimates to test the model in 4 warm-water and 4 cold-water systems. Model accuracy was high in 3 test systems (deviations between the model-predicted densities and empirically observed densities <30%), moderate in 3 test systems (deviations 75–111%), and low in 2 systems (deviations >150%). Model precision was low (e.g., the interquartile ranges of model-predicted densities encompassed ~1 order of magnitude), but appropriate for predicting fish densities at coarse spatial and temporal scales. We concluded that the model is a potentially useful and efficient tool, and we provide recommendations for applying the model. In particular, we emphasize that the model is scalable, and therefore, well-suited for estimating fish densities at large spatial scales. We also point out that the model is a carrying capacity model, and therefore, can be used to predict fish densities in undisturbed systems or to approximate reference conditions.

I investigated the effectiveness of activity traps for macroinvertebrate monitoring in shallow, heavily vegetated drainage ditches and explored 2 ways to optimize the use of activity traps for monitoring purposes. I tested the effects of trapping duration (48, 96, and 168 h) and use of attractants (bait and preconditioned leaves). The number of taxa and individuals captured increased with trapping duration. Based on the taxon accumulation curves, deployment times of 48 h and 96 h were equally efficient in capturing new taxa, but a trapping duration of 168 h was much more efficient and resulted in a larger number of taxa collected with every new sample added. Of the attractants offered in the traps, only bait caused differences in the macroinvertebrate assemblage recorded. After 48 h, more predators were captured in traps with bait than in control traps and traps with preconditioned leaves. This effect disappeared with longer trapping duration. Because of their relatively low labor requirements and high level of standardization, activity traps appear to be a valuable tool in lentic biodiversity surveys, especially when deployed for a longer period than has usually been reported. The use of bait is advisable only if capture of specific taxa is required and not for standard monitoring purposes.

In a previous study, we found that underlying geology, acidic precipitation, and acidic mine drainage (AMD) associated with coal extraction in the Monongahela River basin, West Virginia, interact to produce discrete water-quality types among streams, including: high-quality reference, soft, hard, transitional, and severely impaired AMD types. In this study, we tested the prediction that this discrete impairment template would produce correspondingly discrete benthic macroinvertebrate assemblages. Furthermore, we posited that unique macroinvertebrate genera would serve as strong indicators of each water-quality type. Macroinvertebrate assemblages were strongly influenced by stream water chemistry, and tests of compositional similarity identified statistical links with water-quality types. However, assemblage composition was highly variable within and among water-quality types, a finding that provides greater support for continuous than for discrete structure. High variability in assemblage composition among streams of the same water-quality type could not be explained by differences in physical habitat or by variation in water chemistry. Benthic invertebrate assemblages also exhibited significant nestedness, but only AMD assemblages were clear subsets of reference-type assemblages. Furthermore, indicator species analysis found relatively few genera that were indicators for specific water-quality types, but they were strong for reference streams (e.g., Epeorus, Dolophilodes), soft-water streams (e.g., Simulium, Leuctra), and hard-water streams (e.g., Ectopria). Therefore, little evidence was found for a direct correspondence between benthic macroinvertebrate assemblages and the discrete water-quality template. We suggest that metacommunity dynamics might prevent a strong correspondence despite the localized effects of water chemistry. Consequently, use of benthic macroinvertebrate assemblage data to diagnose specific water-quality stressors might continue to be difficult until we factor in the effects of invertebrate dispersal and assemblage dynamics at a watershed scale.

Mayflies (Ephemeroptera) are important to the food web of most stream, river, and lake ecosystems and are critical to water-quality monitoring programs. They are widespread and ancient (predate dinosaurs) and have primitive reproductive systems and the shortest adult life spans of all insects. Here we formulate and test the hypothesis that facultative parthenogenesis occurs as a widespread adaptation in most, if not all, mayflies. A rare form of reproduction, facultative parthenogenesis combines the short-term advantages of parthenogenesis, which doubles reproductive output, with the long-term advantages of genetic variation associated with sex. For 7 species of bisexual mayflies, we show that their eggs hatched whether fertilized or not and that unfertilized eggs took longer to develop and hatch. However, once hatched, larvae produced via parthenogenesis grew and developed to the adult in a manner similar to that of larvae produced sexually. In addition, for all study species, female adults produced parthenogenetically were diploid and could reproduce either sexually (producing males and females equally) or parthenogenetically (producing mostly females and some males). Males produced parthenogenetically were viable and occurred either as 1^st generation (F1) offspring or as offspring of F1 gynandromorphs (intersex individuals). Last, we show that parthenogenetic descendants lose ~10 to 22% of their genetic variation as a result of each parthenogenetic generation. We use these findings and observations and other published data to support the hypothesis that most, if not all, mayflies are facultatively parthenogenetic. We propose that this ability increases mayfly reproductive success by giving eggs from unmated or incompletely fertilized females a 2^nd chance. We then formulate 3 additional hypotheses: 1) parthenogenetic development of mayfly eggs proceeds automatically soon after emergence but can be and normally is preempted by fertilization, and that this mechanism both helps to compensate for and is enabled by the peculiarities of life history and primitive reproductive biology in this ancient group; 2) that facultative parthenogenesis enhances mayfly dispersal and gene flow because it enables virgin females to transfer genes within and among catchments and to form bisexual populations in new habitat; and 3) that mayflies remain bisexual despite being able to reproduce without males because of lost genetic variation associated with the parthenogenetic process.

Ecosystems generally are not stable over long periods of time and are subject to disturbances of different frequencies and intensities. As a result, natural communities usually are not in equilibrium, and temporal snapshots can show different stages of community assembly. Nonequilibrium is especially common in temporary aquatic systems where desiccation continuously resets the development of communities and hydroperiod is an important determinant of species richness and community structure. We investigated effects of disturbance regime on community assembly of aquatic invertebrates in a cluster of 36 temporary rock pools in central South Africa. Pools were assigned to 4 categories based on their long-term disturbance regime and were sampled 4 times during 1 inundation. Disturbance modulated the relative importance of deterministic and stochastic processes driving community assembly and reduced both α and β diversity. Premature desiccation truncated community development. Initial communities were dominated by resident large branchiopods (fairy shrimp and clam shrimp). Arrival of flying colonists, including many predators, caused a drastic change in community composition and initiated a new successional phase dominated by cladocerans. For residents, the link between local environment and community structure was strongest early during inundation and decreased during community assembly. For flying colonists, variation in arrival time and habitat selection led to a better match between communities and environment later during inundation. We conclude that the combination of colonization–extinction dynamics and niche partitioning in space and time determines metacommunity structure in this extreme type of aquatic habitat.

Invertebrates are often used in biological monitoring of soil and water ecosystems. Because of the huge number of invertebrate species, sample processing (sorting and identification) is a labor-intensive and often difficult task that is prone to error. These errors can bias assessment results, which often are used by environmental managers to guide funding decisions for costly restoration measures. However, quality control of assessment results is not implemented in many freshwater monitoring programs. We conducted the first audit of an official European freshwater monitoring program based on 414 macroinvertebrate samples from streams and rivers in Germany. The samples were collected by personnel at 7 different commercial environmental laboratories using the European Union (EU) Water Framework Directive protocol. We audited 12% of all samples at 3 different levels: 1) a sorting audit, 2) an identification audit, and 3) a total audit based on both sorting and identification. The sorting audit revealed that 29% of the specimens and every 5^th taxon (20.6%) had been overlooked by the primary analyst. Differences in sorting were correlated with taxon body size (r  =  0.61, p < 0.001). The identification audit showed that >30% of taxa differed between the results of the primary analysts and auditors. Taxa considered difficult to identify were not more prone to error than were taxa considered easier to identify. Primary analysts and auditors assigned 34% of audited samples to different quality classes in ≥1 of 3 assessment modules (organic pollution, acidification, and general degradation). For 16% of the samples, these changes resulted in a different final ecological assessment. Such a high rate of differences between primary analysts and auditors could lead to ineffective allocation of several million Euros. Our results clearly illustrate the need for adequate quality control and auditing in freshwater monitoring.

Coal-mine development is occurring at a rapid rate in the central Appalachians, but few tools exist to assess the consequences of cumulative effects of mining to downstream aquatic resources. We constructed and applied an index of mining intensity (MI) to the Lower Cheat River basin, northern West Virginia. Our objectives were to: 1) determine if the MI could be used to predict stream-water quality and biological conditions, 2) quantify the extent to which geology and the geographic position of mines modulate the effects of mining on in-stream conditions, and 3) identify thresholds of MI that produce quantifiable changes to benthic macroinvertebrate communities. We quantified water chemistry, habitat quality, and benthic macroinvertebrate communities from May 2002 to May 2003 in 39 stream segments randomly distributed across a range of MI, coal geology, elevation, and watershed area. We sampled benthic macroinvertebrates at an additional 41 validation sites in May 2002. The MI was positively correlated with dissolved metals (r  =  0.65–0.85) and negatively correlated with ecological condition metrics (r  =  0.49–0.78), including total richness, Ephemeroptera, Plecoptera, Trichoptera richness, and the West Virginia Stream Condition Index. Coal geology and distance from mining had a significant interactive effect on benthic macroinvertebrate responses. Streams draining watersheds with Freeport coal geology had significantly poorer water quality and ecological condition than streams draining watersheds with similar MI but with Kittanning coal geology. Mining effects on stream conditions diminished as distance from the nearest mining activities upstream increased to a distance of ∼10 km. Changepoint analysis provided evidence of threshold effects of mining on benthic macroinvertebrate communities in Freeport coal watersheds but not in Kittanning coal watersheds. Abrupt reductions in ecological condition occurred at MI values as low as 1 to 5% of maximum intensity, and ecological impairment to streams became almost certain at MI >18 to 20%. Our results provide evidence of an interactive effect of landuse intensity, underlying geology, and the spatial arrangement of disturbance on the degree of impairment to receiving water bodies. The thresholds we identified could be used by water-resource managers to protect and restore stream conditions in actively mined watersheds of the central Appalachian region.

We tested whether the functional structures of Mediterranean and temperate western European fish communities responded similarly along environmental gradients. The species pools of the 2 regions were quite different, with few species common to both regions. Each species was assigned to 1 trait for each of 6 guilds considered. We aggregated occurrences or densities of the species sharing the same traits and then computed 26 metrics describing the functional structure of fish assemblages. For each metric, we fitted and then compared 3 nested models. The 1^st model related the metric to environmental variables without taking into account the region. Therefore, the response was assumed to be the same between regions. The 2^nd model related the metric to the environmental variables with the region as an additive parameter. Therefore, the response was assumed to be similar between regions but with a constant deviation between them. The 3^rd model took into account all interactions between the environmental variables and region. Therefore, the response to the environmental gradient was assumed to be different in the 2 regions. For the 17 metrics finally tested, 11 metrics responded similarly to environmental gradients but generally showed a constant deviation between the 2 regions, and responses of 6 metrics differed between the regions. Our results highlight the roles played by biogeographical factors and the environment on current community structure in 2 neighboring but ecologically distinct regions.

The effects of wildfire can alter the structure of stream insect assemblages. Post-fire shifts to dominance by r-strategist taxa could drive increases in productivity of primary consumer and predatory insects, but this possibility has not been explicitly investigated. Likewise, the extent and duration of such effects might be mediated by fire severity, but this hypothesis also has not been evaluated. We report results from a comparative study that examined the mid-term (5–10 y post-fire) effects of wildfire of varying severity on stream insect assemblage composition and productivity measured in terms of benthic larval biomass and flux of emerging adults. We compared benthos and emergence in a suite of 2^nd- to 3^rd-order, unburned streams to those that had experienced low-severity and high-severity wildfire in wilderness watersheds of central Idaho. Reaches that experienced high-severity burn had the greatest biomass of r-strategist, generalist primary consumers that included Chironomidae, Baetis, and Simuliidae. The greatest biomass of predatory insects, such as Rhyacophila, occurred in reaches that experienced high-severity burn. Differences in composition of emerging insects were more pronounced in some time periods than others, with greatest emergence from high-severity reaches in early summer. High-severity reaches consistently had the greatest emergence flux. Reaches that experienced low-severity burn had the least emergence, and unburned reaches had intermediate emergence flux. Our results suggest that burn severity might drive differences in aquatic insect assemblages and their productivity.

The origin of organic C supporting zoobenthic communities in 8 boreal lakes with different concentrations of dissolved organic C (DOC) was assessed by stable-isotope analysis. Profundal zoobenthos was depleted in ¹³C compared to littoral zoobenthos, and this difference increased with decreasing DOC concentration. The δ¹³C of littoral zoobenthos suggested reliance on benthic algae, whereas depleted ¹³C of profundal zoobenthos could be explained by contributions from allochthonous and autochthonous C sources. In deeper lakes, profundal zoobenthos diets also included C processed by methanotrophic bacteria. Littoral zoobenthos δ¹³C decreased with increasing DOC concentration in the lake water. Our results suggest that littoral benthic fauna are mainly supported by benthic algae in low-DOC lakes and by phytoplankton and allochthonous organic C in high-DOC lakes and that this difference is a result of light absorbance and energy supply by allochthonous organic C. Increasing allochthonous DOC inputs, as expected in a warmer and wetter climate, might reduce benthic algal production and alter the organic C base for benthic food webs in lake ecosystems.

Climate change is predicted to increase the frequency of rain-on-snow events in the winter and spring in mountain streams. Flooding caused by such events can drastically alter stream geomorphology, but much less is known about how such floods affect aquatic life. A record flood in early January 1997 afforded an opportunity to carry out a case study of benthic invertebrate community responses to this flood in 14 eastern Sierra streams sampled in the summers before and after the event. All sites were exposed to the flood. Study sites included 8 exposed to livestock grazing disturbance (test) and 6 where grazing was absent or minimal (reference). Reference sites had more riffle habitat, coarser substrata (cobbles), and greater relative abundances of many Ephemeroptera, Plecoptera, and Trichoptera (EPT) genera, whereas test sites had higher alkalinity, more pool habitats and fine substrata, lower diversity of EPT taxa, and higher densities of chironomid genera and elmid beetles. From 1996 to 1997 (before vs after the flood), the densities of total invertebrates and many taxa increased in most sites, and significantly (10×) more at test than reference sites. At all sites, bank stability and riparian vegetation coverage decreased and roots were exposed on the stream banks. At many sites, fine sediments decreased and average substrate sizes increased. Examination of 1996 to 1997 vectors in nonmetric multidimensional scaling (NMDS) space indicated differences in responses at reference vs test sites. Macroinvertebrate assemblages at reference sites changed relatively little, whereas the relative abundances of certain midges and Hydropsyche increased at test sites. Densities of filterers and small gatherers of fine particulate organic matter increased at test sites, and densities of filterers decreased at reference sites. Changes from 1996 to 1997 suggested that the winter rain-on-snow event had little effect in reference streams but increased the densities of small collector invertebrates at disturbed sites where fine sediments were flushed out and particulate organic resources increased. Given the predicted effects of climate change on mountain stream hydrology, these results indicate that benthic invertebrate communities in small pristine streams might be resilient to the effects of large rain-on-snow floods, but that small, mobile collector invertebrates in some degraded streams might increase in response to sediment flushing and increases in particulate organic matter.

Climate change is expected to alter freshwater communities and accelerate extinction, but the exact processes are poorly known. Here, we appraise interannual variation between 2 sympatric planarians (Crenobia alpina and Phagocata vitta) in upland Welsh streams over 25 y during which 1 of this pair (C. alpina) disappeared. We tested 3 nonexclusive hypotheses involving: 1) long-term changes in stream chemistry, 2) interspecific competition, and 3) climatic variation or directional change to explain this apparent local extinction. Several lines of evidence revealed potential exploitation competition between C. alpina and P. vitta. Coexistence was confined to conditions with high prey abundance (recorded as the abundance of Ephemeroptera, Plecoptera, and Trichoptera) and summer temperatures <12.5°C, whereas P. vitta dominated at sites with higher temperature, greater discharge, and lower prey abundance. The loss of C. alpina in the Llyn Brianne experimental catchments coincided with the largest-ever positive amplification of the North Atlantic Oscillation (NAO) in 1989 to 1994, accompanied by increased stream temperature, increased winter discharge, 2 summer droughts, and markedly reduced prey abundance. We suggest that interspecific competition and this prolonged climatic event acted in concert to favor P. vitta over C. alpina. Since its local loss, summer stream temperatures have generally exceeded the favorable range for C. alpina and, coupled with weak dispersal ability, probably explain its continued absence. Our data are consistent with the prediction that extreme climatic events will affect small, fluctuating populations. Nevertheless, this case study demonstrates clear difficulties in identifying unequivocally the exact climatic processes causing extinction where: 1) evidence is confined to weak inference, 2) responses to complex climatic events are nonlinear, 3) interactions occur among species or between climate and ecological processes, and 4) assessments are made retrospectively following extinctions.

Aquatic ecosystems and their fauna are vulnerable to a variety of climate-related changes. Benthic macroinvertebrates are used frequently by water-quality agencies to monitor the status of aquatic resources. We used several regionally distributed state bioassessment data sets to analyze how climate change might influence metrics used to define ecological condition of streams. Many widely used, taxonomically based metrics were composed of both cold- and warm-water-preference taxa, and differing responses of these temperature-preference groups to climate-induced changes in stream temperatures could undermine assessment of stream condition. Climate responsiveness of these trait groups varied among states and ecoregions, but the groups generally were sensitive to changing temperature conditions. Temperature sensitivity of taxa and their sensitivity to organic pollution were moderately but significantly correlated. Therefore, metrics selected for condition assessments because taxa are sensitive to disturbance or to conventional pollutants also were sensitive to changes in temperature. We explored the feasibility of modifying metrics by partitioning components based on temperature sensitivity to reduce the likelihood that responses to climate change would confound responses to impairment from other causes and to facilitate tracking of climate-change-related taxon losses and replacements.

We analyzed long-term changes in macroinvertebrate communities in a Mediterranean temporary stream in southern Portugal over 15 y (1993–2008) at 10 locations with 3 degrees of physicochemical disturbance (reference, high disturbance, and mild disturbance). We related year-to-year variation of macroinvertebrate communities to long-term (59 y) information on precipitation and temperature. Our goals were to: 1) determine the stability of macroinvertebrate communities in the stream, 2) establish the influence of physicochemical disturbance on community stability, 3) assess the influence of climate change on the macroinvertebrate communities, and 4) assess the interactive effects of climate change and disturbance level on macroinvertebrate communities. Community structure varied naturally from year to year, but changes in taxon richness and evenness were much stronger and more unpredictable in disturbed than in other sites. In the long term, the more diverse (reference) and the poorest (disturbed) communities were stable, whereas communities affected by mild disturbance slowly decreased in taxon richness (slope  =  −0.07, r²  =  0.38). This decrease could be a response to the continuous stress or to climate change. The multivariate patterns over time of invertebrate communities at mildly disturbed sites were the only patterns significantly correlated with climatic patterns. In the past 59 y in this Mediterranean area (southeastern Europe), winter temperature has increased 1°C and precipitation has decreased 1.5 mm/d. Marked changes in community composition (70–80% Bray–Curtis dissimilarity) occurred only in years of extremely low precipitation or temperature. In years of climatic extremes and at chemically disturbed sites, Orthocladiinae and Simuliidae became dominant. In this stream, a shift in community equity occurs before species elimination. This shift might be useful as an early warning for biodiversity loss because of disturbance or climate change. We recommend continued sampling of reference sites for monitoring purposes so that effects of climate change can be established and so that contemporary human disturbance can be assessed relative to an adjusted reference condition.

Analyses of long-term data are an important component of climate-change research because they can help further our understanding of the effects of climate change and can help establish expectations for biological responses to future climate changes. We used macroinvertebrate data to assess whether biological trends associated with directional climate change could be detected in routine biomonitoring data from Maine, North Carolina, and Utah. We analyzed data from 8 long-term biomonitoring sites that had 9 to 22 y of data, and focused on thermal-preference metrics based on cold- and warm-water-preference trait groups. The thermal-preference metrics were derived primarily from weighted-average or generalized-linear-model inferences based on data from each state database and are region specific. Long-term trends varied across sites and regions. At some sites, the thermal-preference metrics showed significant patterns that could be interpreted as being related to directional climate change, whereas at others, patterns were not as expected or were not evident. The strongest trends occurred at 2 Utah sites that had ≥14 y of data. At these sites, cold-water taxa were negatively correlated with air temperature, and, when years were grouped into hottest- and coldest-year samples, were strongly reduced in the hottest-year samples. Results suggest that thermal-preference metrics show promise for application in a biomonitoring context to differentiate climate-related responses from other stressors.

Climate change is expected to have strong effects on mediterranean-climate regions worldwide. In some areas, these effects will include increases in temperature and decreases in rainfall, which could have important implications for biological assessment programs of aquatic ecosystems. We used a consistently collected, 20-y benthic macroinvertebrate data set from 4 sites along 2 small northern California streams to examine potential climate-change effects on aquatic communities. The sites represented unique combinations of stream order and flow intermittency. The North Coast benthic macroinvertebrate index of biotic integrity (B-IBI) developed for northern California streams was not influenced by temperature extremes (cool and warm) or precipitation extremes (wet and dry). Other common indices and metrics used in biological monitoring studies, such as the ratio of observed to expected taxa (O/E), % Ephemeroptera, Plecoptera, and Trichoptera (EPT) individuals, and total richness were unaffected by temperature and precipitation variability. For future monitoring of climate-change effects on small streams, we developed a local climate-change indicator that is composed of the presence/absence of 9 macroinvertebrate taxa, identified to genus level. This indicator detected significant differences between years that were grouped based on temperature, precipitation, and a combination of temperature and precipitation. It also detected significant differences between groups in an external data set including 40 reference sites throughout the San Francisco Bay area, a result that suggests this indicator could be used at larger spatial scales in this region. Two biological trait categories found in large, long-lived organisms decreased with increasing temperature and decreasing precipitation at the most intermittent site. This result indicates that climate change might selectively affect taxa with certain traits. The robustness of the North Coast B-IBI and other common indices and metrics to temperature and precipitation variability demonstrates their continued applicability for examining water quality under future climate-change scenarios, but suggests that they probably will not be good indicators for detecting climate-change effects. The effects of climate change in mediterranean-climate streams can be monitored effectively within the framework of existing biological assessment programs by using regional indicators based on specific taxa identified to the generic level and information on their species traits.

Forecasting responses of benthic community structure and function to anthropogenic climate change is an emerging scientific challenge. Characterizing benthic species by biological attributes (traits) that are responsive to temperature and streamflow conditions can support a mechanistic approach for assessing the potential ecological responses to climate change. However, nonclimatic environmental factors also structure benthic communities and may mitigate transient climatic conditions, and these must be considered in evaluating potential impacts of climate change. Here we used macroinvertebrate and environmental data for 279 reference-quality sites spanning 12 states in the western US. For each sampling location, we described 45 environmental variables that spanned reach to catchment scales and that represented contemporary climate drivers, hydrologic metrics, and nonclimatic habitat features, as well as purely spatial metrics. We described benthic community composition at each site in terms of 7 species traits, including those considered sensitive to temperature increases and streamflow changes. All combined environmental variables explained 67% of the total trait variation across the sites, and catchment-scale climatic and hydrologic variables independently accounted for 19%. Sites were clustered into 3 community types based on trait composition, and a classification-tree analysis confirmed that climatic and hydrologic variables were important in partitioning these groups. Sensitivity of benthic communities to projected climate change was assessed by quantifying the proportion of taxa at sites having the traits of either cold stenothermy or obligate rheophily. Regression-tree analysis showed that temperature and hydrologic variables mostly accounted for the differences in proportion of sensitivity traits across the sites. We examined the vulnerability of sites to climate change by superimposing regional-scale projections of late-21^st-century temperature and runoff change on the spatial distribution of temperature- and runoff-sensitive assemblages. Sites with high proportions of cold stenotherms and obligate rheophiles occur throughout the western US, but the degree of temperature and runoff change is projected to be greatest for reference sites in the Upper Colorado River and Great Basin. Thus, our results suggest that traits-based sensitivity coupled with intraregional variation in projected changes in temperature and runoff will cause reference sites in the western US to be differentially vulnerable to future climate change.

Climate change is gradual and long-term, consistently collected data are required to detect resulting biological responses and to separate such responses from local effects of human activities that monitoring programs usually are designed to assess. The reference-condition approach is commonly used in freshwater assessments that use predictive modeling, but a consistent reference condition is required to maintain the relevance and integrity of results over the long term. We investigated whether external influences, such as climate change, inhibited clear interpretation of bioassessment results in a study design using reference vs test sites. Macroinvertebrates were collected from 16 sites (11 sites affected by ski resorts and 5 reference sites) on 5 streams in 4 seasons each year from 1994 to 2008 within Kosciuszko National Park, Australia. We analyzed trends over 15 y to address questions regarding climate-change and macroinvertebrate bioindicators of stream condition (observed/expected [O/E] taxa; Stream Invertebrate Grade Number Average Level [SIGNAL] 2 scores; Simpson's Diversity; Ephemeroptera, Plecoptera, Trichoptera [EPT] richness ratio; and Oligochaeta abundance). Climate became slightly warmer and less humid (p < 0.0001), but no significant relationships between climate variables and bioindicators were evident. All bioindicators consistently distinguished between test and reference sites in all seasons. All bioindicators except for O/E taxa scores differed among streams (regardless of site type). O/E taxa are inherently adjusted for specific stream characteristics, and, thus, were robust to differences in stream type while remaining sensitive to reference and test site variation. Generally, reference and test sites did not respond differently to any gradual climate changes. Furthermore, the reference sites sampled through time remained in a condition equivalent to the previously defined reference condition and provided a valid comparison for current test sites of unknown condition. The bioindicators used here were insensitive to the small but significant changes in climate detected over the 15-y study. However, extreme climate-related events (such as severe drought and extensive bushfire) were detected by the chosen bioindicators at both reference and test sites. Ecological outcomes of climate change can be accounted for only by an appropriate study design that includes standardized sampling of fixed sites (both test and reference) over long periods.

Phytoplankton and littoral invertebrate assemblages in 4 boreal lakes recovering from acidification and 4 minimally disturbed reference lakes studied over 2 decades were used to determine the pathways and trajectories of change under the influence of climatic variability. Assemblage composition (species presence–absence data) but not dominance patterns (invertebrate abundance/phytoplankton biovolume) of acidified lakes became more similar to those of reference lakes (distance decreased with time), indicating that detection of recovery varies as a function of chosen metrics. Acidified lakes had more pronounced shifts in assemblage composition than did reference lakes. The most marked differences were noted for phytoplankton assemblages. Assemblages in acidified lakes had mean between-year Euclidean distances almost 2× greater than those of assemblages in reference lakes. Trends in water chemistry showed unequivocal recovery, but responses of phytoplankton and invertebrate assemblages, measured as between-year shifts in assemblage composition, were correlated with interannual variability in climate (e.g., North Atlantic Oscillation, water temperature) in addition to decreased acidity. The finding that recovery pathways and trajectories of individual acidified lakes and the environmental drivers explaining these changes differed among assemblages shows that biological recovery is complex and the influence of climatic variability is poorly understood.

Despite the uncertainties in the rate of climate change, the Atlantic zone of northwestern Europe is expected to experience warmer, wetter winters and wetter summers than at present. Summer precipitation probably will depend on short, heavy rain showers between dry periods. Changes in the amount, frequency, and intensity of precipitation are expected to change stream discharge patterns, especially in rainwater-fed lowland streams, which will shift toward more dynamic flow regimes. Indices of discharge dynamics were used to assess the effect of changes in climate through changes in hydrology and land and water use on natural lowland stream macroinvertebrate communities. Discharge dynamics were significantly correlated with macroinvertebrate community structure, current velocity, and organic material preference. Our results demonstrate important influences of dynamic discharge regimes and extreme flows on macroinvertebrate community structure. Predictions of the ecological effects of climate change and of changes in land and water use indicate impaired ecological conditions in lowland streams of the Atlantic zone of northwestern Europe. Scenario tests involving different climate and landuse options suggest that current restoration practices and planned restoration activities can positively interact to reduce negative effects of climate change on lowland stream ecosystems.

Southern Florida's Everglades are at the front line of potential negative effects on aquatic ecosystems from climate change and associated sea-level rise. A diversity of aquatic habitats supports a rich assemblage of aquatic and terrestrial wildlife, including 36 vertebrates and 26 plant species federally listed as endangered, threatened, or candidate species. Anticipated climate-change trends for southern Florida include increased weather uncertainty with more droughts, higher temperatures, and an increased number of more intense storms. Hydrologic regimes, temperature, and CO₂ have been strongly correlated with plant community structure, coral and fish abundance and diversity, and higher trophic-level responses. Higher levels of variability in extreme climatic events, such as droughts, have the potential to destabilize aquatic communities. Sea-level rise is expected to be 0.8 to 2 m over the next century, a serious problem in a landscape that rises only 5 cm/km from Florida Bay inland. Wading birds and other wildlife species dependent on fresh water are likely to decline because of salt-water overwash and inundation. In addition to causing habitat loss, saltwater inundation of the peat substrate of Everglades freshwater wetlands may increase C emissions from sequestered C released as peat is destroyed and freshwater plant communities die. Identification of those species and habitats most at risk and ways to increase habitat and landscape resilience to large-scale environmental change will be critical for maintaining a diverse and productive Everglades.