Extent, properties, and landscape setting of geographically isolated wetlands in urban southern New England watersheds

We assessed the extent and characteristics of geographically isolated wetlands (i.e., wetlands completely surrounded by upland) in a series of watersheds in the urban northeast US. We applied a previously developed index of urbanization to a sample of 10 watersheds selected at random from a set of 30 watersheds whose boundaries lay within the borders of Rhode Island, USA. The index of urbanization in our sample watersheds ranged over more than an order of magnitude and increased with increasing amount of urban land use in the watersheds (r ² = 0.51, F = 8.22, P = 0.02). The density of isolated wetlands in the watersheds averaged 1.93 ± 0.21 wetlands km⁻² and comprised 38.2 ± 1.77% of all wetlands. Isolated wetlands were smaller than those connected to other waters (non-isolated), and accounted for 6.01–16.5% of the total wetland area in the watersheds. The area of isolated wetlands as a percent of all wetland area significantly increased with increasing watershed urbanization (r ² = 0.62, F = 12.9, P = 0.007). Isolated wetlands were predominantly deciduous forested wetlands, and urban land cover in the 50 m buffer surrounding isolated wetlands was significantly higher than in the 50 m surrounding non-isolated wetlands. The proportion of urban land cover was greater in a 150 than a 50 m buffer surrounding the wetlands. Our results suggest that an increase in the index of urbanization of 50 will result in 7% of the watershed’s wetlands being lost from federal protection. These findings indicate that the process of urbanization, along with accompanying habitat fragmentation, may result in an increase in the vulnerability of wetlands to loss and degradation and therefore has implications for the management and conservation of geographically isolated wetlands.     

Drainage Size, Stream Intermittency, and Ecosystem Function in a Sonoran Desert Landscape

Understanding the interactions between terrestrial and aquatic ecosystems remains an important research focus in ecology. In arid landscapes, catchments are drained by a channel continuum that represents a potentially important driver of ecological pattern and process in the surrounding terrestrial environment. To better understand the role of drainage networks in arid landscapes, we determined how stream size influences the structure and productivity of riparian vegetation, and the accumulation of organic matter (OM) in soils beneath plants in an upper Sonoran Desert basin. Canopy volume of velvet mesquite (Prosopis velutina), as well as overall plant cover, increased along lateral upland–riparian gradients, and among riparian zones adjacent to increasingly larger streams. Foliar δ¹³C signatures for P. velutina suggested that landscape patterns in vegetation structure reflect increases in water availability along this arid stream continuum. Leaf litter and annual grass biomass production both increased with canopy volume, and total aboveground litter production ranged from 137 g m⁻² y⁻¹ in upland habitat to 446 g m⁻² y⁻¹ in the riparian zone of the perennial stream. OM accumulation in soils beneath P. velutina increased with canopy volume across a broad range of drainage sizes; however, in the riparian zone of larger streams, flooding further modified patterns of OM storage. Drainage networks represent important determinants of vegetation structure and function in upper Sonoran Desert basins, and the extent to which streams act as sources of plant-available water and/or agents of fluvial disturbance has implications for material storage in arid soils.     

Riparian zones as havens for exotic plant species in the central grasslands

In the Central Grasslands of the United States, we hypothesized that riparian zones high in soil fertility would contain more exotic plant species than upland areas of low soil fertility. Our alternate hypothesis was that riparian zones high in native plant species richness and cover would monopolize available resources and resist invasion by exotic species. We gathered nested-scale vegetation data from 40 1 m²subplots (nested in four 1000 m² plots) in both riparian and upland sites at four study areas in Colorado, Wyoming, and South Dakota (a total of 320 1 m² subplots and 32 1000 m² plots). At the 1 m² scale, mean foliar cover of native species was significantly greater (P<0.001) in riparian zones (36.3% ± 1.7%) compared to upland sites (28.7% ± 1.5%), but at this small scale there were no consistent patterns of native and exotic species richness among the four management areas. Mean exotic species cover was slightly higher in upland sites compared to riparian sites (9.0% ± 3.8% versus 8.2% ± 3.0% cover). However, mean exotic species richness and cover were greater in the riparian zones than upland sites in three of four management areas. At the 1000 m² scale, mean exotic species richness was also significantly greater (P<0.05) in riparian zones (7.8 ± 1.0 species) compared to upland sites (4.8 ± 1.0 species) despite the heavy invasion of one upland site. For all 32 plots combined, 21% of the variance in exotic species richness was explained by positive relationships with soil % silt (t =1.7, P=0.09) and total foliar cover (t = 2.4, P=0.02). Likewise, 26% of the variance in exotic species cover (log₁₀ cover) was explained by positive relationships with soil % silt (t =2.3, P=0.03) and total plant species richness (t = 2.5, P=0.02). At landscape scales (four 1000 m² plots per type combined), total foliar cover was significantly and positively correlated with exotic species richness (r=0.73, P<0.05) and cover (r=0.74, P<0.05). Exotic species cover (log₁₀ cover) was positively correlated with log₁₀% N in the soil (r=0.61, P=0.11) at landscape scales. On average, we found that 85% (±5%) of the total number of exotic species in the sampling plots of a given management area could be found in riparian zones, while only 50% (±8%) were found in upland plots. We conclude that: (1) species-rich and productive riparian zones are particularly invasible in grassland ecosystems; and (2) riparian zones may act as havens, corridors, and sources of exotic plant invasions for upland sites and pose a significant challenge to land managers and conservation biologists.     

Riparian control on NO3 −, DOC, and dissolved Fe concentrations in mountainous streams, northern Japan

We evaluated (1) the longitudinal pattern of stream chemistry and (2) the effects of the riparian zone on this longitudinal pattern for nitrate (NO₃ ⁻), dissolved organic carbon (DOC), and total dissolved iron (Fe). We selected two small watersheds; the “southern watershed” had an extending riparian wetland and the “northern watershed” had a narrow riparian area. Stream NO₃ ⁻ concentrations decreased from the spring to outlet of both watersheds. In the southern watershed, stream DOC concentration decreased from the spring to midstream and then increased to the outlet. Stream Fe concentration in the southern watershed longitudinally increased. On the other hand, the northern watershed exhibited no longitudinal pattern for DOC and Fe concentrations. In both watersheds, while NO₃ ⁻ concentrations in the soil and ground water were lower than those in the stream waters, DOC and Fe concentrations exhibited the opposite patterns. The longitudinal decreases of NO₃ ⁻ concentrations in both streams and increase of stream Fe in the southern watershed mainly resulted from the inflow of the soil and ground water to the stream. The decrease in stream DOC from the spring to midstream in the southern watershed was due to the deep groundwater having low DOC, while the subsequent increase to the surrounding soil and ground water. Moreover, considerations of stream solute flow with soil and ground water chemistry suggested other mechanisms adding NO₃ ⁻ and removing/diluting DOC and Fe, especially for the northern watershed; coexistence of oxidizing and reducing conditions in the riparian zone might control the longitudinal concentration change in the stream water chemistry.     

Relationship between Riparian Vegetation and Stream Benthic Communities at Three Spatial Scales

We examined the influence of riparian vegetation on macroinvertebrate community structure in streams of the Upper Thames River watershed in southwestern Ontario. Thirty-three μ-basins (129–1458 ha) were used to identify land cover variables that influenced stream macroinvertebrates. Micro-basins represented the entire drainage area of study streams and were similar in stream order (first, second) and land cover (agricultural or forest; no urban). We described the structure and composition of riparian vegetation and benthic macroinvertebrate communities at the outflow reach. The nature of the land cover was quantified for the stream network buffer (30 m) and the whole μ-basin. The objective of this study was to measure the magnitude and nature of the relationship between the riparian vegetation and benthic macroinvertebrate community at the outflow reach, stream network buffer, and whole μ-basin scales. Taxon richness (including total number of Ephemeroptera, Plecoptera, and Trichoptera taxa) and Simpson’s diversity of the macroinvertebrate community all increased with increased tree cover in the riparian zone at the outflow reach scale. Simpson’s equitability was lower with greater agricultural land cover in the stream network buffer. No relationship between the macroinvertebrate community and land cover was found at the whole μ-basin scale. Analysis of the influence of land cover on stream communities within a spatial hierarchy is important for understanding the interactions of stream ecosystems with their adjacent landscapes.     

Phosphorus regulates stream injury by filamentous green algae,DO, and pH with thresholds in responses

Nutrient concentrations, benthic algal biomass, dissolved oxygen (DO), and pH were measured in 70 or more streams during spring and summer in the Illinois River Watershed (IRW), which crosses the Oklahoma and Arkansas (USA) border, to determine whether injury to streams occurred and if that injury was related to spreading poultry waste on fields. Definitions of injury were based on Oklahoma water quality regulations and scientific literature. Phosphorus and nitrogen concentrations were each independently related to poultry house density (PHD) in watersheds and percent urban land use in watersheds. In addition, phosphorus and nitrogen concentrations were unusually high compared to regions with similar geology and hydrology. Molar N:P ratios were high and indicated that phosphorus was the most likely limiting nutrient. Phosphorus concentrations, as well as PHD and urban land use, were related to algal biomass during spring, but were less related during summer. A threshold response in cover of stream bottoms by nuisance filamentous green algae (NFGA: Cladophora, Rhizoclonium, and Oedogonium) during spring was observed at 27 μg TP l⁻¹ using regression tree analysis. Great increases in average NFGA cover (from 4 to 36% cover) occurred with relatively small increases in TP concentration at the 27 μg TP l⁻¹ threshold. Average concentrations of DO, variability in DO, and pH during spring were positively related to TP, chlorophyll a, and NFGA cover. Minimum DO during spring and early morning DO during summer were negatively related to TP concentration. Spring pH and summer DO frequently violated water quality requirements for protecting biodiversity that were established by the state of Oklahoma. We conclude that poultry house operations as well as urban activities, independently and interactively, pollute IRW streams with phosphorus, which resulted in injury to aesthetic condition and the potential for injury of biodiversity.     

A Stable Isotope Tracer Study of the Influences of Adjacent Land Use and Riparian Condition on Fates of Nitrate in Streams

The influence of land use on potential fates of nitrate (NO₃ ⁻) in stream ecosystems, ranging from denitrification to storage in organic matter, has not been documented extensively. Here, we describe the Pacific Northwest component of Lotic Intersite Nitrogen eXperiment, phase II (LINX II) to examine how land-use setting influences fates of NO₃ ⁻ in streams. We used 24 h releases of a stable isotope tracer (¹⁵NO₃-N) in nine streams flowing through forest, agricultural, and urban land uses to quantify NO₃ ⁻ uptake processes. NO₃ ⁻ uptake lengths varied two orders of magnitude (24–4247 m), with uptake rates (6.5–158.1 mg NO₃-N m⁻² day⁻¹) and uptake velocities (0.1–2.3 mm min⁻¹) falling within the ranges measured in other LINX II regions. Denitrification removed 0–7% of added tracer from our streams. In forest streams, 60.4 to 77.0% of the isotope tracer was exported downstream as NO₃ ⁻, with 8.0 to 14.8% stored in wood biofilms, epilithon, fine benthic organic matter, and bryophytes. Agricultural and urban streams with streamside forest buffers displayed hydrologic export and organic matter storage of tracer similar to those measured in forest streams. In agricultural and urban streams with a partial or no riparian buffer, less than 1 to 75% of the tracer was exported downstream; much of the remainder was taken up and stored in autotrophic organic matter components with short N turnover times. Our findings suggest restoration and maintenance of riparian forests can help re-establish the natural range of NO₃ ⁻ uptake processes in human-altered streams.     

Nitrogen Transformations in Flowpaths Leading from Soils to Streams in Amazon Forest and Pasture

The modification of large areas of tropical forest to agricultural uses has consequences for the movement of inorganic nitrogen (N) from land to water. Various biogeochemical pathways in soils and riparian zones can influence the movement and retention of N within watersheds and affect the quantity exported in streams. We used the concentrations of NO₃ ⁻ and NH₄ ⁺ in different hydrological flowpaths leading from upland soils to streams to investigate inorganic N transformations in adjacent watersheds containing tropical forest and established cattle pasture in the southwestern Brazilian Amazon Basin. High NO₃ ⁻ concentrations in forest soil solution relative to groundwater indicated a large removal of N mostly as NO₃ ⁻ in flowpaths leading from soil to groundwater. Forest groundwater NO₃ ⁻ concentrations were lower than in other Amazon sites where riparian zones have been implicated as important N sinks. Based on water budgets for these watersheds, we estimated that 7.3–10.3 kg N ha⁻¹ y⁻¹ was removed from flowpaths between 20 and 100 cm, and 7.1–10.2 kg N ha⁻¹ y⁻¹ was removed below 100 cm and the top of the groundwater. N removal from vertical flowpaths in forest exceeded previously measured N₂O emissions of 3.0 kg N ha⁻¹ y⁻¹ and estimated emissions of NO of 1.4 kg N ha⁻¹ y⁻¹. Potential fates for this large amount of nitrate removal in forest soils include plant uptake, denitrification, and abiotic N retention. Conversion to pasture shifted the system from dominance by processes producing and consuming NO₃ ⁻ to one dominated by NH₄ ⁺, presumably the product of lower rates of net N mineralization and net nitrification in pasture compared with forest. In pasture, no hydrological flowpaths contained substantial amounts of NO₃ ⁻ and estimated N removal from soil vertical flowpaths was 0.2 kg N ha⁻¹ y⁻¹ below the depth of 100 cm. This contrasts with the extent to which agricultural sources dominate N inputs to groundwater and stream water in many temperate regions. This could change, however, if pasture agriculture in the tropics shifts toward intensive crop cultivation.     

A multi-scale assessment of the occurrence of exotic plants on the Olympic Peninsula,Washington

Abstract. Exotic plants were surveyed in 208 plots within the Dungeness and Hoh river watersheds on the Olympic Peninsula, Washington, USA. Landscape patch types included uplands (clearcuts, young and mature forests) and riparian zones (cobble bars, shrub patches, riparian forests, and alder flats). Patterns of exotic plant invasions were assessed between watersheds, between riparian and upland areas, among patch types, and within clearcuts. 52 exotic plant species were encountered, accounting for 23% of the flora in each watershed. In both watersheds, exotic species richness was approximately 33% greater in riparian zones than in uplands, and mean number and cover of exotic species were > 50% greater in riparian zones than in uplands. Among landscape patch types, richness and mean number and cover of exotics was highest in young riparian patches, intermediate in clearcuts and riparian forests, and lowest in young and mature forests. The exception to this was Hoh alder flats, which had the highest mean cover of exotic plants. Cover of exotic plants peaked in uplands 3 to 7 yr after clearcutting, then decreased with increased canopy closure. Disturbance type and time since disturbance were major factors influencing invasibility. Landscape patch size, position within watershed (distance from patch to human population centers, major highway, or river mouth), and environmental variables (slope, aspect, and elevation) were not important indicators of landscape patch invasibility. Riparian zones facilitated movement of exotic plants through landscapes, but did not appear to act as sources of exotic plants for undisturbed upland areas.     

Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon

This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon, a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO₃−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH₄−N) concentrations were very low (usually < 0.005 mg L⁻¹) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged (defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds that were all older than 10 years still lost significantly more N, primarily as NO₃−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal scales in a region of relatively low anthropogenic N input.     

Impact of riparian buffer zones on water quality and associated management considerations

Recent attention has focused on riparian forest buffer systems for filtering sediment, nutrients, and pesticides entering from upland agricultural fields. This paper summarizes the results of a field monitoring study done in Tokachikawa watershed in Hokkaido, Japan, Cisadane, Cianten and Citamyang sub-watersheds in Indonesia and Cauvery watershed, India to quantify the impact of riparian buffer zones on changes in stream water quality. A watershed approach was used to compare land use indicators – uplands, forests, riparian forest, livestock areas – to a wide range of surface water physical and chemical properties. Stream water physical property values increased from upstream to the confluence point, influenced by the upland and livestock land use activities. The greatest reduction in impairment of water quality was observed in buffer zones located along higher order streams where the gradient is very low, leading to slow groundwater movement. The lower stream water temperature in riparian buffer zones suggests that the shading effect is most pronounced in this area of the watershed. The results demonstrate the positive impact of forest buffer zones in reducing the influence of agricultural nutrients and chemicals on surface stream waters. Design and management considerations for establishing riparian zone land use are discussed.     

SPATIAL PATTERNS AND ECOLOGICAL DETERMINANTS OF BENTHIC ALGAL ASSEMBLAGES IN MID-ATLANTIC STREAMS, USA

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid-Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid-Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom-based stream classification did not correspond to Omernik's ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid-Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse-scale factors, such as land cover/use in watersheds, and by site-specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid-Atlantic streams.     

Spatial and temporal patterns of nitrogen concentrations in pristine and agriculturally-influenced prairie streams

Long-term data on nitrogen chemistry of streams draining Konza Prairie Biological Station (Konza), Kansas were analyzed to assess spatial and temporal patterns and examine the influence of agricultural activity on these patterns. Upland watersheds of Konza are predominantly tallgrass prairies, but agricultural fields and riparian forests border the lower reaches of the streams. We have up to 11 years of data in the relatively pristine upland reaches and 4 years of data on wells and downstream reaches influenced by fertilized croplands. Seasonal and spatial patterns in total nitrogen (TN) concentrations were driven largely by changes in the nitrate (NO₃ ^–) concentrations. A gradient of increasing NO₃ ^– concentrations occurred from pristine upland stream reaches to the more agriculturally-influenced lowland reaches. Nitrate concentrations varied seasonally and were negatively correlated with discharge in areas influenced by row-crop agriculture (p = 0.007). The NO₃ ^– concentrations of stream water in lowland reaches were lowest during times of high precipitation, when the relative influence of groundwater drainage is minimal and water in the channel is primarily derived from upland prairie reaches. The groundwater from cropland increased stream NO₃ ^– concentrations about four-fold during low-discharge periods, even though significant riparian forest corridors existed along most of the lower stream channel. The minimum NO₃ ^– concentrations in the agriculturally influenced reaches were greater than at any time in prairie reaches. Analysis of data before and after introduction of bison to four prairie watersheds revealed a 35% increase of TN concentrations (p < 0.05) in the stream water channels after the introduction of bison. These data suggest that natural processes such as bison grazing, variable discharge, and localized input of groundwater lead to variation in NO₃ ^– concentrations less than 100-fold in prairie streams. Row-crop agriculture can increase NO₃ ^– concentrations well over 100-fold relative to pristine systems, and the influence of this land use process over space and time overrides natural processes.     

Reconstructing the historical trophic status of northwestern Pennsylvania lakes using GIS

From each of 46 watersheds in glaciated northwestern Pennsylvania we estimated phosphorus export (kg P/ha/yr) from weekly or twice-weekly measured stream phosphorus concentrations and measured stream discharges, and determined land covers using GIS. Simple and step-down multiple regression analyses yielded models that explained 24% of the variation in P export using land cover within whole watersheds, and 64% of the variation using land cover within 200 m riparian buffers. We used these models to predict P loading to seven lakes and found that predicted lake [P] was consistent with measured lake [P]. To estimate pre-settlement lake [P] we reapplied the P export models with the assumption that human-impacted land covers were originally forests. Predicted (hindcast) pre-settlement lake [P] indicated that six of the seven lakes were edaphically mesotrophic (10 < [P] < 20 μg/l). Lake remediation targets set on the assumption that area lakes were historically oligotrophic ([P] < 10 μg/l) will be unattainable.     

Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: nitrous oxide fluxes

Fluxes of N₂O at the soil surface, dissolved N₂O in near-surface groundwater, and potential N₂O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N₂O fluxes were highest at topographic breaks in the landscape (≃ 40–300 μg N₂O-N m⁻² h⁻¹). At other locations in the riparian zone and hillslope, fluxes were lower (⩽ 2 μg N₂O-N m⁻² h⁻¹). This pattern of surface N₂O fluxes was persistent. In the Bisley watershed, mean suface N₂O fluxes were lower (<40 μg N₂O-N m⁻² h⁻¹) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N₂O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO₃ to N₂O (and perhaps N₂). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N₂O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N₂O flux     

Importance of scale,land‐use,and stream network properties for riparian plant communities along an urban gradient

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Stormflow Dynamics of Dissolved Organic Carbon and Total Dissolved Nitrogen in a Small Urban Watershed

We examined patterns of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) loading to a small urban stream during baseflow and stormflow. We hypothesized that lower DOC and TDN contributions from impervious surfaces would dilute natural hydrologic flowpath (i.e., riparian) contributions during storm events in an urban watershed, resulting in lower concentrations of DOC and TDN during storms. We tested these hypotheses in a small urban watershed in Portland, Oregon, over a 3-month period during the spring of 2003. We compared baseflow and stormflow chemistry using Mann–Whitney tests (significant at p<0.05). We also applied a mass balance to the stream to compare the relative significance of impervious surface contributions versus riparian contributions of DOC and TDN. Results showed a significant increase in stream DOC concentrations during stormflows (median baseflow DOC = 2.00 mg l⁻¹ vs. median stormflow DOC = 3.46 mg l⁻¹). TDN streamwater concentrations, however, significantly decreased with stormflow (median baseflow TDN = 0.75 mg l⁻¹ vs. median stormflow TDN = 0.56 mg l⁻¹). During storms, remnant riparian areas contributed 70–74% of DOC export and 38–35% of TDN export to the stream. The observed pattern of increased DOC concentrations during stormflows in this urban watershed was similar to patterns found in previous studies of forested watersheds. Results for TDN indicated that there were relatively high baseflow nitrogen concentrations in the lower watershed that may have partially masked the remnant riparian signal during stormflows. Remnant riparian areas were a major source of DOC and TDN to the stream during storms. These results suggest the importance of preserving near-stream riparian areas in cities to maintain ambient carbon and nitrogen source contributions to urban streams.     

Do non-native earthworms in Southeast Alaska use streams as invasional corridors in watersheds harvested for timber?

Exotic earthworms from Europe and Asia have invaded previously earthworm-free areas of North America where they consume leaf litter, mix soil horizons, and alter nutrient cycling. Primarily, earthworm introductions occur through human activities; we hypothesized that the combination of logging (i.e., road construction and soil disturbance) and stream transport (i.e., hydrochory) allows earthworms to invade new ecosystems and spread within watersheds. On Prince of Wales Island, AK, we surveyed riparian zones in 11 watersheds with varying timber harvest intensity for terrestrial oligochaetes. Additionally, common invasive earthworms were experimentally submerged in a local stream to test for tolerance to prolonged immersion: all taxa survived immersion for at least 6 days. Using principal components analysis, watershed and harvest variables describing the watersheds upstream of our sampled riparian areas were reduced to two principal components describing harvest intensity (PC1) and harvest style (PC2). Logistic models successfully predicted earthworm abundance (r ²  = 0.70) from PC1, which indicated that watersheds with older, intense upstream timber harvest contained larger earthworm populations. Earthworm species richness was best predicted by PC2 (r ²  = 0.39), which suggested that earthworm communities in watersheds containing large clear-cut stands were more species-rich. Collectively, these results suggest that (1) invasive earthworms may use streams for dispersal and (2) upstream introductions via timber harvest can initiate downstream earthworm invasions. Hydrochory would allow invasive earthworms to spread at rates (tens of km d⁻¹) that are much greater than previously reported rates of terrestrial spread (5–10 m y⁻¹). Effective control of exotic earthworms in riparian zones will require watershed-level management and surveillance.     

Evaluation of nutrient retention in four restored Danish riparian wetlands

During the last 15–20 years, re-establishment of freshwater riparian wetlands and remeandering of streams and rivers have been used as a tool to mitigate nutrient load in downstream recipients in Denmark. The results obtained on monitoring four different streams and wetland restoration projects are compared with respect to hydrology, i.e. flow pattern and discharge of ground or surface water, retention of phosphorus (P), and removal of nitrogen (N). Furthermore, the monitoring strategies applied for quantifying the post-restoration nutrient retention are evaluated. The four wetland restoration projects are the Brede River restoration (including river valley groundwater flow, remeandering and inundation), Lyngbygaards River restoration (groundwater flow, irrigation with drainage water, inundation with river water and remeandering), Egeskov fen (fen re-establishment and stream remeandering) and Egebjerg Meadows (fen restoration and hydrological reconnection to Store Hansted River). Retention of phosphorus varied between 0.13 and 10 kg P ha⁻¹ year⁻¹, while the removal of nitrogen varied between 52 and 337 kg N ha⁻¹ year⁻¹. The monitoring strategy chosen was not optimal at all sites and would have benefitted from a knowledge on local hydrology and water balances in the area to be restored before planning for the final monitoring design. Furthermore, the outcome concerning P retention would have benefitted from a more frequent sampling strategy.     

Relating land cover to stream properties in southern Chilean watersheds: trade-off between geographic scale, sample size, and explicative power

Several studies relating land cover to stream properties have used sample sizes of more than 100 watersheds, but the variance that they explain is moderate to low (R ² less than 50%), limiting the predictive value of these studies when their models are applied to watersheds that were not included in the models’ development. We hypothesize that this is due to the increases in variation that occur with increases in sample size and in the geographic scales of the areas in which the watersheds are distributed. Land cover alone cannot explain all of that variation; more predictors must be considered. Conversely, models with high explicative power would require relatively small sample sizes distributed over small areas. This hypothesis was evaluated sampling 17 watersheds from southern Chile’s Lake Region, for which we developed regressive models between land cover/watershed area/precipitation/geomorphology and stream properties (i.e., conductivity, temperature). With a maximum n = 15 watersheds, on a regional scale, a poorly explained variation in hydrologic variables (mean 37–49%) was obtained. The R ² increased slightly, to 45–52%, when precipitation was included as a predictor. In half of the cases analyzed, the models improved when geomorphology was considered as an additional predictor (60–66%), supporting our hypothesis. Furthermore, when our analysis was restricted to a narrower latitudinal span (n = 9), the R ² was much stronger (68–87%) when only land cover and watershed area were included as predictors. These percentages also increased when more predictors were incorporated. Nevertheless, a portion of unexplained variance remained that would require the consideration of more predictors, such as geology and edaphology. The documented trade-off provides evidence that argues against the spatial generality of land cover/stream property models.