Aggregation of organic matter occurs commonly in the water column of streams and rivers. Aggregates are formed by physicochemical or biological mechanisms, and these aggregations result in the transformation of suspended matter into much larger units. Microaggregates are formed by spontaneous assembly of organic molecules and resemble single particles. In contrast, macroaggregates (flocs and snow) are formed by binding of components with exopolymers (EPS), and they vary in size, shape, and porosity. Fecal pellets are a 3^rd type of aggregate and usually are bound by EPS but are more compact than flocs or snows. Transport of aggregates to the substratum plays a significant role in the use of organic matter by microorganisms and other benthic biota, and we need to integrate these processes into our studies of lotic ecosystems. Understanding aggregation is essential to basic science, and it plays a role in planning for conservation and management at a time when flowing waters and their catchments are increasingly threatened.

The complex effects of global climate change on freshwater ecosystems limit our ability to predict biological responses in a standard way and may compromise ecosystem management with respect to potential changes. I present a theoretical framework that shows the usefulness of resurrection ecology for standardizing cross-system comparisons of ecological responses to global climate change. Resurrection ecology makes use of plant seed and animal resting-egg (propagule) banks that integrate past environmental histories in the gene pools of their organisms. Resurrected organisms that have undergone different periods of dormancy can be studied comparatively using evolutionary/genetic and experimental approaches. Both approaches combined can provide insights into how the dimensions of species' ecological niches have shifted over time and could help reveal whether direct effects of climate change (increased temperatures and CO₂ concentrations and hydrological alterations) or other anthropogenic stressors (e.g., contamination, landuse change) have caused microevolution. Insights gained from resurrection ecology could be used to manage gene flow between populations and to help prevent extinctions of threatened populations. These insights could be used to help manage ecosystem structure and function and maintain ecological sustainability. However, our ability to apply results from resurrection ecology to organisms that do not have long-term dormancy stages in their life cycles may be limited, and the usefulness of resurrection ecology will have to be evaluated along gradients of hydroperiod and flood frequency, which may determine rates of microevolution in aquatic ecosystems.

Many studies testing hypotheses about the effects of leaf litter diversity on litter processing rates contain a systematic bias that stems from mathematically incorrect assumptions. Assuming that the mean of single-species processing rates adequately describes noninteracting mixed-species rates will consistently underestimate or miss positive interactions between species. Further, the slope of log-transformed mass loss of litter mixtures vs time is affected by the duration of the experiment. Simple solutions to these errors include separating species from litter mixtures after incubation, or comparing masses remaining rather than comparing slopes.

The reaeration coefficient (k) is an essential and sensitive component of the equations used to calculate whole-stream metabolism (WSM) for a stream reach by the open-channel method. However, the empirical methods used to estimate k (e.g., by propane or sulfur hexafluoride evasion rates) are time consuming and costly. We reasoned that the reaeration rate (the product of k and the dissolved O₂ deficit or surplus) and sound level (noise) are related to turbulence in a stream and, thus, sound level should be related to reaeration. We used a simple and inexpensive sound level meter to measure sound levels at a fixed height (30 cm) above key geomorphic features (e.g., pools, riffles, cascades) in several streams in Alaska and Vermont. We calculated a feature-weighted average sound pressure for selected stream reaches based on the proportion of these geomorphic features within each reach. We calculated k in the Alaskan stream reaches based on propane evasion rates and found a strong linear relationship to the feature-weighted average sound pressure (n = 11, r² = 0.94, p < 0.001). We concluded that the sound pressure method, which requires 0.5 h to complete and relies on inexpensive instrumentation, provides estimates of k that are comparable to estimates from the more resource-intensive volatile gas-evasion method. In the Vermont stream reaches, we were able to create a sound pressure–stage rating curve, similar to a hydrologic rating curve. Combining the k–sound pressure relationship with the sound pressure–stage rating curve holds promise as a way to estimate reaeration continuously, at modest cost and effort, and on a time scale similar to that on which dissolved O₂, temperature, and light values are recorded to calculate WSM.

Delivery of water, sediments, nutrients, and organic matter to stream ecosystems is strongly influenced by the catchment of the stream and can be altered greatly by upland soil and vegetation disturbance. At the Fort Benning Military Installation (near Columbus, Georgia), spatial variability in intensity of military training results in a wide range of intensities of upland disturbance in stream catchments. A set of 8 streams in catchments spanning this upland disturbance gradient was selected for investigation of the impact of disturbance intensity on hydrodynamics and nutrient uptake. The size of transient storage zones and rates of NH₄ uptake in all study streams were among the lowest reported in the literature. Upland disturbance did not appear to influence stream hydrodynamics strongly, but it caused significant decreases in instream nutrient uptake. In October 2003, coarse woody debris (CWD) was added to ½ of the study streams (spanning the disturbance gradient) in an attempt to increase hydrodynamic and structural complexity, with the goals of enhancing biotic habitat and increasing nutrient uptake rates. CWD additions had positive short-term (within 1 mo) effects on hydrodynamic complexity (water velocity decreased and transient storage zone cross-sectional area, relative size of the transient storage zone, fraction of the median travel time attributable to transient storage over a standardized length of 200 m, and the hydraulic retention factor increased) and nutrient uptake (NH₄ uptake rates increased). Our results suggest that water quality in streams with intense upland disturbances can be improved by enhancing instream biotic nutrient uptake capacity through measures such as restoring stream CWD.

Organic matter inputs, transport, and storage, ecosystem metabolism, and organic C turnover length were measured in a forested Mediterranean 3^rd-order stream (Fuirosos) over a period of 3 y. Meteorological patterns influenced the organic matter dynamics of Fuirosos through 2 pathways: summer weather and flood pulses. Summer rains affected the timing of litter fall and the extent of flow intermittency, which gave rise to accumulations of organic matter in the streambed in dry years. With the onset of flow, these organic matter accumulations were the energy sources for considerable ecosystem respiration (30 g O₂ m⁻² d⁻¹). Interflood periods punctuated by flood pulses determined cycles of steady accumulation and abrupt removal of stored organic matter. During the interflood periods, the efficiency of organic matter processing increased continuously. The seasonal changes in the riparian forest influenced the ecological consequences of the flood pulses. Resilience of the ecosystem, measured in terms of gross primary production and ecosystem respiration, strongly depended on the supply of benthic organic matter and light availability, both of which were seasonally variable in the deciduous riparian forest.

Leaf-litter assays have advanced understanding of decomposition processes in both terrestrial and aquatic ecosystems. Some shortcomings inherent in the technique may be overcome through use of a cotton-strip assay. Key assumptions for using cotton strips as proxies for natural leaves are: 1) decomposition rates of the 2 materials are related, and 2) the materials decay in a similar way when exposed to the same environmental conditions. These assumptions were tested by comparing cotton-strip decomposition (loss of tensile strength and mass) and leaf decomposition (mass loss) across different floodplain habitats of the Tagliamento River (northeastern Italy). Patterns of loss of cotton-strip tensile strength and leaf mass were broadly comparable across river channels, ponds, and terrestrial sites. Differences between river channels and ponds were greater for loss of cotton-strip tensile strength than leaf mass, indicating that, in some situations, loss of cotton-strip tensile strength may be more sensitive to differing environmental conditions than loss of leaf mass. Loss of cotton-strip mass was less sensitive than loss of either tensile strength or leaf mass. Although combined data from all floodplain sites and additional sites in Swiss streams yielded a curvilinear relationship between loss of cotton-strip tensile strength and mass, the slope was extremely steep in the range of 20 to 30% mass loss (corresponding to 0 to 95% loss in tensile strength), indicating that inferring one variable from the other is unreliable. Leaf mass loss was significantly correlated with loss of tensile strength in fine- and coarse-mesh bags in ponds and in coarse-mesh bags in terrestrial sites. However, these correlations were relatively weak (r = 0.50–0.63), suggesting that loss of tensile strength did not accurately reflect leaf mass loss. Thus, the cotton-strip assay should not be used uncritically as a surrogate for leaf-litter assays, but it has potential as a standardized method to measure organic-matter decomposition in fluvial settings and as a functional indicator for stream assessment.

Periphyton is a key component of shallow littoral zones of lakes and streams because it is an important source of primary production and a food resource for herbivores. Meiofauna are abundant in periphyton, but macroinvertebrate grazer (macrograzers) effects on periphytic meiofauna have not been studied so far. We used a spatially structured field experiment (hierarchical nested design consisting of 3 subsites at each of 3 sites) in Lake Erken (Sweden) to investigate the effect of macrograzers on epilithic meiofauna and algae in periphyton by controlling macrograzer access to littoral periphyton communities. Overall, we found a strong negative effect of macrograzer presence on algal biomass and some evidence for negative macrograzer effects on meiofaunal abundance and community composition. The impact of macrograzers on both algae and meiofauna were highly variable between sites and subsites. The largest spatial differences were for macrograzer effects on meiofaunal abundance and composition. We also investigated the ability of macrograzers to reduce spatial heterogeneity of periphyton biomass, but the presence of macrograzers did not alter the variation in algal biomass and associated meiofauna among replicates. We conclude that strong local variability in algal biomass and meiofauna abundance exists between neighboring sites even in the presence of strong overall macrograzer effects. This local variability could be based on factors known to cause spatial heterogeneity, such as hydrodynamics, nutrients, substrate characteristics (size, texture, exposure), or biotic interactions.

The effects of nutrient enrichment on the responses of alpine stream macroinvertebrates to disturbance were studied using 2 field experiments. In experiment 1, individual stones were physically disturbed in 3 different streams at intervals of 0 (i.e., no disturbance), 4, 8, and 16 d over a 32-d period. In experiment 2, nutrients were added to 1 of 2 channels in the same stream, and the same disturbance regime as in experiment 1 was applied in both channels. The stones were collected at the end of each experiment. Periphyton chlorophyll a and ash-free dry mass (experiment 2 only), macroinvertebrate diversity and density, and densities and relative abundances of baetid mayflies, nemourid stoneflies, and chironomid and simuliid dipterans were determined for each stone. In addition, C, N, and P contents of periphyton, benthic organic matter, grazing mayflies, and detritivorous stoneflies and dipterans were determined for each channel in experiment 2. In experiment 1, disturbance frequency affected the relative abundances of baetids, nemourids, chironomids, and simuliids, but no other variable was significantly affected. In experiment 2, disturbance frequency effects in the unfertilized channel were similar to the effects observed in experiment 1, whereas no significant disturbance effects were observed in the fertilized channel. Nutrient enrichment caused a shift in community structure in the fertilized channel, and chironomids constituted >85% of the assemblage. Periphyton, grazing mayflies, and detritivorous chironomids had significantly higher P concentrations and lower molar N:P ratios in the fertilized than in the unfertilized channel. Periphyton N content also was higher in the fertilized channel than in the unfertilized channel. Nutrient enrichment appeared to change the disturbance responses of stream macroinvertebrates by changing the community structure of primary consumers.

We conducted a field experiment to evaluate the effects of habitat modification by aggregations of the case-building caddisfly Goera japonica Banks (Trichoptera:Goeridae) on the size structure and composition of benthic macroinvertebrate assemblages in a Japanese stream. We compared macroinvertebrate assemblages between cobbles with (intact treatment) and without (removal treatment) case aggregations of G. japonica. The total biomass of macroinvertebrates and Margalef's index did not differ between the treatments, but total abundance and taxon richness were higher in the intact treatment than in the removal treatment. Mean body length of macroinvertebrates was significantly smaller in the intact than in the removal treatment. Abundance of individuals in small (<1.55 mm body length) and medium (1.55–3.00 mm) size classes was higher in the intact treatment than in the removal treatment, but abundance of individuals in the large (>3.00 mm) size class did not differ between treatments. For 5 of the 9 dominant macroinvertebrate species, case aggregations facilitated colonization of individuals in small and medium size classes, but not of individuals in the large size class. Assemblage composition changed with habitat modification by the case aggregations. The interstitial spaces created by case aggregations of G. japonica would be at a spatial scale suitable for smaller individuals. Our results suggest that case aggregations of G. japonica cases facilitate colonization by smaller individuals of certain species. This size-dependent and species-dependent facilitation indicates that the aggregation of case-building caddisflies can change the size structure and composition of benthic macroinvertebrate assemblages.

The long-term effects of an alien species may differ from transient effects that occur shortly after its invasion of a new ecosystem. Conservationists fear that the invasion of North America by the zebra mussel since 1985 may lead to the extinction of many populations and species of native bivalves. The appearance of zebra mussels in the Hudson River estuary in 1991 was followed by steep declines (65–100%) in population size of all species of native bivalves between 1992 and 1999. The body condition of all unionids and growth and recruitment of young unionids also declined significantly. Initial declines in population size and body condition were correlated primarily with the filtration rate of the zebra mussel population but not with fouling of native bivalves by zebra mussels. However, samples taken since 2000 have shown that populations of all 4 common native bivalves have stabilized or even recovered, although the zebra mussel population has not declined. The mechanisms underlying this apparent reversal of fortune are not clear. Recruitment and growth of young mussels have shown limited recovery, but the body condition of adults has not. We found no evidence that spatial refuges contributed to this reversal of population declines. Simple statistical models project now that native bivalves may persist at population densities about an order of magnitude below their preinvasion densities. These results offer a slender hope that zebra mussels may coexist with unionids and sphaeriids in North America, as they do in Europe.

The lengths of marked specimens of the freshwater mussel, Eastern Elliptio (Elliptio complanata [Lightfoot 1786]), were monitored annually in 3 lakes in Rhode Island, USA, from 1991 to 2005. Mussels growing in Worden Pond showed a change in mean shell length of only 4.3 mm over 14 y, whereas mussel growth in 2 nearby lakes was 3 to 8× greater than growth in Worden Pond over the same time period. L_∞, the length at which shell growth stops, was significantly different (p < 0.001) among lakes and ranged from 60.5 to 87.4 mm. Transplant experiments revealed that mussels moved to Worden Pond stopped growing, whereas mussels moved from Worden Pond to the 2 other lakes grew at rates similar to the rates observed for resident mussels in the 2 lakes. Standard water-quality measures did not explain the observed growth cessation and lower condition indices of mussels in Worden Pond. Our growth data are consistent with food limitation. The consistent slow growth of E. complanata in Worden Pond, without high mortality, and its ability to increase growth when placed in environments more favorable than Worden Pond, suggests both growth plasticity and longevity in these animals.

Three-quarters of the world's crayfish fauna are found in the US and Canada. Small natural ranges, habitat disturbance, and introduced crayfish species threaten many species, and nearly ½ are imperiled. Naturally small ranges are considered the leading factor for crayfish vulnerability to loss, yet species with small ranges have received little research attention. Orconectes saxatilis is a rare crayfish species with a range restricted to the upper Kiamichi River watershed in southeastern Oklahoma. We examined the distribution, habitat use, and life-history characteristics of O. saxatilis and 2 sympatric crayfish species in the upper Kiamichi River watershed to determine factors that might limit its distribution. Surveys for O. saxatilis expanded its known range and confirmed its restriction to tributaries of the upper Kiamichi River. Orconectes saxatilis showed a strong affinity for riffles, contrary to previous data, whereas Orconectes palmeri longimanus, a regionally abundant sympatric species, showed an equally strong affinity for pools. Tributaries of the upper Kiamichi River are intermittent, and surface flow typically ceases in late summer and early autumn. During dry periods when habitat was limited to disconnected pools, O. saxatilis aestivated beneath cobbles and boulders in dry riffles. The strict use of riffles by O. saxatilis and its need for habitat conducive to aestivation probably contribute to its small range and put this species at risk. Year-round monitoring of populations susceptible to imperilment is needed to make informed conservation decisions. For O. saxatilis and other imperiled crayfish species, conservation efforts should emphasize identification of habitat types required for species survival, avoiding alterations to those habitat types, and protecting natural flow regimes.

We experimentally tested for systematic biases in techniques commonly used to study behavior of larval aquatic insects. We determined whether larval Zygoptera responded to the presence of an observer and whether live observation missed some behaviors. We found significant differences between behaviors recorded during live observations and behaviors videotaped in the absence of an observer. All behaviors, except Rotate, were exhibited less frequently in the presence of an observer. These results suggest that larvae respond to the presence of observers as if they were predators. Live observation also missed some behaviors. The duration of Crawl Forward, which can be very subtle, and the frequency of Rotate, which can be very rapid and is easily missed, were greater when recorded from the videotape than by a live observer. Wherever possible, use of video recording systems is preferable over reliance on live observations.

The effects of measurement errors on biological inferences of stream temperature and bedded fine sediment were investigated. Single variable and multivariate logistic regression models were used to relate the occurrences of different macroinvertebrate genera and observed temperature and fine sediment. Next, a simulation and extrapolation method (SIMEX) was used to adjust regression model coefficients for the effects of measurement errors in the temperature and fine-sediment observations. It was assumed that the persistence of different stream organisms was related to long-term average environmental conditions, so measurement error in this analysis was interpreted broadly as including all the variability associated with estimating long-term averages from single measurements. On average, correcting for measurement error narrowed the breadth of genus–environment relationships and shifted optimum values toward the mean of the observations. Accounting for measurement error also improved the predictive accuracy of some of the inference models. Inferences of temperature based on single-variable SIMEX models were 28% more accurate than inferences based on naive models that used uncorrected observations. However, inferences of fine sediment based on single-variable SIMEX models were only slightly more accurate than inferences based on naive models. Multivariate models, in which both stream temperature and fine sediment were modeled simultaneously, exhibited the strongest improvement in predictive performance when measurement error was taken into account. The accuracy of temperature inferences improved by 39%, whereas fine-sediment inferences improved by 8%.