Flows, droughts, and aliens: factors affecting the fish assemblage in a Sierra Nevada, California, stream

The fishes of Martis Creek, in the Sierra Nevada of California (USA), were sampled at four sites annually over 30 years, 1979-2008. This long-term data set was used to examine (1) the persistence and stability of the Martis Creek fish assemblage in the face of environmental stochasticity; (2) whether native and alien fishes responded differently to a natural hydrologic regime (e.g., timing and magnitude of high and low flows); and (3) the importance of various hydrologic and physical habitat variables in explaining the abundances of native and alien fish species through time. Our results showed that fish assemblages were persistent at all sample sites, but individual species exhibited marked interannual variability in density, biomass, and relative abundance. The density and biomass of native fishes generally declined over the period of study, whereas most alien species showed no significant long-term trends. Only alien rainbow trout increased in both density and biomass at all sites over time. Redundancy analysis identified three hydrologic variables (annual 7-day minimum discharge, maximum winter discharge, and number of distinct winter floods) and two habitat variables (percentage of pool habitat and percentage of gravel substrate) that each explained a significant portion of the annual variation in fish assemblage structure. For alien taxa, their proportional contribution to the total fish assemblage was inversely related to mean annual streamflow, one-day maximum discharge in both winter and spring, and the frequency of springtime floods. Results of this study highlight the need for continuous annual monitoring of streams with highly variable flow regimes to evaluate shifts in fish community structure. Apparent successes or failures in stream management may appear differently depending on the time series of available data.     

Natural flow regimes, nonnative fishes, and native fish persistence in arid-land river systems

Escalating demands for water have led to substantial modifications of river systems in arid regions, which coupled with the widespread invasion of nonnative organisms, have increased the vulnerability of native aquatic species to extirpation. Whereas a number of studies have evaluated the role of modified flow regimes and nonnative species on native aquatic assemblages, few have been conducted where the compounding effects of modified flow regimes and established nonnatives do not confound interpretations, particularly at spatial and temporal scales that are relevant to conservation of species at a range-wide level. By evaluating a 19-year data set across six sites in the relatively unaltered upper Gila River basin, New Mexico, USA, we tested how natural flow regimes and presence of nonnative species affected long-term stability of native fish assemblages. Overall, we found that native fish density was greatest during a wet period at the beginning of our study and declined during a dry period near the end of the study. Nonnative fishes, particularly predators, generally responded in opposite directions to these climatic cycles. Our data suggested that chronic presence of nonnative fishes, coupled with naturally low flows reduced abundance of individual species and compromised persistence of native fish assemblages. We also found that a natural flow regime alone was unlikely to ensure persistence of native fish assemblages. Rather, active management that maintains natural flow regimes while concurrently suppressing or excluding nonnative fishes from remaining native fish strongholds is critical to conservation of native fish assemblages in a system, such as the upper Gila River drainage, with comparatively little anthropogenic modification.     

Flow regulation and fragmentation imperil pelagic-spawning riverine fishes.

Flow regulation and fragmentation of the world's rivers threaten the integrity of freshwater ecosystems and have resulted in the loss or decline of numerous fish species. Pelagic-spawning fishes (pelagophils) are thought to be particularly susceptible to river regulation because their early life stages (ichthyoplankton) drift until becoming free-swimming, although the extent of transport is largely unknown. Transport velocity and distance were determined for passively drifting particles, which mimicked physical properties of ichthyoplankton, in two large, regulated rivers (Rio Grande and Pecos River) of the arid Southwest United States. Particle drift data were incorporated into celerity-discharge equations (r2 > 0.90; P < 0.001), and reach-specific transport velocity was modeled as a function of discharge. Transport velocities of particles exceeded 0.7 m/s in all river reaches during typical spawning flows (i.e., reservoir releases or rainstorms) and were greatest in highly incised and narrow channel reaches. Mean transport distance of particles released in the Pecos River during sustained reservoir flows (141.1 km; 95% CI = 117.0-177.5 km) was significantly longer than during declining reservoir flows that mimicked a natural rainstorm (52.4 km; 95% CI = 48.8-56.5 km). Mean transport distance of particles in the Rio Grande during sustained reservoir flows was 138.7 km (95% CI = 131.0-147.2 km). There are 68 dams and 13 reservoirs that fragment habitats and regulate flow in the Rio Grande Basin (Rio Grande and Pecos River) in areas historically occupied by pelagophils. While the basin historically provided 4029 km of free-flowing riverine habitat, reservoir habitat now represents > 10% of the longitudinal distance. Only five unfragmented nonreservoir reaches > 100 km remain in the Rio Grande, and two remain in the Pecos River. Pelagophils were extirpated from all reservoirs and from nearly all short, fragmented reaches (< 100 km) of the Rio Grande Basin, but at least some fraction persisted in all longer reaches (> 100 km). The recovery and long-term persistence of pelagophils in regulated rivers, including those in this study, will likely depend on reestablishment and protection of long unfragmented reaches coupled with mimicry of the natural flow regime.     

The Brown Trout Population at Loch Fleet Eight Years After Liming

At the start of the Loch Fleet Project in 1984, the Loch and the upper 7 km of its efferent stream were found to be devoid of trout (Salmo trutta) as a result of acidification. Following the liming treatments applied to the catchment, from 1986 the formerly toxic water quality conditions (pH ∼ 4.5, calcium ∼ 1 mgl^-1, elevated aluminium and heavy metal levels) were eliminated, and trout were reintroduced on two occasions, in 1987 and 1988. A total of 520 fish were stocked, at a combined density equivalent to 5.5 kg ha^-1. Surveys of the loch and stream populations were carried out annually until 1993 to monitor their development, using a range of techniques, including electrofishing, gill-netting, seine-netting, spawner trapping and mark-release recapture methods. Length and scale- analysis were used to investigate fish growth.

The trout population in Loch Fleet expanded rapidly as a result of natural spawning in the loch's main feeder stream, augmented by the use of an artificial spawning bed which was constructed at the loch outlet in 1990. in mid-1983 the stock density, estimated by mark-recapture census methods, had increased to 24.9 kg ha^-1. Poor recruitment in the years 1991-93, however, reduced the rate of expansion and resulted in a population comprising mainly older individuals. the poor recruitment in these years was not fully explained but was not caused by water quality and was most likely a result of fry washout by spring spates.

Fish growth rates were high initially and were estimated on the basis of the Elliott trout growth model to be optimal for the prevailing water temperature regime of the loch. By 1991, growth rates had fallen, probably as a result of competition for food, but showed signs of recovery towards the end of the study period in 1993, following the period of lower population densities of young fish.

Trout rapidly repopulated the loch's outlet stream after 1987 but have remained sparse and have shown no signs of spawning within most of the stream. Water analyses have shown that the liming of the Loch Fleet catchment has minimal impact on downstream waters when flows are high, so that potentially toxic acid episodes have not been prevented.     

Fish assemblages composition in a natural, then regulated, stream: A quantitative long-term study

This paper tests the hypothesis how effectively fish assemblage composition was shaped by local climate changes and by engineering impact in 1989. This was possible due to the monitoring protocols of the present study, which allowed estimation of the magnitude of anthropogenic changes from changes naturally occurring in nature. Fish were sampled at the end of every growing season (October) for 23 years at five contiguous sites in a stream, before (1979-1988) and after (1989-2001) regulation. In each sample, six successive electrofishing passes were used to calculate the density and mean biomass for assemblage analysis using the Zippin model. During the study, the natural, meandering stream with pools, riffles, and a moderate canopy was modified into a straight stream of uniform width and depth, stripped of all vegetation. The output layer of a self-organizing map (SOM, the artificial neural network algorithm) applied in this study for site similarity analysis was partitioned into six subclusters placed in two main clusters. Subclusters in the upper part of the SOM were occupied chiefly by regulated stream samples and those in the lower part of the SOM by natural stream samples. Subclusters in the middle position, contained both natural (19) and regulated (20) samples in nearly equal proportion. In addition, the SOM contained one subcluster with sites only from the regulated period and another with only natural sites. Differences between subclusters were attributed to differences in climate, with some differences profound. Warming of the local weather, which became most evident in the 1990s, may have resulted in changes in fish assemblages. This is shown in the SOM, in which samples from the 1980s with cold years dominate the bottom of the SOM, whereas those from the 1990s and later are at the top. Subclusters dominated by regulated or natural sites were not always significantly different when the number of species and diversity indices were considered. Clear differences between the regulated and natural samples involved qualitative characteristics and mainly concerned assemblage composition. They were also confirmed by significant IndVal values (indicator species) and neither mixed subcluster contained important species in their assemblages.     

The Brown Trout Population at Loch Fleet Eight Years After Liming

Abstract

At the start of the Loch Fleet Project in 1984, the Loch and the upper 7 km of its efferent stream were found to be devoid of trout (Salmo trutta) as a result of acidification. Following the liming treatments applied to the catchment, from 1986 the formerly toxic water quality conditions (pH ～ 4.5, calcium ～ 1 mgl^?1, elevated aluminium and heavy metal levels) were eliminated, and trout were reintroduced on two occasions, in 1987 and 1988. A total of 520 fish were stocked, at a combined density equivalent to 5.5 kg ha^?1. Surveys of the loch and stream populations were carried out annually until 1993 to monitor their development, using a range of techniques, including electrofishing, gill-netting, seine-netting, spawner trapping and mark-release recapture methods. Length and scale- analysis were used to investigate fish growth.

The trout population in Loch Fleet expanded rapidly as a result of natural spawning in the loch's main feeder stream, augmented by the use of an artificial spawning bed which was constructed at the loch outlet in 1990. in mid-1983 the stock density, estimated by mark-recapture census methods, had increased to 24.9 kg ha^?1. Poor recruitment in the years 1991–93, however, reduced the rate of expansion and resulted in a population comprising mainly older individuals. the poor recruitment in these years was not fully explained but was not caused by water quality and was most likely a result of fry washout by spring spates.

Fish growth rates were high initially and were estimated on the basis of the Elliott trout growth model to be optimal for the prevailing water temperature regime of the loch. By 1991, growth rates had fallen, probably as a result of competition for food, but showed signs of recovery towards the end of the study period in 1993, following the period of lower population densities of young fish.

Trout rapidly repopulated the loch's outlet stream after 1987 but have remained sparse and have shown no signs of spawning within most of the stream. Water analyses have shown that the liming of the Loch Fleet catchment has minimal impact on downstream waters when flows are high, so that potentially toxic acid episodes have not been prevented.     

Evolution of a stream ecosystem in recently deglaciated terrain

Climate change and associated glacial recession create new stream habitat that leads to the assembly of new riverine communities through primary succession. However, there are still very few studies of the patterns and processes of community assembly during primary succession for stream ecosystems. We illustrate the rapidity with which biotic communities can colonize and establish in recently formed streams by examining Stonefly Creek in Glacier Bay, Alaska (USA), which began to emerge from a remnant glacial ice mass between 1976 and 1979. By 2002, 57 macroinvertebrate and 27 microcrustacea species had become established. Within 10 years of the stream's formation, pink salmon and Dolly Varden charr colonized, followed by other fish species, including juvenile red and silver salmon, Coast Range sculpin, and sticklebacks. Stable-isotope analyses indicate that marine-derived nitrogen from the decay of salmon carcasses was substantially assimilated within the aquatic food web by 2004. The findings from Stonefly Creek are compared with those from a long-term study of a similarly formed but older stream (12 km to the northeast) to examine possible similarities in macroinvertebrate community and biological trait composition between streams at similar stages of development. Macroinvertebrate community assembly appears to have been initially strongly deterministic owing to low water temperature associated with remnant ice masses. In contrast, microcrustacean community assembly appears to have been more stochastic. However, as stream age and water temperature increased, macroinvertebrate colonization was also more stochastic, and taxonomic similarity between Stonefly Creek and a stream at the same stage of development was <50%. However the most abundant taxa were similar, and functional diversity of the two communities was almost identical. Tolerance is suggested as the major mechanism of community assembly. The rapidity with which salmonids and invertebrate communities have become established across an entire watershed has implications for the conservation of biodiversity in freshwater habitats.     

Indirect upstream effects of dams: consequences of migratory consumer extirpation in Puerto Rico.

Large dams degrade the integrity of a wide variety of ecosystems, yet direct downstream effects of dams have received the most attention from ecosystem managers and researchers. We investigated indirect upstream effects of dams resulting from decimation of migratory freshwater shrimp and fish populations in Puerto Rico, USA, in both high- and low-gradient streams. In high-gradient streams above large dams, native shrimps and fishes were extremely rare, whereas similar sites without large dams had high abundances of native consumers. Losses of native fauna above dams dramatically altered their basal food resources and assemblages of invertebrate competitors and prey. Compared to pools in high-gradient streams with no large dams, pool epilithon above dams had nine times more algal biomass, 20 times more fine benthic organic matter (FBOM), 65 times more fine benthic inorganic matter (FBIM), 28 times more carbon, 19 times more nitrogen, and four times more non-decapod invertebrate biomass. High-gradient riffles upstream from large dams had five times more FBIM than did undammed riffles but showed no difference in algal abundance, FBOM, or non-decapod invertebrate biomass. For epilithon of low-gradient streams, differences in basal resources between pools above large dams vs. without large dams were considerably smaller in magnitude than those observed for pools in high-gradient sites. These results match previous stream experiments in which the strength of native shrimp and fish effects increased with stream gradient. Our results demonstrate that dams can indirectly affect upstream free-flowing reaches by eliminating strong top-down effects of consumers. Migratory omnivorous shrimps and fishes occur throughout the tropics, and the consequences of their declines upstream from many tropical dams are likely to be similar to those in Puerto Rico. Thus, ecological effects of migratory fauna loss upstream from dams encompass a wider variety of species interactions and biomes than the bottom-up effects (i.e., elimination of salmonid nutrient subsidies) recognized for northern temperate systems.     

Effects of flow regimes altered by dams on survival, population declines, and range-wide losses of California river-breeding frogs

Widespread alteration of natural hydrologic patterns by large dams combined with peak demands for power and water delivery during summer months have resulted in frequent aseasonal flow pulses in rivers of western North America. Native species in these ecosystems have evolved with predictable annual flood-drought cycles; thus, their likelihood of persistence may decrease in response to disruption of the seasonal synchrony between stable low-flow conditions and reproduction. We evaluated whether altered flow regimes affected 2 native frogs in California and Oregon (U.S.A.) at 4 spatial and temporal extents. We examined changes in species distribution over approximately 50 years, current population density in 11 regulated and 16 unregulated rivers, temporal trends in abundance among populations occupying rivers with different hydrologic histories, and within-year patterns of survival relative to seasonal hydrology. The foothill yellow-legged frog (Rana boylii), which breeds only in flowing water, is more likely to be absent downstream of large dams than in free-flowing rivers, and breeding populations are on average 5 times smaller in regulated rivers than in unregulated rivers. Time series data (range = 8 - 19 years) from 5 populations of yellow-legged frogs and 2 populations of California red-legged frogs (R. draytonii) across a gradient of natural to highly artificial timing and magnitude of flooding indicate that variability of flows in spring and summer is strongly correlated with high mortality of early life stages and subsequent decreases in densities of adult females. Flow management that better mimics natural flow timing is likely to promote persistence of these species and others with similar phenology.     

Regeneration of Native Trees in the Presence of Invasive Saltcedar in the Colorado River Delta, Mexico

Abstract:  Many riparian zones in the Sonoran Desert have been altered by elimination of the normal flood regime; such changes to the flow regime have contributed to the spread of saltcedar ( Tamarix ramosissma Ledeb.), an exotic, salt-tolerant shrub. It has been proposed that reestablishment of a natural flow regime on these rivers might permit passive restoration of native trees, without the need for aggressive saltcedar clearing programs. We tested this proposition in the Colorado River delta in Mexico, which has received a series of large-volume water releases from U.S. dams over the past 20 years. We mapped the vegetation of the delta riparian corridor through ground and aerial surveys (1999–2002) and satellite imagery (1992–2002) and related vegetation changes to river flood flows and fire events. Although saltcedar is still the dominant plant in the delta, native cottonwood (  Populus fremontii S. Wats.) and willow ( Salix gooddingii C. Ball) trees have regenerated multiple times because of frequent flood releases from U.S. dams since 1981. Tree populations are young and dynamic (ages 5–10 years). The primary cause of tree mortality between floods is fire. Biomass in the floodplain, as measured by the normalized difference vegetation index on satellite images, responds positively even to low-volume (but long-duration) flood events. Our results support the hypothesis that restoration of a pulse flood regime will regenerate native riparian vegetation despite the presence of a dominant invasive species, but fire management will be necessary to allow mature tree stands to develop.     

Differential movement patterns and site fidelity among trophic groups of reef fishes in a Hawaiian marine protected area

We tracked the long-term movements of 70 parrotfishes, surgeonfishes and goatfishes captured inside a small (1.3 km²) marine protected area (MPA: Kealakekua Bay Marine Life Conservation District, Hawaii) by implanting them with small transmitters and deploying underwater monitoring devices inside the bay and along 100 km of the adjacent west Hawaii coastline. Individual fish were detected inside Kealakekua Bay for up to 612 days but many were detected for much shorter periods (median = 52 days). There were species-specific differences in the scale of movements and habitats used, but most fish utilized between 0.2 and 1.6 km of coastline, and individuals of each species showed some degree of diel habitat shift. A wide variety of reef fishes captured inside the MPA swam back and forth across an MPA boundary intersecting continuous reef (i.e., this boundary was porous to reef fish movements), but only 1 of 11 species tagged crossed a wide sandy channel inside Kealakekua Bay suggesting that this feature may function as a natural barrier to movements. Results indicate relatively small MPAs (<2 km of coastline) could provide effective, long-term protection for multi-species assemblages of reef fishes provided that boundaries are situated along major habitat breaks (e.g., large sand channels between reefs) that may serve as natural barriers to reef fish movements. It is crucial that a multi-species approach be used when assessing MPA effectiveness.     

An Evaluation of Restoration Efforts in Fishless Lakes Stocked with Exotic Trout

Abstract: Detrimental effects of introduced fishes on native amphibian populations have prompted removal of introduced cutthroat ( Oncorhynchus clarki ), rainbow ( Oncorhynchus mykiss ), and brook trout (  Salvelinus fontinalis ) from naturally fishless lakes at Mt. Rainier National Park, Washington ( U.S.A.). Using paleolimnological indicators (diatoms, invertebrates, and sediment characteristics) in eight 480-year-old sediment cores from eight lakes, we (1) derived estimates of baseline environmental conditions and natural variation, (2) assessed the effects of stocking naturally fishless lakes, and (3) determined whether lakes returned to predisturbance conditions after fish removal (restoration). Diatom floras were relatively stable between 315 and 90 years before present in all lakes; we used this time period to define lake-specific "baseline" conditions. Dissimilarity analyses of diatoms revealed sustained, dramatic changes in diatom floras that occurred approximately 80 years ago (when fish were introduced) in four of five stocked lakes, whereas the diatom floras in two unstocked lakes had not changed significantly in the last 315 years. Diatoms were not preserved in an eighth lake. State changes also occurred in two lakes over 200 years before European settlement of the Pacific Northwest. Preserved invertebrate densities fluctuated dramatically over time in all cores, providing a poor reference for assessing the effects of fishes. Nevertheless, fish-invertebrate interactions have been demonstrated in other paleolimnological studies and may be useful for lower-elevation or more productive lakes. Because diatom communities have not returned to predisturbance assemblages in restored lakes, even 20–30 years after fish removal, we conclude that Mt. Rainier lakes were not successfully restored by the removal of fishes.     

Development of a coupled reservoir operation and water diversion model: Balancing human and environmental flow requirements

Maintaining a natural flow regime helps preserve the health of riverine ecosystems. Conventional studies on reservoir operations have focused mainly on identifying optimal operational schemes for satisfying human water demands. To systematically reflect the ecological effects of both natural and human-induced hydrologic alterations, water diversions downstream of the reservoirs should be considered as well. This research focused on a coupled reservoir operation and water diversion (CROWD) model, created through the integration of a reservoir operation model and a water diversion model. The proposed model considers both human and environmental flow requirements, and represents a compromise that balances ecological protection (preservation of the natural flow regime of a river) and human needs (reduced water shortages). In the reservoir operation model, the reservoir space is divided into three zones and different operating rules are developed for directing reservoir operation when water levels are in different zones; in the water diversion model, different water users are assigned different supply priorities with the instream flows no more than the minimum environmental flows having the highest priority; and the two models are coupled by the water mass balance between the two hydraulic facilities. The non-dominated-sorting genetic algorithm II (NSGA-II) was used to determine the parameters of the developed CROWD model and the model was applied to support the joint operational management of the Tanghe Reservoir and the Liaoyang Diversion in the Tang river basin, China. The resulting reservoir operation and water diversion schemes indicate that the CROWD model is useful for optimizing the operation of reservoirs and water diversion schemes. Moreover, it helps to analyze tradeoffs between human and environmental water needs, resulting in solutions that reduce the risk of water shortages and minimize ecological integrity disturbances.     

Life history theory predicts fish assemblage response to hydrologic regimes

The hydrologic regime is regarded as the primary driver of freshwater ecosystems, structuring the physical habitat template, providing connectivity, framing biotic interactions, and ultimately selecting for specific life histories of aquatic organisms. In the present study, we tested ecological theory predicting directional relationships between major dimensions of the flow regime and life history composition of fish assemblages in perennial free-flowing rivers throughout the continental United States. Using long-term discharge records and fish trait and survey data for 109 stream locations, we found that 11 out of 18 relationships (61%) tested between the three life history strategies (opportunistic, periodic, and equilibrium) and six hydrologic metrics (two each describing flow variability, predictability, and seasonality) were statistically significant (P < or = 0.05) according to quantile regression. Our results largely support a priori hypotheses of relationships between specific flow indices and relative prevalence of fish life history strategies, with 82% of all significant relationships observed supporting predictions from life history theory. Specifically, we found that (1) opportunistic strategists were positively related to measures of flow variability and negatively related to predictability and seasonality, (2) periodic strategists were positively related to high flow seasonality and negatively related to variability, and (3) the equilibrium strategists were negatively related to flow variability and positively related to predictability. Our study provides important empirical evidence illustrating the value of using life history theory to understand both the patterns and processes by which fish assemblage structure is shaped by adaptation to natural regimes of variability, predictability, and seasonality of critical flow events over broad biogeographic scales.     

Assessment of Young Dong tributary and Imgok Creek impacted by Young Dong coal mine, South Korea

An initial reclamation of the Young Dong coal mine site, located in northeastern South Korea, was completed in 1995. Despite the filling of the adit with limestone, acid rock drainage (ARD) enters Young Dong tributary and is then discharged to Imgok Creek. This ARD carries an average of 500 mg CaCO(3)/l of mineral acidity, primarily as Fe(II) and Al. Before spring runoff, the flow of Imgok Creek is 3.3-4 times greater than that of the tributary and has an alkalinity of 100 mg CaCO(3)/l, which is sufficient to eliminate the mineral acidity and raise the pH to about 6.5. From April through September 2008, there were at least two periods of high surface flow that affects the flow of ARD from the adit. Flow of ARD reaches 2.8 m(3)/min during spring runoff. This raised the concentrations of Fe and Al in the confluence with Imgok Creek. However, by 2 km downstream the pH of the Imgok Creek is 6.5 and only dissolved Fe is above the Korean drinking water criteria (0.30 mg/l). This suggests only a minor impact of Young Dong Creek water on Imgok Creek. Acid digestion of the sediments in Imgok Creek and Young Dong Tributary reveals considerable abundances of heavy metals, which could have a long-term impact on water quality. However, several water-based leaching tests, which better simulate the bioavailable metals pool, released only Al, Fe, Mn, and Zn at concentrations exceeding the criteria for drinking water or aquatic life.     

Light-mediated thresholds in stream-water nutrient composition in a river network

The elemental composition of solutes transported by rivers reflects combined influences of surrounding watersheds and transformations within stream networks, yet comparatively little is known about downstream changes in effects of watershed loading vs. in-channel processes. In the forested watershed of a river under a mediterranean hydrologic regime, we examined the influence of longitudinal changes in environmental conditions on water-column nutrient composition during summer base flow across a network of sites ranging from strongly heterotrophic headwater streams to larger, more autotrophic sites downstream. Small streams (0.1-10 km2 watershed area) had longitudinally similar nutrient concentration and composition with low (approximately 2) dissolved nitrogen (N) to phosphorus (P) ratios. Abrupt deviations from this pattern were observed in larger streams with watershed areas > 100 km2 where insolation and algal abundance and production rapidly increased. Downstream, phosphorus and silica concentrations decreased by > 50% compared to headwater streams, and dissolved organic carbon and nitrogen increased by approximately 3-6 times. Decreasing dissolved P and increasing dissolved N raised stream-water N:P to 46 at the most downstream sites, suggesting a transition from N limitation in headwaters to potential P limitation in larger channels. We hypothesize that these changes were mediated by increasing algal photosynthesis and N fixation by benthic algal assemblages, which, in response to increasing light availability, strongly altered stream-water nutrient concentration and stoichiometry in larger streams and rivers.     

Flood regime and leaf fall determine soil inorganic nitrogen dynamics in semiarid riparian forests.

Flow regulation has reduced the exchange of water, energy, and materials between rivers and floodplains, caused declines in native plant populations, and advanced the spread of nonnative plants. Naturalized flow regimes are regarded as a means to restore degraded riparian areas. We examined the effects of flood regime (short [SIFI] vs. long [LIFI] inter-flood interval) on plant community and soil inorganic nitrogen (N) dynamics in riparian forests dominated by native Populus deltoides var. wislizenii Eckenwalder (Rio Grande cottonwood) and nonnative Tamarix chinensis Lour. (salt cedar) along the regulated middle Rio Grande of New Mexico. The frequency of inundation (every 2-3 years) at SIFI sites better reflected inundation patterns prior to the closure of an upstream dam relative to the frequency of inundation at LIFI sites (> or =10 years). Riparian inundation at SIFI sites varied from 7 to 45 days during the study period (April 2001-July 2004). SIFI vs. LIFI sites had higher soil moisture but greater groundwater table elevation fluctuation in response to flooding and drought. Rates of net N mineralization were consistently higher at LIFI vs. SIFI sites, and soil inorganic N concentrations were greatest at sites with elevated leaf-litter production. Sites with stable depth to ground water (approximately 1.5 m) supported the greatest leaf-litter production. Reduced leaf production at P. deltoides SIFI sites was attributed to drought-induced recession of ground water and prolonged inundation. We recommend that natural resource managers and restoration practitioners (1) utilize naturalized flows that help maintain riparian groundwater elevations between 1 and 3 m in reaches with mature P. deltoides or where P. deltoides revegetation is desired, (2) identify areas that naturally undergo long periods of inundation and consider restoring these areas to seasonal wetlands, and (3) use native xeric-adapted riparian plants to revegetate LIFI and SIFI sites where groundwater elevations commonly drop below 3 m.     

Assessing stream restoration effectiveness at reducing nitrogen export to downstream waters

The degradation of headwater streams is common in urbanized coastal areas, and the role these streams play in contributing to downstream pollution is a concern among natural resource managers and policy makers. Thus, many urban stream restoration efforts are increasingly focused on reducing the downstream flux of pollutants. In regions that suffer from coastal eutrophication, it is unclear whether stream restoration does in fact reduce nitrogen (N) flux to downstream waters and, if so, by how much and at what cost. In this paper, we evaluate whether stream restoration implemented to improve water quality of urban and suburban streams in the Chesapeake Bay region, USA, is effective at reducing the export of N in stream flow to downstream waters. We assessed the effectiveness of restored streams positioned in the upland vs. lowland regions of Coastal Plain watershed during both average and stormflow conditions. We found that, during periods of low discharge, lowland streams that receive minor N inputs from groundwater or bank seepage reduced in-stream N fluxes. Furthermore, lowland streams with the highest N concentrations and lowest discharge were the most effective. During periods of high flow, only those restoration projects that converted lowland streams to stream-wetland complexes seemed to be effective at reducing N fluxes, presumably because the design promoted the spillover of stream flow onto adjacent floodplains and wetlands. The observed N-removal rates were relatively high for stream ecosystems, and on the order of 5% of the inputs to the watershed. The dominant forms of N entering restored reaches varied during low and high flows, indicating that N uptake and retention were controlled by distinctive processes during different hydrological conditions. Therefore, in order for stream restoration to effectively reduce N fluxes exported to downstream waters, restoration design should include features that enhance the processing and retention of different forms of N, and for a wide range of flow conditions. The use of strategic designs that match the dominant attributes of a stream such as position in the watershed, influence of groundwater, dominant flow conditions, and N concentrations is crucial to assure the success of restoration.     

Are soil mite assemblages structured by the identity of native and invasive alien grasses?

Associations between plants and animals in aboveground communities are often predictable and specific. This has been exploited for the purposes of estimating the diversity of animal species based on the diversity of plant species. The introduction of invasive alien plants into an ecosystem can result in dramatic changes in both the native plant and animal assemblages. Few data exist at the species level to determine whether belowground animal assemblages share the same degree of association to plants. The hypotheses that soil mites (Acari) form assemblages specifically associated with different native grass species in an unmanipulated natural ecosystem and that invasive alien grasses will impact soil mite assemblage composition in this setting were tested. Soil mites sampled beneath five native and two invasive alien species of grasses at the Konza Prairie Biological Station, Kansas, USA, were similarly abundant, species rich, diverse, and taxonomically distinct. No mite species had affinities for a specific grass species. There was no evidence from analysis of similarity, canonical correspondence analysis, or a nonparametric assemblage analysis that the assemblage composition of soil mites was specific to grass species. Results suggest that soil mite assemblages were more related to characteristics of the plant assemblage as a whole or prevailing soil conditions. The most recent invasive alien grass did not support a successionally younger mite fauna, based on the ratio of mesostigmatid to oribatid mites, and neither of the two invasive grasses influenced mite assemblage structure, possibly because they had not yet substantially altered the soil environment. Our results suggest that extrapolations of soil mite diversity based on assumptions of plant specificity would be invalid.     

Ecological Impact of Introduced Crayfish on Benthic Fishes in a British Lowland River

Pacifastacus leniusculus (Dana), a native crayfish of western North America, was introduced into the U.K. in 1976. Our study examined some interactions between P. leniusculus and benthic fish in a British lowland river, the River Great Ouse. In a river survey an inverse correlation was found between the abundance of crayfish and the two dominant benthic fishes, bullhead (Cottus gobio L.) and stone loach (   Noemacheilus barbatulus [L.]) in six riffles. The benthic fishes were least abundant in the riffle nearest the original site of crayfish introduction and gradually increased in abundance both up and down river as crayfish abundance decreased. The hypotheses that crayfish compete with bullheads and stone loach for shelter and prey on fish were tested by laboratory experiments in an outdoor artificial stream (6 × 2 m) with recirculating water and 12 artificial shelters on the bottom. In competition experiments 12 fish of one species were alternatively kept alone and with 12 crayfish for 3-day cycles lasting a total of 12 days. The results showed that crayfish out-competed both fish species for shelter. Predation was measured by keeping 24 fish of each species alone and with 36 crayfish for 10 days respectively in the artificial stream. The mortalities of both fish species were significantly higher when crayfish were present. The loss of fish could be partly due to predation because crayfish guts contained the remains of some lost fish and they were observed preying on both fishes in a tank. In the river crayfish lived at high densities reaching ≥ to 20 m⁻² in riffles, and they continued to disperse. This may lead to a great reduction in benthic fish abundance if not local extinctions.