Nutrient uptake in streams draining agricultural catchments of the midwestern United States

1. Agriculture is a major contributor of non‐point source pollution to surface waters in the midwestern United States, resulting in eutrophication of freshwater aquatic ecosystems and development of hypoxia in the Gulf of Mexico. Agriculturally influenced streams are diverse in morphology and have variable nutrient concentrations. Understanding how nutrients are transformed and retained within agricultural streams may aid in mitigating increased nutrient export to downstream ecosystems. 2. We studied six agriculturally influenced streams in Indiana and Michigan to develop a more comprehensive understanding of the factors controlling nutrient retention and export in agricultural streams using nutrient addition and isotopic tracer studies. 3. Metrics of nutrient uptake indicated that nitrate uptake was saturated in these streams whereas ammonium and phosphorus uptake increased with higher concentrations. Phosphorus uptake was likely approaching saturation as evidenced by decreasing uptake velocities with concentration; ammonium uptake velocity also declined with concentration, though not significantly. 4. Higher whole‐stream uptake rates of phosphorus and ammonium were associated with the observed presence of stream autotrophs (e.g. algae and macrophytes). However, there was no significant relationship between measures of nutrient uptake and stream metabolism. Water‐column nutrient concentrations were positively correlated with gross primary production but not community respiration. 5. Overall, nutrient uptake and metabolism were affected by nutrient concentrations in these agriculturally influenced streams. Biological uptake of ammonium and phosphorus was not saturated, although nitrate uptake did appear to be saturated in these ecosystems. Biological activity in agriculturally influenced streams is higher relative to more pristine streams and this increased biological activity likely influences nutrient retention and transport to downstream ecosystems.     

Stream ecosystem properties and processes along a temperature gradient

We surveyed macrophyte community structure and measured community metabolism and nutrient uptake along a temperature gradient (9.7–17.4°C) in four Icelandic streams influenced by geothermal heating. The study streams are part of the geothermal area in Hengill that is uniquely characterised by streams with comparable water chemistry despite the geothermal influence. Stream metabolism was studied applying the diurnal upstream–downstream dissolved oxygen change technique. Nutrient uptake was studied by adding solutions of nitrogen and phosphorus together with a conservative tracer. Rates of primary production (GPP) and uptake of nitrate–N and phosphate-P increased with increasing stream temperature. GPP was 20 times higher (up to 12.99 g O₂ m⁻² day⁻¹) and rates of nutrient uptake were up to 30-times higher (up to 22.99, 13.31 and 7.94 mg m⁻² h⁻¹ for ammonium, nitrate and phosphate, respectively) in the warmest streams compared with the coldest. Furthermore, macrophytes, when present, were strongly controlling ecosystem processes. Our study implies that temperature may affect stream ecosystem processes both directly (i.e. physiologically) and indirectly (i.e. by changing other structural parameters).     

Seasonality of macrophytes and interaction with flow in a New Zealand lowland stream

Introduced submerged macrophytes have come to dominate many shallow water bodies in New Zealand, and are a common component of many lowland streams. We investigated the seasonal variation of macrophyte abundance, its influence on flow and channel volume, and the implications of this on stream habitat and functioning in Whakapipi Stream, a typical lowland stream draining a predominantly agricultural catchment.Abundance of macrophytes over the summer was primarily controlled by the phenological cycles of the two dominant species. Mean minimum total macrophyte biomass (36 g m^–2) and cover (7%) occurred in winter (June and August, respectively), and mean maximum biomass (324 g m^–2), and cover (79%) occurred in late summer (March and February respectively). Egeria densa comprised the majority of both cover and biomass during the study period, except early summer (December) when Potamogeton crispus was prevalent in the shallow stream reaches.Macrophyte beds had a major impact on summer stream velocities, reducing average velocities by an estimated 41%. Stream cross-sectional area was maintained at relatively stable levels similar to that recorded over winter, when stream discharge was in the order of seven times greater. The mean velocity distribution coefficient (), and Manning's roughness coefficient (n) were dependent on and displayed a positive linear relationship with macrophyte abundance. The velocity distribution coefficient is recommended as a better indicator of macrophyte effects on velocity in natural streams, as it does not assume uniform velocity, channel depth and slope within the stream reach.Our study shows that submerged macrophytes play an important structuring role within the stream during the summer period, where macrophyte beds act as semi-permeable dams, retarding flow velocities and increasing stream depth and cross-sectional area. This promotes habitat heterogeneity by creating a greater range of flow velocity variation, and also provides large stable low-flow areas. Other likely ecosystem effects resulting from macrophyte/velocity interactions include increased sedimentation, potential for nutrient processing and increased primary production, both by macrophytes and attached epiphyton. The complex architecture of submerged macrophytes and their influence on stream flow may also provide an increased diversity of habitat for other aquatic biota. We propose that management of degraded lowland streams such as the Whakapipi Stream to maintain stretches with moderate quantities of submerged macrophytes interspersed with shaded areas would optimise stream health during low summer flows.     

Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream

1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow-channel system. Water was supplied from a nutrient-rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams.
2. Four submerged macrophyte species were tested, Elodea canadensis , Callitriche cophocarpa , Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de-rooted plants.
3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa , indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue-P concentration in response to open-water enrichment.
4. The open-water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams.     

Nitrogen cycling and dynamics in a macrophyte‐rich stream as determined by a release

1. A tracer release study was conducted in a macrophyte‐rich stream, the River Lilleaa in Denmark. The objectives of the study were to compare uptake rates per unit area of by primary producers and consumers in macrophyte and non‐macrophyte habitats, estimate whole‐stream uptake rates of and compare this to other stream types, and identify the pathways and estimate the rate at which enters the food web in macrophyte and non‐macrophyte habitats. 2. Macrophyte habitats had four times higher primary uptake rates and an equal uptake rate by primary consumers per unit habitat area as compared to non‐macrophyte habitats. These rates represent the lower limit of potential macrophyte effects because the rates will be highly dependent on macrophyte bed height and mean bed height in the River Lilleaa was low compared to typical bed heights in many lowland streams. Epiphytes accounted for 30% of primary uptake in macrophyte habitats, illustrating a strong indirect effect of macrophytes as habitat for epiphytes. N flux per unit habitat area from primary uptake compartments to primary consumers was four times lower in macrophyte habitats compared to non‐macrophyte habitats, reflecting much greater biomass accrual in macrophyte habitats. Thus, we did not find higher N flux from macrophyte habitats to primary consumers compared to non‐macrophyte habitats. 3. Whole‐stream uptake rate was 447 mgN m^?2 day^?1. On a habitat‐weighted basis, fine benthic organic matter (FBOM) accounted for 72% of the whole‐stream uptake rate, and macrophytes and epiphytes accounted for 19 and 8%, respectively. 4. We had expected a priori relatively high whole‐stream N uptake in our study stream compared to other stream types mainly due to generally high biomass and the macrophyte’s role as habitat for autotrophic and heterotrophic organisms, but our results did not confirm this. In comparison with other release study streams, we conclude that nutrient concentration is the overall controlling factor for N uptake rates across streams, mostly as a result of high biomass of primary uptake compartments in streams with high nutrient concentrations in general and not in macrophyte streams in particular. 5. Our results indicate that macrophytes play an important role in the longer‐term retention of N and thus a decrease in net downstream transport during the growing season compared to streams without macrophytes, through direct and indirect effects on the stream reach. Direct effects are high uptake efficiency, low turnover rate (partly due to no direct feeding on macrophytes) and high longevity. An indirect effect is increased sedimentation of FBOM in macrophytes compared to non‐macrophyte habitats and streams which possibly also increase denitrification. Increased retention with macrophyte presence would decrease downstream transport during the growing season and thus the N loading on downstream ecosystems.     

Effects of aquatic plant management on stream metabolism and oxygen balance in streams

1. In unshaded, nutrient-rich streams, prolific growth of stream macrophytes often results in flows that over-top the banks and in high primary production and respiration that may result in extreme diel variations in dissolved oxygen. Consequently, water protection authorities commonly remove macrophytes periodically.
2. We investigated the effect of plant removal on stream metabolism and oxygen balance in two Swiss streams with a high macrophyte biomass. We monitored the concentration of dissolved oxygen before and after macrophytes were removed by cutting and dredging, and calculated rates of gross primary production and ecosystem respiration by means of diel oxygen curves.
3. The removal of plants, which had reached a dry biomass of 320–420 g m−2 immediately before plant removal, had a different impact on stream metabolism in the two streams. In the first (plants removed in May), neither primary production nor ecosystem respiration were significantly affected. In the second (plants removed in late July), gross primary production and ecosystem respiration were reduced by about 70%. In this latter stream gross primary production increased in the first 2 weeks after plant removal but never recovered to pre-disturbance levels.
4. The removal of plants coincided with only a moderate increase in nocturnal oxygen concentration (+1 mg L−1). This, and the rapid partial recovery of stream metabolism in the second stream, suggests that an increase in the oxygen concentration after plant cutting is transient in unshaded, nutrient-rich streams.     

Riverine macrophytes control seasonal nutrient uptake via both physical and biological pathways

1. In low-gradient, macrophyte-rich rivers, we expect that the significant change in macrophyte biomass among seasons will strongly influence both biological activity and hydraulic conditions resulting in significant effects on nutrient dynamics. Understanding seasonal variation will improve modelling of nutrient transport in river networks, including annual estimations of export, which could optimise decision-making and management outcomes.
2. We explored the relationships among seasonal differences in reach-scale nutrient uptake, macrophyte abundance, solute transport and transient storage in the River Gudenå (Denmark), a large macrophyte-rich river. We used the minimal pulse addition technique to measure uptake of ammonium, nitrate, soluble reactive phosphorus, as well as reach-scale metabolism, and surface transient storage in spring, summer, and autumn.
3. We found that riverine uptake changed among seasons and was linked to macrophyte biomass via both biological activity, reflected in reach-scale metabolism, and through physical processes, as solute transport was influenced by longitudinal dispersion. In this macrophyte-rich river, seasonal changes in macrophyte biomass affected contact time between the water and biota, which influenced ammonium and soluble reactive phosphorus uptake. Using stoichiometric scaling of reach-scale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake.
4. In summary, riverine nutrient uptake was not static, highlighting the importance of seasonality, with significant implications for modelling of nutrient export in river networks. Moreover, current management strategies that remove macrophyte biomass (i.e. weed cutting and dredging) will short-circuit the positive effects of enhanced nutrient uptake resulting from abundant macrophytes in rivers.

     

Controlled release experiments to determine the effects of shade and plants on nutrient retention in a lowland stream

Understanding nutrient uptake and retention in streams remains an important challenge for lotic scientists. In this study a series of pulse and continuous releases of dissolved nutrients were made to shaded and unshaded (reference) reaches of a small lowland stream to determine whether suppression of macrophyte growth by riparian shade impaired nutrient retention. The nutrients were dissolved reactive phosphorus (DRP), total ammoniacal nitrogen (NH₄–N) and nitrate nitrogen (NO₃–N). Nutrient reductions ranged from 100% of DRP when stream water was anoxic, to 5–10% for NH₄–N and NO₃–N in the reference reach. Nutrient removals were affected by travel times in each reach. Percentage removals of NH₄–N (46 ± 10) and NO₃–N (52 ± 14) were higher in the shaded reach than in the swifter moving reference reach (15 ± 8 and 16 ± 10, respectively). DRP (%) removals were 75± 7 and 57 ± 12 for the shaded and reference reaches, respectively. The presence of emergent marginal macrophytes (Persicaria hydropiper) increased stream velocity in the reference reach by reducing the effective channel cross-section area. Shading reduced plant biomass, increased the channel cross-section and lowered velocity in the experimental reach, effecting dramatic reductions in nutrient concentrations over short distances. The opposite effect is more typical for larger, swifter streams having dense stands of submerged macrophytes, where lowering channel plant biomass will cause increased velocities and lower relative nutrient losses. Riparian shade does not necessarily impair nutrient uptake from small streams. Where invasive marginal species such as P. hydropiper dominate headwater streams shade may be beneficial to the protection of downstream waters from eutrophication. Where reduction of nutrient fluxes from small streams is a key objective for protection of downstream waters, active management of streams should seek to increase travel times, allowing greater potential for nutrient uptake. This will need to be weighed against the need for effective drainage in pastoral areas where reduced travel times are usually sought.     

Factors influencing biomass and nutrient content of the submersed macrophyte Egeria densa Planch. in a pampasic stream

We identified factors influencing biomass and nutrient content in E. densa in an enriched pampean stream of Argentina. Physical (current velocity, temperature), chemical (pH, conductivity, dissolved oxygen, nutrient content in water and sediments), and biological variables (biomass and nutrient content of E. densa, biomass of periphyton and other macrophytes) were estimated at each sampling occasion, and mean monthly values estimated. Biomass and nutrient content in E. densa were correlated with these physical-chemical and biological variables. Biomass was positively correlated with ammonium in stream water (P<0.05) and sediment total nitrogen (P<0.01). Nitrogen showed a positive relationship with ammonium (P<0.01), and a negative one with nitrate and periphyton biomass (P<0.05). Phosphorus was positively correlated with soluble reactive phosphorus (P<0.01). The growth of other macrophyte species in the stream seemed to influence E. densa biomass, probably through competition for light. Current velocity was low and not significantly related with E. densa biomass, however, a flood at the beginning of the study washed the macrophyte stand downstream.     

Primary producers in a Pampean stream: temporal variation and structuring role

Low current velocities, high nutrient levels, the lack of riparian forest vegetation, and the development of dense and rich macrophyte communities characterize Pampean streams. The objective of this study was to describe the main physical, chemical, and biological characteristics of a headwater Pampean stream as well as to analyze the role of macrophytes and phytobenthos. The study was conducted in a stream considered to be not much disturbed by human activities. Samples of water and organisms (macrophytes, benthic algae and invertebrates) were taken monthly for 14 months in two sampling stations, in fast flow and slow flow sites. Macrophyte biomass and diversity increased in spring and summer, and they decreased in autumn, when the plant community was greatly affected by an important flood. Phytobenthos biomass was lower in late summer, possibly due to the establishment of a dense cover of the floating macrophyte Lemna gibba L. Density of amphipods and gastropods greatly increases in spring and summer, jointly with the macrophyte development. Analysis of correlation showed that current velocity is the most important factor influencing macrophyte biomass and phytobenthos structure, while depth, nutrients, and herbivores are linked factors. Pampean streams could be considered systems dynamically fragile, because habitat heterogeneity is generated by aquatic vegetation, a substratum that varies along time.     

The effects of storms,heavy rain,and sedimentation on the shallow coral reefs of St. John,US Virgin Islands

Efforts to limit plant growth in streams by reducing nutrients would benefit from an understanding of the relative influences of nutrients, streamflow, light, and other potentially important factors. We measured macrophytes, benthic algae, nutrients in water and sediment, discharge, and shading from 30 spring-fed or runoff-influenced streams in the upper Snake River basin, ID, USA. We hypothesized that in hydrologically stable, spring-fed streams with clear water, macrophyte and benthic algae biomass would be a function of bioavailable nutrients in water or sediments, whereas in hydrologically dynamic, runoff-influenced streams, macrophyte and benthic algae biomass would further be constrained by flow disturbance and light. These hypotheses were only partly supported. Nitrogen, both in sediment and water, was positively correlated with macrophyte biomass, as was loosely sorbed phosphorus (P) in sediment. However, P in water was not. Factors other than nutrient enrichment had the strongest influences on macrophyte species composition. Benthic algal biomass was positively correlated with loosely sorbed sediment P, lack of shade, antecedent water temperatures, and bicarbonate. These findings support the measurement of bioavailable P fractions in sediment and flow histories in streams, but caution against relying on macrophyte species composition or P in water in nutrient management strategies for macrophytes in streams.     

Macrophytes in Urban Stream Rehabilitation: Establishment, Ecological Effects, and Public Perception

Efforts to rehabilitate degraded urban streams generally focus on improving physical habitat and rarely include reestablishing biota such as macrophytes. Our objectives in this study were to propagate and transplant native macrophytes into a South Island, New Zealand, urban stream undergoing rehabilitation, assess macrophyte survival and growth, and determine whether native macrophytes suppress non-native macrophytes and/or enhance stream invertebrate communities. Effects of native macrophytes on invertebrates and non-native macrophytes were assessed after transplanting patches of native macrophytes into a 230-m-long stream section. A 100-m-long section upstream was left unplanted for subsequent comparisons. Following the study, a survey was conducted to gauge public opinion about the rehabilitation project and determine whether macrophytes were prominent in perceptions of stream health. In the first growing season, native macrophyte cover in the planted stream section increased from 1.5 to 20%, and then decreased during winter. Regrowth from rhizomes led to rapid aboveground growth during the second year, when cover reached 51%. Non-native macrophytes colonized the stream the first year, but native macrophytes appeared to limit the spread of non-natives, which were absent in the planted section by the second spring. Native macrophyte establishment did not enhance invertebrate communities as predicted; few invertebrate metrics differed significantly between the planted and unplanted sections. Pollution- and sediment-tolerant invertebrate taxa were abundant in both sections, suggesting that invertebrate colonization was limited by water quality or sedimentation, not macrophyte composition. Survey respondents considered the stream to be visually and ecologically improved after rehabilitation, and macrophyte establishment was generally considered positive or neutral.     

Effects of wood removal on stream habitat and nitrate uptake in two northeastern US headwater streams

Forested headwater streams play an important role in watershed nutrient dynamics, and wood is thought to be a key factor influencing habitat structure and nitrate-nitrogen dynamics in many forested streams. Because wood in streams can promote nitrogen uptake through denitrification, we hypothesized that nitrate uptake velocities would decrease following wood removal. We measured stream characteristics and nitrate uptake velocities before and after wood manipulation experiments conducted at Hubbard Brook Experimental Forest, NH, and the Sleepers River watershed, VT. The mean size of stream substrates and the amount of riffle habitat increased following wood removal. In contrast to our expectations, summer nitrate uptake velocities increased in the wood removal treatments relative to the reference treatments, possibly because wood removal increased the availability of stable substrates for periphyton growth, therefore increasing nitrate demand in these streams. Our results highlight that effects of wood on stream ecosystems occur through multiple pathways and suggest that the relative importance of these pathways may vary seasonally.     

Macrophyte diversity and composition in relation to substratum characteristics in regulated and unregulated Danish streams

1. The objective of the present study was to examine how the physical stream environment in regulated and unregulated lowland streams affects the diversity and distribution of macrophyte communities. We analysed the abundance, distribution and composition of macrophytes, together with physical parameters, in seven regulated and seven unregulated unshaded Danish stream reaches. 2. Total macrophyte coverage was similar in the regulated and unregulated streams, but species richness and Shannon diversity were higher in the unregulated streams. Overall, we found fifty-two different species in the regulated stream reaches and sixty-two in the unregulated stream reaches. The spatial distribution of macrophytes on the stream bottom was more heterogeneous in the unregulated streams. 3. We found positive correlations between the coverage and diversity of macrophytes and the coverage of coarse-textured substratum types on the stream bottom, as well as between macrophyte coverage and diversity and substratum heterogeneity. We also found that the macrophytes were more heterogeneously distributed where substratum heterogeneity was greater. 4. The species growing both submerged and emergent were more abundant in the regulated streams, whereas species growing only submerged were more abundant in the unregulated streams. Species growing submerged, species growing both submerged and emergent, and species only growing emergent segregated differently in a canonical correspondence analysis ordination. The submerged species were primarily associated with coarser-textured substrata, whereas species growing both submerged and emergent, and species growing only emergent were associated with finer-textured substrata. 5. The most abundant species in the regulated streams, Sparganium emersum, accounting for almost one-third of the total macrophyte coverage, was primarily associated with clay and sandy bottom substrata, whereas the most abundant species in the unregulated streams, Batrachium peltatum, was primarily associated with gravel and stony substrata.     

Patch- and reach-scale dynamics of a macrophyte-invertebrate system in a New Zealand lowland stream

Abundant growths of macrophytes are a common feature of streams in open lowland areas of New Zealand during summer, but the values of these to aquatic biota are poorly understood. We studied the temporal dynamics of, and associations amongst, elements of a macrophyte-invertebrate system to provide an improved information base for lowland stream management. The biomass of macrophytes increased significantly over the four quarterly sampling occasions from 43.8 g m^-2 in June to 370.8 g m^-2 in March; biomass was dominated by Egeria densa on all dates, except in December when Potamogeton crispus was dominant. We did not detect strong associations between epiphyton biomass and invertebrate abundance in our study, but this may reflect the fact that we sampled loosely-adhering epiphyton on young, surface-reaching shoots whereas invertebrates were collected from macrophytes growing through the water column. Density of some invertebrate species per gram dry weight of plant material varied by macrophyte type, with the chironomids Tanytarsus vespertinus and Naonella forsythi displaying positive correlations with Egeria and Potamogeton biomass, respectively. The shrimp Paratya curvirostris accounted for 50% of phytophilous invertebrate biomass, with Chironomidae the only other group to comprise more than 9%. Abundance of total phytophilous invertebrates displayed a positive linear relationship with macrophyte biomass in a sample (0.1 m²), and a humped relationship with species richness, such that highest numbers of taxa occurred at macrophyte biomass levels around 400 g dw m^-2. Our study suggests that intermediate macrophyte biomass levels are likely to enhance macroinvertebrate biodiversity in sandy-bottomed lowland streams. This revised version was published online in July 2006 with corrections to the Cover Date.     

Macrophyte growth and sediment phosphorus and nitrogen in a Canadian prairie river

1. The role of sediment phosphorus (P) and nitrogen (N) in regulating growth of rooted macrophytes in a Canadian prairie river was investigated by means of in situ observations and artificial stream fertilization experiments.
2. Biomass and percentage cover of rooted macrophytes in the South Saskatchewan River increased downstream of a municipal sewage treatment plant, with maximum abundance occurring between 25 and 100 km downstream of the outfall. Biomass in the river was related to sediment P but not N concentration, although sites of maximum biomass did not coincide with sites of maximum sediment P concentration.
3. Artificial stream experiments revealed that while biomass was unaffected by addition of N to the sediment, it was enhanced by the addition of P to sediments, and further enhanced with the addition of N and P together, indicating a primarily P‐limited system, with secondary N limitation when P is in excess.
4. Macrophyte biomass increased linearly with increasing sediment P concentration in the artificial streams, and tissue P concentration peaked at ≈ 400 μg g^–1. Biomass did not respond to increasing sediment N concentration, and only a weak relationship was observed between tissue N and sediment N, with maximum tissue N corresponding to ≈ 140 μg g^–1 sediment exchangeable N.
5. A lack of concurrence between the sediment P concentration producing maximum biomass and tissue P concentration in situ vs. under experimental conditions indicates that other environmental factors have an important role in regulating macrophyte growth in rivers. Thus, while nutrient control may be one element in a river macrophyte control programme, a holistic ecosystem approach should be adopted to account for the other factors that may affect the growth of rooted plants.     

Inter‐regional comparison of land‐use effects on stream metabolism

1. Rates of whole‐system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first‐order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban‐influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land‐use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land‐use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land‐use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land‐use categories showed reduced inter‐regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.     

Using Macrophytes in Urban Stream Rehabilitation: A Cautionary Tale

Native macrophytes were transplanted into a small urban stream as part of a rehabilitation program, that also meandered the previously channeled stream, naturalized stream banks, and planted native riparian vegetation. Transplanted macrophytes minimized spread of introduced macrophytes and were viewed beneficially by residents, as was the stream rehabilitation. We transplanted the native macrophyte Myriophyllum triphyllum into five larger streams dominated by exotic macrophytes—some of which were weeded prior to transplanting—to see whether Myriophyllum could prevent regrowth of weeded plants. Transplanted Myriophyllum plants were washed away in two streams, reflecting high shear stresses there. Myriophyllum cover in the other streams decreased as weeded plants regrew. Our attempt at eliminating exotic macrophytes in patches in large streams was unsuccessful. Furthermore, council authorities weeded other experimental sections following complaints from residents of excess macrophyte growth. This problem highlighted conflicting multiple values placed on urban streams by managers and the public. A repeat survey of residents living near the original rehabilitated stream showed that many respondents were now critical of excessive plant growth—both in‐stream and riparian. A recurring comment made concerned the apparent lack of maintenance to the stream, giving it an untidy appearance. Difficulties with propagating and transplanting native macrophytes into larger streams, coupled with a negative perception of native vegetation (both in‐stream and riparian) if it looks unmanaged, suggest that planting macrophytes or riparian plants as part of urban stream rehabilitation programs may be more problematic than realized.     

Assessment of European streams with diatoms, macrophytes, macroinvertebrates and fish: a comparative metric-based analysis of organism response to stress

1. Periphytic diatoms, macrophytes, benthic macroinvertebrates and fish were sampled with standard methods in 185 streams in nine European countries to compare their response to degradation. Streams were classified into two main stream type groups (i.e. lowland, mountain streams); in addition, the lowland streams were grouped into four more specific stream types. 2. Principal components analysis with altogether 43 environmental parameters was used to construct complex stressor gradients for physical–chemical, hydromorphological and land use data. About 30 metrics were calculated for each sample and organism group. Metric responses to different stress types were analysed by Spearman Rank Correlation. 3. All four organism groups showed significant response to eutrophication/organic pollution gradients. Generally, diatom metrics were most strongly correlated to eutrophication gradients (85% and 89% of the diatom metrics tested correlated significantly in mountain and lowland streams, respectively), followed by invertebrate metrics (91% and 59%). 4. Responses of the four organism groups to other gradients were less strong; all organism groups responded to varying degrees to land use changes, hydromorphological degradation on the microhabitat scale and general degradation gradients, while the response to hydromorphological gradients on the reach scale was mainly limited to benthic macroinvertebrates (50% and 44% of the metrics tested correlated significantly in mountain and lowland streams, respectively) and fish (29% and 47%). 5. Fish and macrophyte metrics generally showed a poor response to degradation gradients in mountain streams and a strong response in lowland streams. 6. General recommendations on European bioassessment of streams were derived from the results.     

Macrophyte communities in unimpacted European streams: variability in assemblage patterns, abundance and diversity

Macrophytes are an important component of aquatic ecosystems and are used widely within the Water Framework Directive (WFD) to establish ecological quality. In the present paper we investigated macrophyte community structure, i.e., composition, richness and diversity measures in 60 unimpacted stream and river sites throughout Europe. The objectives were to describe assemblage patterns in different types of streams and to assess the variability in various structural and ecological metrics within these types to provide a basis for an evaluation of their suitability in ecological quality assessment. Macrophyte assemblage patterns varied considerably among the main stream types. Moving from small-sized, shallow mountain streams to medium-sized, lowland streams there was a clear transition in species richness, diversity and community structure. There was especially a shift from a predominance of species-poor mosses and communities dominated by liverwort in the small-sized, shallow mountain streams to more species-rich communities dominated by vascular plants in the medium-sized, lowland streams. The macrophyte communities responded to most of the features underlying the typological framework defined in WFD. The present interpretation of the WFD typology may not, however, be adequate for an evaluation of stream quality based on macrophytes. First and most important, by using this typology we may overlook an important community type, which is characteristic of small-sized, relatively steep-gradient streams that are an intermediate type between the small-sized, shallow mountain streams and the medium-sized, lowland streams. Second, the variability in most of the calculated metrics was slightly higher when using the pre-defined typology. The consistency of these results should be investigated by analysing a larger number of sites. Particularly the need of re-defining the typology to improve the ability to detect impacts on streams and rivers from macrophyte assemblage patterns should be investigated. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.