Light, nutrient availability, and flow are strong factors controlling the elemental composition and biomass of epilithon in temperate stream ecosystems. However, comparatively little is known about these relationships in tropical streams. We investigated how gradients of light and nutrient availability, seasonality, and habitat influenced epilithon biomass, chlorophyll a, and nutrient ratios in montane streams of Trinidad, West Indies. We sampled 4 focal tributaries of a single river, 2 of which had canopies experimentally thinned, every other month over a 2-y period to observe temporal dynamics and light effects on epilithon. We also sampled 18 sites across Trinidad's Northern Range Mountains once each in a wet and dry season to examine the effects of naturally occurring differences in light and dissolved nutrient availability on epilithic characteristics. We found greater chlorophyll a concentrations in habitats with greater light availability, but the effect of light on epilithon stoichiometry differed between the site-survey and focal-tributary data. In general, epilithic C∶nutrient ratios decreased with increasing dissolved nutrient concentrations, but relationships between nutrient availability and biomass probably were obscured by naturally high dissolved N and P concentrations in many of the streams. Season and habitat type had profound effects on epilithon variables. Biomass and % C generally decreased in riffles and under wet-season conditions. These results suggest multiple controls for the quantity and quality of stream epilithon and have important implications for in-stream consumers.

We explored the resilience of the invasive New Zealand mudsnail Potamopyrgus antipodarum to fluctuating winter freezing and near-freezing temperature cycles in laboratory tests. Our goal was to provide data to confirm field observations of mortality and presumed mortality in stream habitats with fluctuating freezing to near-freezing temperatures. We tested individuals from 2 locations with distinctly different thermal regimes and population densities. One location had low snail densities and water temperatures with strong diel and seasonal water variation. The other location had high snail densities and nearly constant water temperatures. Groups of individuals from both locations were tested in each of 3 laboratory-created diel thermal cycles around nominal temperatures of 0, 2, or 4°C. Mortality occurred in cycles around 0°C in both populations, and little to no mortality occurred at temperatures >0°C. Individuals from both sources held in diel 0°C cycles for 72 h showed 100% mortality. Our findings support observations from published field studies that survival was limited in infested habitats subject to freezing temperatures.

Much is known about invertebrate community traits in basins across Europe, but no comprehensive description of traits exists for the continental US. Little is known about the trait composition of invertebrates in reference or least-disturbed basins of the US, how trait composition varies among ecoregions, or how consistently traits respond to land use. These elements are essential to development of trait-based tools for conservation and assessment of biological integrity. We compared invertebrate traits of least-disturbed basins among ecoregions of the US. Benthic invertebrate data (presence/absence) from 1987 basins were translated into 56 binary traits (e.g., bivoltine, clinger). Basins were classified as least-disturbed, agricultural, or urban, and grouped into 9 ecoregions. Landuse, climatic, physiographic, and hydrologic data were used to describe ecoregions and to evaluate least-disturbed basin quality. The unique habitat template of each ecoregion selected for trait compositions in least-disturbed basins that differed among ecoregions. Among the traits examined, life-history (e.g., voltinism, development) and ecological traits (e.g., rheophily, thermal preference) differed most among ecoregions. Agricultural and urban land uses selected for trait compositions that differed from least-disturbed, but the extent of the differences depended on ecoregion and quality of the least-disturbed basins. No trait compositions unique to specific land uses were found. However, a disturbance syndrome was observed in that the magnitude and direction of trait responses to urban and agricultural land uses were consistent among ecoregions. Each ecoregion had a unique trait composition, but trait compositions could be used to aggregate ecoregions into 3 broad regions: Western Mountains, Plains and Lowlands, and Eastern Highlands. Our results indicate that large-scale trait-based assessment tools for the US will require calibration to account for regional differences in the trait composition of basins and in the quality of least-disturbed basins.

We investigated how dissolved inorganic N (DIN) inputs from a wastewater treatment plant (WWTP) effluent are processed biogeochemically by the receiving stream. We examined longitudinal patterns of NH₄ and NO₃⁻ concentrations and their ¹⁵N signatures along a stream reach downstream of a WWTP. We compared the δ¹⁵N signatures of epilithic biofilms with those of DIN to assess the role of stream biofilms in N processing. We analyzed the δ¹⁵N signatures of biofilms coating light- and dark-side surfaces of cobbles separately to test whether light constrains functioning of biofilm communities. We sampled during 2 contrasting periods of the year (winter and summer) to explore whether changes in environmental conditions affected N biogeochemical processes. The study reach had a remarkable capacity for transformation and removal of DIN, but the magnitude and relevance of different biogeochemical pathways of N processing differed between seasons. In winter, assimilation and nitrification influenced downstream N fluxes. These processes were spatially segregated at the microhabitat scale, as indicated by a significant difference in the δ¹⁵N signature of light- and dark-side biofilms, a result suggesting that nitrification was mostly associated with dark-side biofilms. In summer, N processing was intensified, and denitrification became an important N removal pathway. The δ¹⁵N signatures of the light- and dark-side biofilms were similar, a result suggesting less spatial segregation of N cycling processes at this microhabitat scale. Collectively, our results highlight the capacity of WWTP-influenced streams to transform and remove WWTP-derived N inputs and indicate the active role of biofilms in these in-stream processes.

Incoming solar radiation is an important driver of aquatic ecosystem processes, such as gross primary production and photodegradation of organic matter. Rates of incoming solar radiation can be estimated in several ways, but their utility is limited for collecting a large number of replicate samples needed to quantify variation in light availability within and among streams. We evaluated the utility of 2 photodegrading organic dyes (rhodamine WT [RWT] and fluorescein) for measuring light exposure, especially at the level of the stream bed. We attached vials with known concentrations of the RWT or fluorescein to the stream bed and used regressions of concentration vs accumulated light to estimate photodegradation rates. Initial concentrations of RWT (20–100 µg/L) did not affect rate of photodegradation, but RWT decay rates were 93% slower in the dark than in the light. We also tested fluorescein, which degrades faster than RWT when exposed to light and is stable when kept in the dark. On average, RWT degraded at a slower rate (3.5 µg L⁻¹ d⁻¹) than fluorescein (40.8 µg L⁻¹ d⁻¹) when exposed to similar levels of light accumulation. Water temperature did not affect the decay rate of RWT at 10 or 20°C, but RWT did not decay significantly at 30°C, a result suggesting that high temperatures might affect decay rates differently than lower temperatures. Water temperature did not affect the decay rate of fluorescein. The strength of this method is that it enables researchers to integrate light measurements into a single value. Researchers can deploy multiple arrays within a reach to develop a relative measure of incoming light that has the potential to cover large spatial and temporal scales.

We assessed the effects of road deicer (NaCl) on life-history traits (survival, emergence, development time, sex ratio, and size at emergence) of Chironomus riparius under laboratory conditions mimicking near-freezing overwintering temperatures and warmer temperature fluctuations during early spring. We added 1-d-old larvae to culture chambers and acclimated them for 1 wk at 22°C. We decreased the temperature over a period of 2 d to 1–2°C. After 1 wk, we added deicing salt (NaCl) to the chambers over a period of 4 d to expose the larvae (as 2^nd instars) to 0, 5, or 10 g/L salt (from deicer) concentrations. We then raised the temperature over a period of 1–2 d to fluctuate between 8–12°C and monitored adult emergence. All larvae in the 10-g/L chambers died when the temperature reached 12°C. The number of individuals that survived and the number that emerged were significantly lower, and development time was significantly longer in the 5- than in the 0-g/L chambers. Males and females emerged synchronously, sex ratio was 1∶1, and females were significantly larger than males at 0 g/L. Males emerged only slightly before females, sex ratio was male skewed (2∶1), and females were the same size as males at 5 g/L. Male size did not differ between 0 and 5 g/L, but females were significantly smaller at 5 than at 0 g/L. Road deicers can have significant lethal and sublethal population-level effects on chironomids in roadside wetlands, but these effects might be reduced if chironomids are present in nearby unaffected habitats that can act as source populations in late spring, summer, and autumn.

Tar spot is a common leaf disease of Acer caused by Rhytisma spp. (Ascomycetes). Initial symptoms are small green-yellow spots. These infected areas turn into black fungal stromata, which resemble blobs of tar. Stromata with apothecia overwinter and release ascospores in the following spring. We measured rates of decomposition of leaf litter of infected and uninfected Norway maple (Acer platanoides). In addition, we assessed changes in leaf-litter N and P and rates of sporulation of aquatic hyphomycetes. We incubated leaf disks from areas covered with tar spot, disks touching but not overlapping with tar spot (adjacent disks), and disks from uninfected leaves in a stream. The exponential decay rate of uninfected disks was 50% greater than those for infected and adjacent disks. Initial N and P concentrations were highest in infected disks. N and P concentrations dropped in all treatments during the 1^st wk in the stream and subsequently increased. Conidium release by aquatic hyphomycetes in the laboratory was lowest from infected disks and highest from uninfected disks. Cumulative sporulation over 10 wk of stream exposure increased from infected (109 conidia mg⁻¹ d⁻¹) to adjacent (215 conidia mg⁻¹ d⁻¹) to uninfected (521 conidia mg⁻¹ d⁻¹) disks. Similarity of fungal communities was highest between adjacent and uninfected disks (71.5%) and lowest between infected and uninfected leaves (51.9%).

We used in situ nutrient-enrichment experiments at sites in 4 Texas streams spanning a steep nutrient gradient to test the hypothesis that periphyton photoautotrophs and heterotrophs are differentially limited by N and P, respectively. We contrasted the responses of total periphyton biomass (ash-free dry mass [AFDM]), photoautotrophic biomass (chlorophyll a [CHLA]), % CHLA of AFDM [% CHLA]), alkaline phosphatase activity (APA), and nitrogenase activity (NA) to ambient N and P concentrations among sites and among nutrient-diffusing substrata (NDS) (control, N, P, and N P) within sites. Periphyton biomass (AFDM and CHLA) increased steadily with increasing ambient N and P among sites. Both APA and NA were relatively high on the control treatments at the lowest nutrient site and decreased with increasing ambient N and P until falling to negligible levels at the highest nutrient site. Within sites, P enrichment decreased APA but did not increase % CHLA or CHLA/cm². P enrichment did increase AFDM/cm² at one of the low nutrient sites. N enrichment reduced NA and increased % CHLA and CHLA/cm² without increasing AFDM/cm². N P enrichment resulted in the highest % CHLA and CHLA/cm² at the 3 lowest nutrient sites but did not increase AFDM/cm². Combined, these findings suggest that photoautotrophs in periphyton communities may be more limited than heterotrophs by N or become more competitive for P under elevated N. The high levels of APA and NA on the controls coupled with minimal difference in AFDM among nutrient treatments at most sites despite increases in % CHLA on N and N P treatments provide experimental evidence that differential nutrient limitation probably occurs in periphyton communities. Our results support the view that the single-limiting-nutrient paradigm may lead to overly simplistic interpretations of periphyton responses to nutrient enrichment.

Population genetic studies of freshwater invertebrate taxa in New Zealand and South America are currently few despite the geologically and climatically dynamic histories of these regions. The focus of our study was a comparison of the influence on realized dispersal of 2 closely related nonbiting midges (Chironomidae) of population fragmentation on these separated austral land masses. We used a 734-base pair (bp) fragment of cytochrome c oxidase subunit I (COI) to investigate intraspecific genetic structure in Naonella forsythi Boothroyd in New Zealand and Ferringtonia patagonica Edwards in Patagonia. We proposed hypotheses about their potential dispersal and, hence, expected patterns of genetic structure in these 2 species based on published patterns for the closely related Australian taxon Echinocladius martini Cranston. Genetic structure revealed for both N. forsythi and F. patagonica was characterized by several highly divergent (2.0–10.5%) lineages of late Miocene–Pliocene age within each taxon that were not geographically localized. Many were distributed widely. This pattern differed greatly from population structure in E. martini, which was typified by much greater endemicity of divergent genetic lineages. Nevertheless, diversification of lineages in all 3 taxa appeared to be temporally congruent with the onset of late Miocene glaciations in the southern hemisphere that may have driven fragmentation of suitable habitat, promoting isolation of populations and divergence in allopatry. We argue that differences in realized dispersal post-isolation may be the result of differing availability of suitable habitat in interglacial periods.

Input, trapping, and retention of coarse particulate organic matter are among the key determinants of resource availability for consumers in forested headwater streams. Retention efficiency is regulated by the interaction between bed roughness and hydrological characteristics. We examined short-term retention of leaf analogs in artificial flumes, where we manipulated streambed structure and discharge using a factorial design. The substrate structures used are widely applied in stream restoration and represent a gradient of increasing bed complexity: channelized streams, channelized streams moss, boulder additions, boulders moss, addition of wood (low-wood and high-wood). We assessed whether, and by how much, the retentive efficiency of a stream is enhanced by various restoration structures and whether the effect of discharge on retention efficiency is similar in channelized and differently restored streams. All restoration structures enhanced leaf retention considerably, and even channelized streams containing mosses were highly retentive. At low discharge, channelized streams lacking mosses retained leaf litter less effectively than did the restored streams, whereas retention differed little among restoration treatments. At high discharge, most restoration structures were much less retentive, and differences in retentiveness between restored and channelized streams became insubstantial, except that streams with abundant wood remained highly retentive at all discharges. Our results suggest that all types of structures added to improve stream retentiveness may enhance leaf retention, and thereby contribute to organic matter dynamics of a stream, but the addition of wood of variable sizes is the best long-term solution to forested stream restoration.

Dispersal is an essential process in metapopulation and metacommunity dynamics. Most studies of aquatic invertebrate dispersal in streams have focused on in-stream drift of larvae. However, understanding aerial dispersal is important for predicting community assembly in isolated habitats after disturbance or stream restoration. We used artificial pools placed at 3 distances (5, 75, and 250 m) from 1 perennial and 1 ephemeral arid-land stream to examine aerial-dispersal dynamics of aquatic invertebrates over a 6-wk period in summer 2009. We also conducted a 2-wk experiment to examine the relationship between daily rainfall and disperser abundance at the perennial site. Sixty-six aquatic invertebrate taxa (including many Coleoptera and Diptera and fewer Hemiptera, Ephemeroptera, Trichoptera, and noninsect taxa) colonized the artificial pools. They represented ⅓ of taxa documented from neighboring perennial streams. Abundance and species richness declined with distance away from both streams. This result suggests that ephemeral stream channels may serve as important aerial dispersal corridors for aquatic invertebrates even when no surface water is present. Mean species richness tripled after 58 mm of rain during the 4^th wk of the experiment. Data from the 2-wk experiment highlighted the role of rainfall as a dispersal cue in this system. Amount of daily rainfall explained 48 to 77% of the variation in disperser abundance at 5, 75, and 250 m from the perennial site. We used spatiotemporal dispersal patterns observed in our study to identify 5 modes of aerial dispersal among 56 taxa: 1) widespread common, 2) widespread haphazard, 3) range-restricted, 4) cue-limited, and 5) infrequent. Classification of specific aerial-dispersal modes provides a conceptual framework for modeling spatially explicit community responses to disturbance, stream restoration, and climate-change-induced habitat contraction or expansion.

Nestedness measures the degree to which species assemblages are proper subsets of larger assemblages within a metacommunity. We analyzed nestedness in 20 freshwater data sets, each for a single metacommunity within a drainage basin. First, we examined the degree of nestedness across local assemblages and identified major environmental correlates of nestedness at sites. Second, we examined whether nestedness showed concordance among trophic levels. Third, we examined whether nested and idiosyncratic species (i.e., species that have unpredictable gaps in their distribution) differed ecologically and showed dissimilar distribution patterns along geographical and environmental gradients. Degrees of nestedness, obtained using a metric by Almeida-Neto et al. (2008), ranged from 22.9 to 57.0, with 11 matrices showing significant nestedness. Lake bacteria had a higher degree of nestedness than did phyto- or zooplankton. The degree of nestedness was related mainly to water temperature, conductivity, and trophic status of the water, results suggesting that habitat quality may be an important predictor of nestedness, but was only weakly concordant among trophic levels within metacommunities. However, averaged across all metacommunities, nestedness values between phytoplankton and zooplankton and between phytoplankton and bacteria were positively correlated. Nested and idiosyncratic species did not differ in their spatial and environmental distribution patterns. Freshwater planktonic assemblages were characterized by a relatively low degree of nestedness. The low congruence in nestedness among trophic levels suggests that assemblages typically show independent gradients of nestedness. This result indicates that assemblages respond dissimilarly to driving forces, such as habitat quality, or that nestedness in freshwaters might be related to stochastic colonization. Conserving only the most diverse sites may not protect rare species because of the relatively low degree of nestedness in fresh waters.

Currently available tools for studying plant litter decomposition and invertebrate consumption in aquatic ecosystems have at least 2 major limitations: 1) the difficulty of manipulating litter chemical composition to provide mechanistic insights into attributes of litter quality controlling decomposition rate, and 2) lack of a standardized litter that hampers comparisons of results among studies. These limitations point to a need for a standard litter surrogate with adjustable chemical composition. We propose using a decomposition and consumption tablet (DECOTAB) consisting of cellulose powder embedded in an agar matrix to evaluate decomposition and consumption rates in aquatic environments. We describe the preparation of DECOTABs and demonstrate some applications in laboratory microcosms and outdoor mesocosms. A leaf shredder, the isopod Asellus aquaticus, and a collector-gatherer, the nonbiting midge larva Chironomus riparius, readily consumed DECOTABs, leading to massive mass loss of the tablets within 21 d (∼90%). The isopod also consumed DECOTABs amended with extracts of riparian plants and soil to create a chemically complex source of organic matter. Our results highlight the potential utility of DECOTABs to assess invertebrate contributions to organic matter decomposition in aquatic systems. In the absence of invertebrates, exposure of basic and complex DECOTABs to microorganisms resulted in significant mass loss within 21 d (10–25%), and addition of an antibiotic and fungicide suppressed microbial decomposition, suggesting that the tablets are useful for studying microbial processes. Complex tablets decomposed faster than the basic tablets, a result illustrating the importance of chemical composition of organic material for microbial decomposers. DECOTABs are a novel, versatile tool for addressing long-standing questions in aquatic ecology and environmental assessment.

Alterations to the natural flow regime affect the structure and function of rivers and wetlands and contribute to loss of biodiversity worldwide. Although the effects of flow regulation have been relatively well studied, a lack of synthesis of the ecological consequences of low flows and droughts impedes research progress and our grasp of the mechanistic effects of human-induced water reductions on riverine ecosystems. We identified 6 ecologically relevant hydrological attributes of low flow (antecedent conditions, duration, magnitude, timing and seasonality, rate of change, and frequency) that act within the temporal hierarchy of the flow regime and a spatial context. We synthesized the literature to propose 4 principles that outline the mechanistic links between these low-flow attributes and the processes and patterns within riverine ecosystems. First, low flows control the extent of physical aquatic habitat, thereby affecting the composition of biota, trophic structure, and carrying capacity. Second, low flows mediate changes in habitat conditions and water quality, which in turn, drive patterns of distribution and recruitment of biota. Third, low flows affect sources and exchange of material and energy in riverine ecosystems, thereby affecting ecosystem production and biotic composition. Last, low flows restrict connectivity and diversity of habitat, thereby increasing the importance of refugia and driving multiscale patterns in biotic diversity. These principles do not operate in isolation, and many of the ecological pathways that are affected by low flows are likely to overlap or occur simultaneously, potentially resulting in synergistic and complex effects. Last, we outlined major human-induced threats to low-flow hydrology and how they act upon the ecologically relevant hydrological attributes of low flow to affect potential changes in riverine ecosystem integrity. The mechanistic links described in this synthesis can be used to develop and test hypotheses of low-flow hydrological–ecological response relationships in a cause–effect framework that will have value for both research and river flow management. Continued experimental research and ongoing consolidation of ecological information will improve our understanding and ability to predict consequences of low-flow alteration on river, floodplain, and estuarine ecosystems.

Temporary rivers are shifting mosaics of aquatic and terrestrial habitat driven by hydrologic variability. Advancing wetted fronts (AWFs) that rewet dry river beds are unpredictable events, and knowledge about their composition and role in habitat mosaics is scarce. We collected dead and living terrestrial invertebrates, aquatic invertebrates, and sestonic matter (i.e., suspended sediments and organic matter) entrained by flow in an AWF travelling downstream over a 7-km-long dry reach. We collected samples at 12 sites along the rewetting reach and at 3 sites in the upstream perennial reach of the Albarine River, France. Invertebrates in the AWF were mainly of terrestrial origin and organic matter was essentially coarse (>5 mm). Terrestrial invertebrate density and taxonomic richness and sestonic matter concentration were several orders of magnitude higher in the AWF than in the perennial reach. However, only terrestrial invertebrate taxonomic richness increased longitudinally. At least ⅓ of terrestrial taxa could have survived submersion, and the density and taxonomic richness of these taxa decreased downstream. These results indicate that terrestrially derived material is stored downstream during rewetting where it could greatly stimulate in-stream aquatic productivity and succession in terrestrial invertebrates along the riparian zone. In contrast, entrained aquatic taxa represented ∼15% of the benthic taxa collected 1 mo after rewetting, indicating a low contribution of AWFs to benthic invertebrate succession. In the context of global change and increasing appropriation of water resources by humans, our results suggest that conceptual models of invertebrate dynamic and organic matter processing in rivers should account for dry phases and transitional periods from dry to wet conditions.

Changes to nutrient loads may affect the stoichiometry of urban or suburban stream food webs. We quantified foodweb stoichiometry in 9 detritus-based headwater streams draining forested or suburban watersheds in northeastern, coastal Massachusetts (USA). We measured C, N, and P content (%C, %N, %P) and relative ratios (C∶N∶P) of basal food resources and macroinvertebrate consumers in each stream. Greater dissolved nutrient concentrations in suburban streams did not cause significant changes in overall elemental content (%C, %N, %P) of basal food resources, but their N∶P ratios decreased in high-nutrient streams, indicating that higher nutrients do affect food quality. The nutrient content of most macroinvertebrate taxa consuming these altered food resources was unaffected by suburban nutrient enrichment. However, some taxa deviated from strict homeostasis. Samples of these stoichiometrically variable taxa from the most enriched suburban streams had variable %N, a result suggesting that shifts in %P may not be the only means by which body stoichiometry is altered by nutrient enrichment. Moreover, many of the taxa with nutrient contents that differed across stream groups are typically more abundant in suburban than in forested streams, a result suggesting a potential link between consumer–resource stoichiometry and the tolerance of certain taxa for nutrient enrichment. Overall, our results are consistent with those of other studies on ecosystem properties or functions in urban streams in which the complex nature of urban effects does not always yield general patterns seen in less-affected systems.

We developed a series of models using landuse and water-quality variables to predict sediment P release rates under anoxic conditions in reservoirs. We collected sediment cores from 17 reservoirs in the Central Plains region of the USA, and we measured nutrient release rates under anoxic conditions in laboratory incubation studies. We used corresponding landuse and water-quality data from the reservoirs to develop regression models for predicting P release rates. We used variables that relate directly to trophic state, including % cropland in the watershed, which explained the greatest amount of variation in release rates. P release rates tended to be higher in reservoirs that had greater % cropland in the watersheds. We developed additional predictive models using surface total P concentrations and Secchi disk depths. Trophic state was also a good predictor of release rates because more P was released from hypereutrophic reservoirs than from mesotrophic or eutrophic reservoirs. The median release rates for reservoirs representing different trophic state classes (e.g., mesotrophic, eutrophic, and hypereutrophic) were very similar to those previously reported for natural lakes. Our models can be used to predict sediment release rates in individual reservoirs of concern or to screen a large number of reservoirs to help direct resources to those systems that are most vulnerable to internal loading. Models based on landuse characteristics are particularly valuable because these data can be obtained from computer-based assessments and do not require labor-intensive field sampling.

One of the largest impediments to the conservation of freshwater mussels is the absence of host-fish data. Suitable hosts must be present in sufficient numbers and occur at the appropriate time for successful mussel recruitment. Habitat degradation and fragmentation caused by dams and other anthropogenic alterations may reduce host availability. Host data are lacking for the federally threatened Purple Bankclimber mussel (Elliptoideus sloatianus), which is endemic to the Apalachicola–Chattahoochee–Flint basin (ACF) in Alabama, Florida, and Georgia, and the Ochlockonee basin in Florida and Georgia. We tested 29 fish species in 7 families as potential hosts for Purple Bankclimbers and observed high metamorphosis success (79–89%) with 4 species of sturgeon: Gulf (Acipenser oxyrinchus desotoi), Atlantic (Acipenser oxyrinchus oxyrinchus), Lake (Acipenser fulvescens), and Shortnose (Acipenser brevirostrum). Metamorphosis was less successful with Blackbanded Darters (Percina nigrofasciata) and Halloween Darters (Percina crypta) as hosts (34–36% metamorphosis), and the remainder of the fishes we tested were not suitable hosts. The federally threatened Gulf Sturgeon is the only sturgeon species present in the ACF, but access of this migratory fish to most of the basin is blocked by Jim Woodruff Dam on the Apalachicola River. In the absence of sturgeon upstream of Jim Woodruff Dam, darters appear to have facilitated persistence of this mussel species, but at abundances far lower than historical conditions. This relationship between the Purple Bankclimber and Gulf Sturgeon is the first description of a federally protected fish serving as a host for a federally protected mussel and is an archetypal example of the role of habitat fragmentation in the ecology of listed species. Recovery of the Purple Bankclimber and other mussel species probably will require restoration of habitat connectivity for fish passage.

Current speed could affect algal responses to river eutrophication and flow regulation via its influence on nutrient availability to benthic algae. In laboratory experiments, the thickness of the diffusive boundary layer decreases with increasing current speed, enhancing rates of nutrient mass transfer by molecular diffusion across the layer to the algal cell wall. In rivers and streams, this phenomenon may be masked by grazing and physical losses. We used a 10-wk field experiment to test the hypothesis that the response of benthic algae to nutrient addition was a function of current speed. We deployed clay pavers at the beginning of a 3-mo period of baseflow in the Daly River (tropical Australia) at 5 sites (current speeds <2–98 cm/s). After a 5-wk colonization period, we added nutrients to treatment sites via slow-release fertilizer pellets. After 5 wk, we measured chlorophyll a, algal composition, and macroinvertebrate grazer abundance and composition, nutrient concentrations, and current speed. We measured algal biomass response (BRR) as the ratio of biomass in the nutrient-addition treatment to biomass in the control treatment. BRR increased linearly with current speed between 27 to 98 cm/s and accounted for 99% of biomass variation. At <2 cm/s BRR did not conform to this relationship. The dominance of filamentous chlorophytes, oligotrophic conditions, and weak grazer effects probably contributed to the strong relationship between current speed and BRR. The effect of nutrient pollution on riverine benthic algal biomass could be greatest where current speeds are highest, and flow regulation that reduces current speed could reduce nutrient availability. Algal composition and physiognomy, grazing, and physical losses could reduce this effect. Current-mediated nutrient availability and patchiness of nutrient-driven bottom-up control of benthic algal biomass may influence algal responses to eutrophication and flow regulation.

A growing body of genes-to-ecosystems research has documented the ecosystem-level consequences of intraspecific variation in plants caused in large part by variation in chemical composition. Understanding how this genetic variation in trees might interact with elevated nutrients resulting from increases in anthropogenic deposition may give us insight into how future riparian forests might influence adjacent streams through leaf-litter deposition. We examined the effects of tree genotype, soil nutrient environment, and their interaction on aspen (Populus tremuloides) litter chemistry and aquatic decomposition. We used litter collected from 5 aspen genotypes grown in a common garden under low and high nutrient availability and monitored decomposition over 112 d in a woodland stream. Genotype, environment, and genotype × environment interactions influenced litter chemistry and decomposition dynamics. Genotype and environment both strongly influenced initial litter chemistry, with significant genotype × environment interactions for bound condensed tannins and C∶N. Consistent with effects on litter chemistry, decomposition rates were significantly affected by genotype, environment, and genotype × environment interactions. These results suggest that future changes in intraspecific genetic variation of tree species and deposition of nutrients because of shifts in climate, land use, and related factors may influence decomposition processes in terrestrial systems and in the aquatic systems with which they are coupled via material transfer.

Floodplain rivers have been conceptualized as patchwork mosaics of hydrogeomorphic zones that differ in hydrological, physiochemical, and ecological conditions. However, few investigators have empirically examined the extent to which basic ecological properties of large rivers, such as nutrient concentrations, are patchily distributed or the relationship between such distributions and water-mediated connectivity. We used global and local measures of spatial autocorrelation in 5 reaches of the Upper Mississippi River from 1994–2008 in spring (higher discharge) and summer (lower discharge) to examine distributions of total N (TN), total P (TP), and TN∶TP for evidence of patchiness. TN was distributed as discrete patches regardless of season, but the patches of TP and TN∶TP were more distinct during summer low-flow than spring high-flow conditions. The occurrence of patches of high TN, low TP, and high TN∶TP was highly predictable, generally occurring in backwater, low-velocity (0.1 m/s) environments. Patches of low TN, high TP, and low TN∶TP were less common and less predictable, but were more likely to occur in channels with high current velocities (>0.1 m/s) or in impounded areas with low current velocity, but adjacent to channel environments. These patch distributions are consistent with our current understanding of dominant mechanisms of riverine nutrient cycling and were generally consistent across 5 river reaches that spanned nearly the entire length of a large floodplain river. Watershed characteristics ultimately determine nutrient inputs to river systems, but our results support the idea that local geomorphic and hydraulic conditions influence spatial patterns of nutrient delivery and biochemical transformation and create a patchwork mosaic of nutrient distributions within large floodplain rivers.

Little is known about the effects of waterbirds on benthic macroinvertebrates in nontidal habitats. We conducted 4 waterbird-exclusion experiments in a transformed marshland in Doñana Natural Park (southwestern Spain) and compared benthic macroinvertebrates in 3 × 3-m control (no exclusion) and test plots from which Greater Flamingos Phoenicopterus ruber or all waterbirds were excluded. The 4 experimental periods coincided with the waterbird breeding season (summer), postbreeding period (autumn), wintering period (winter), and pre- and early breeding season (spring). Densities of waterbirds were relatively low compared to densities reported in other studies, but we found strong effects of excluding flamingos and waterfowl on the structure of the benthic community in autumn, winter, and spring. In winter, numbers of polychaetes and chironomids were nearly 2× higher in exclusion than in control plots. The difference in macroinvertebrate biomass among treatments was strongest in winter when benthic dry mass was ∼2× higher in flamingo exclosures than in control plots and ∼2× higher in all-bird exclosures than in flamingo exclosures. Macroinvertebrate richness was 15% lower in control than in all-bird exclosures in winter and 25% lower in control than in all-bird exclosures in spring. Our results suggest that benthic macroinvertebrate communities in this wetland are affected more by predation, bioturbation, and waterbird herbivory than by competition. These findings suggest that waterfowl and the increasing population of flamingos are of great importance in structuring shallow lakes in the Mediterranean region.