Freshwater Biology

SUMMARY 1. Transfer of carbon from freshwater to terrestrial ecosystems can occur through predation on adult aquatic insects, but the significance of this trophic pathway to the energetics of riparian communities is poorly understood. We used stable isotopes of carbon and nitrogen to explore linkages between aquatic insect production and the nutrition of web‐building and free‐living spiders alongside two streams in the North Island of New Zealand.

2. δ¹³C values for riparian tree leaves (means for each site = −32.2 and −30.3‰) were distinct from those of lichens collected from stream channel rocks and instream algae, both of which were similar (−23.4 to −22.4‰). δ¹⁵N values for leaves were similar at both sites (−3.4 and −2.7‰), but algae were considerably more depleted in δ¹⁵N atonesite suggesting significant differences in instream nitrogen sources between the twostreams.

3. Isotope values for potential aquatic prey of spiders indicated that aquatic algal production was their primary carbon source at both sites. Terrestrial invertebrates collected and assumed to be potential prey reflected a range of carbon sources and represented several trophic levels.

4. At one site, δ¹³C values indicated a primarily algae‐aquatic insect pathway of carbon transfer to both web‐building and free‐living spider guilds. The other site appeared to have a primarily terrestrial carbon pathway for the free‐living spider guild, and a mixed aquatic‐terrestrial pathway for the web‐building guild.

5. Overall, web‐building spiders were estimated to obtain around 61% of their body carbon from aquatic production compared with 55% for free‐living spiders. Our findings suggest that consumption of prey derived from aquatic sources can provide significant nutrition for spiders living along some stream channels. This pathway may represent an important feedback mechanism contributing to the energetics of riparian communities at sites where aquatic insect production is high.

SUMMARY. 1. Habitat utilization, as well as inter‐ and intraspecific relations of different size groups of arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjø, south‐east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June‐October 1985.

2. Small individuals (150–230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5–15 m and brown trout at depths of 0–5 m.

3. Larger (>230 mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2‐3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0–12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspecific competition for food.

4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton, whereas brown trout had a more variable diet, consisting of surface insects, zooplankton. aquatic insects and fish.

5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high social position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewarding part of the benthic habitat.

1. Epilimnetic alkaline phosphatase activity (APA) was measured in longitudinal profiles of the canyon‐shaped, eutrophic Sau Reservoir (Catalonia, Spain) during the autumn, winter and spring periods of 1997–2000.

2. The spatial pattern of APA depended on lake circulation. During periods of stable stratification, when the ratio of mixed to euphotic depth (z_mix/z_eu < 1.7) was low and the Chlorophyll a (Chl a) concentration high, APA was also comparatively high (0.5–3.4 μmol L⁻¹ h⁻¹) and located mostly in the >2‐μm size‐fraction. APA increased towards the dam at the same time as the concentration of soluble reactive phosphorus (SRP) decreased. In periods of unstable stratification, deep mixing (z_mix/z_eu > 2.4) and low Chl a concentration, APA was low (<0.1 μmol L⁻¹ h⁻¹) and without longitudinal changes, consistent with a high and stable SRP concentration.

3. A high input of mostly dissolved (in the <0.2‐μm size‐fraction) phosphatases from the river Ter was found in 1997–98. At the river inflow, independently of season and despite a continuously high SRP concentration, APA was approximately 0.7 μmol L⁻¹ h⁻¹ and decreased towards the dam within the inflowing, canyon‐like part of the reservoir.

4. Analysis of saturation kinetics revealed the kinetic heterogeneity of APA. Low‐affinity APA was localised in the >2‐μm (algal) size‐fraction while, in the <2‐μm (picoplankton, mainly bacteria + dissolved) size‐fraction, high‐affinity APA, or a mixture of both, was found. The presence of two kinetic components, whose substrate affinities (i.e. in Michaelis constants, K_m) differed, was confirmed statistically in 13 of 18 cases analysed. The range of K_mH values of the high‐affinity component was 0.15–2.4 μmol L⁻¹, K_mL values of the low‐affinity component ranged from 18 to 275 μmol L⁻¹.

1. The flow regime is a primary determinant of the structure and function of aquatic and riparian ecosystems for streams and rivers. Hydrologic alteration has impaired riverine ecosystems on a global scale, and the pace and intensity of human development greatly exceeds the ability of scientists to assess the effects on a river‐by‐river basis. Current scientific understanding of hydrologic controls on riverine ecosystems and experience gained from individual river studies support development of environmental flow standards at the regional scale.

2. This paper presents a consensus view from a group of international scientists on a new framework for assessing environmental flow needs for many streams and rivers simultaneously to foster development and implementation of environmental flow standards at the regional scale. This framework, the ecological limits of hydrologic alteration (ELOHA), is a synthesis of a number of existing hydrologic techniques and environmental flow methods that are currently being used to various degrees and that can support comprehensive regional flow management. The flexible approach allows scientists, water‐resource managers and stakeholders to analyse and synthesise available scientific information into ecologically based and socially acceptable goals and standards for management of environmental flows.

3. The ELOHA framework includes the synthesis of existing hydrologic and ecological databases from many rivers within a user‐defined region to develop scientifically defensible and empirically testable relationships between flow alteration and ecological responses. These relationships serve as the basis for the societally driven process of developing regional flow standards. This is to be achieved by first using hydrologic modelling to build a ‘hydrologic foundation’ of baseline and current hydrographs for stream and river segments throughout the region. Second, using a set of ecologically relevant flow variables, river segments within the region are classified into a few distinctive flow regime types that are expected to have different ecological characteristics. These river types can be further subclassified according to important geomorphic features that define hydraulic habitat features. Third, the deviation of current‐condition flows from baseline‐condition flow is determined. Fourth, flow alteration–ecological response relationships are developed for each river type, based on a combination of existing hydroecological literature, expert knowledge and field studies across gradients of hydrologic alteration.

4. Scientific uncertainty will exist in the flow alteration–ecological response relationships, in part because of the confounding of hydrologic alteration with other important environmental determinants of river ecosystem condition (e.g. temperature). Application of the ELOHA framework should therefore occur in a consensus context where stakeholders and decision‐makers explicitly evaluate acceptable risk as a balance between the perceived value of the ecological goals, the economic costs involved and the scientific uncertainties in functional relationships between ecological responses and flow alteration.

5. The ELOHA framework also should proceed in an adaptive management context, where collection of monitoring data or targeted field sampling data allows for testing of the proposed flow alteration–ecological response relationships. This empirical validation process allows for a fine‐tuning of environmental flow management targets. The ELOHA framework can be used both to guide basic research in hydroecology and to further implementation of more comprehensive environmental flow management of freshwater sustainability on a global scale.

1. The use of species traits to interpret biological changes in invertebrate assemblages across environmental gradients has been suggested as a method to improve discrimination over existing species composition methods. One reason for greater potential discrimination and predictive ability is the assumed universality of traits across a range of spatial and temporal scales. We explore this assumption by comparing the consistency of stressor–response relationships of a trait characteristic (percent clinging taxa) and a common taxonomy‐based metric [percent Ephemeroptera, Plecoptera and Trichoptera (EPT)] over a stressor gradient of increasing benthic fine sediment.

2. We use invertebrate assemblage and environmental data from three large‐scale surveys that cover the western United States, eastern United States and the Mid‐Atlantic Highlands of the US. These three datasets allow us to compare stressor–response relationships in terms of geographic position (west versus east) and spatial scale (entire east versus a sub‐region of the east). We compare the slopes and intercepts of the two measures of assemblage response.

3. Trait characteristics exhibited more consistent stressor–response relationships than identity characteristics. Stressor–response relationships generated for clinging invertebrate richness had statistically similar slopes over sediment gradients, regardless of spatial scale or geographic location. In contrast, slopes were significantly different for relationships generated with EPT richness over sediment gradients.

4. Results of this study support the hypothesis that trait‐based measures have a more consistent response to a stressor gradient than identity‐based assemblage measures. Choosing consistent measures of community response will facilitate comparisons among assemblages across large spatial scales.

1. The spatial distribution of macroinvertebrate species was examined in relation to hydraulic and sedimentary conditions in a large gravel‐bed river, the Fraser River, Canada. Mean annual discharge in the Fraser River is 2900 m3 s−1 and annual flood discharge, due to snowmelt in May and June, averages 8760 m3 s−1. 
2. Invertebrates were sampled from four water depths (0.2, 0.5, 1.5, 3.0 m) at various levels of discharge that together captured the spatial and temporal variability of the physical habitat. Several hydraulic (near‐bed shear velocity, Boundary Reynolds number, turbulence intensity, depth‐averaged velocity, Froude number, Reynolds number) and substratum variables (mean grain size, Trask's sorting coefficient, Nikuradse's roughness, percentage of fine sediment, and Shields entrainment function) were measured for each sample of macroinvertebrates. Concentrations of fine and coarse particulate organic matter were also assessed. 
3. The physical habitat was characterized by a major gradient of hydraulic conditions that corresponded positively with increasing water depth and accounted for 52% of the total variation in the habitat data. Substratum conditions and the concentration of organic matter explained 24% of the total variation in the habitat data. 
4. The distribution of invertebrates was correlated significantly with hydraulic variables and suggests that hydraulic conditions represent a major physical gradient along which the benthic community is organized. The distribution of organic matter and substratum texture were also important for some species. The spatial distribution of most species reflected morphological and trophic suitability to particular habitat conditions. 
5. Hydraulic stress associated with foraging and maintaining position, as well as organic matter retention in coarse substrata, are probable mechanisms affecting the spatial distribution of macroinvertebrates.

1. The main objectives of our study were (i) to establish the relative impact of pressures that degrade ecological status, especially those caused by agriculture and urbanisation, (ii) to identify regional patterns in these pressure‐impact relationships and (iii) to evaluate the relative weight of the pressures acting at the basin and riparian corridor scales, and the possible buffering effect of riparian areas.

2. We developed large‐scale models linking invertebrate indices of ecological quality to river basin and riparian land cover in France, Slovakia, Estonia and U.K. Invertebrate indices, transformed to ecological quality ratios (EQR), were taken from national monitoring networks. We based the models on partial least squares (PLS) regressions at national and a hydro‐ecoregion (HER) scales. The HERs provided a framework for grouping data in terms of natural river features and human activities.

3. The different national methods provided consistent results that indicated the hierarchy of pressures impacting river invertebrates at the European scale. Artificial land cover (e.g. urban and industrial sites) in the river basin represented the pressure with the most negative impact on invertebrate indices, in all countries and regions.

4. The impact of agricultural land cover was more variable. Arable land had a smaller impact than urban areas and was insignificant in some models. The proportion of arable land in the river basin appeared to be a weak predictor of agricultural impacts by itself; the type of cultivation and intensity as well as the proximity to the river must be taken into account.

5. At the riparian corridor scale, the negative impact of artificial areas or arable land and the positive effects of forests and pastures were demonstrated in many regions. The protective effect of riparian forests against mixed agricultural and urban pressures was demonstrated in three regions in France. Riparian corridors appear to be manageable areas, and these results strongly support the idea of including their restoration in priority actions for achieving good ecological status.

1. We analysed a large number of concurrent samples of macroinvertebrate communities and chemical indicators of eutrophication and organic pollution [total‐P, total‐N, NH₄–N, biological oxygen demand (BOD₅)] from 594 Danish stream sites. Samples were taken over an 11‐year time span as part of the Danish monitoring programme on the aquatic environment. Macroinvertebrate communities were sampled in spring using a standardised kick‐sampling procedure whereas chemical variables were sampled six to 24 times per year per site. Habitat variables were assessed once when macroinvertebrates were sampled.

2. The plecopteran Leuctra showed a significant negative exponential relationship (r² = 0.90) with BOD₅ and occurred at only 16% of the sites with BOD₅ above 1.6 mg L⁻¹. Sharp declines with increasing BOD₅ levels were found for the trichopteran families Sericostomatidae and Glossosomatidae although they appeared to be slightly less sensitive than Leuctra. Other plecopterans such as Isoperla showed a similar type of response curve to Leuctra (negative exponential) but occurred at sites with relatively high concentrations of BOD₅ up to 3–4 mg L⁻¹. In contrast, the response curve of the isopod Asellus aquaticus followed a saturation function reaching a plateau above 3–4 mg L⁻¹ BOD₅ and the dipteran Chironomus showed an exponential increase in occurrence with increasing BOD₅ concentration.

3. Macroinvertebrate occurrence appeared to be related primarily to concentrations of BOD₅, NH₄–N and total‐P whereas there were almost no relationships to total‐N. Occurrence of a number of taxa showed a stronger relationship to habitat conditions (width and substrate) than chemical variables.

4. Important macroinvertebrate taxa are reduced at concentrations of BOD₅ that are normally perceived as indicating unimpacted stream site conditions. Our results confirmed sensitivity/tolerance patterns used by existing bioassessment systems only to some degree.

1. The fundamental importance of freshwater resources, the rapid extinction rate among freshwater species and the pronounced sensitivity of freshwater ecosystems to climate change together signal a pre‐eminent need for renewed scientific focus and greater resources. Against this background, the Freshwater Biological Association in 2008 launched a new series of ‘summit’ Conferences in Aquatic Biology intended to develop and showcase the application of ecological science to major issues in freshwater management.

2. This collection of studies arose from the first summit entitled ‘Multiple Stressors in Freshwater Ecosystems’. Although freshwater science and management are replete with mutiple‐stressor problems, few studies have been designed explicitly to untangle their effects.

3. The individual case studies that follow reveal the wide array of freshwaters affected by multiple stressors, the spatial and temporal scales involved, the species and ecosystem processes affected, the complex interactions between ecology and socioeconomics that engender such effects, the approaches advocated to address the problems and the challenges of restoring affected systems. The studies also illustrate the extent to which new challenges are emerging (e.g. through climate change), but also they develop a vision of how freshwaters might be managed sustainably to offset multiple stressors in future.

4. More generically, these case studies illustrate (i) how freshwaters might be at particular risk of multiple‐stressor effects because of conflicts in water use, and because the hydrological cycle vectors stressor effects so effectively and so extensively; (ii) that dramatic, nonlinear, ‘ecological surprises’ sometimes emerge as multiple‐stressor effects develop and (iii) that good ecology and good ecologists add considerable value to other freshwater disciplines in understanding multiple stressors and managing their effects.

1. Anthropogenic activities can increase fine sediment supply to streams over multiple spatial and temporal extents. Identifying the processes responsible, and the scale at which any effects on stream organisms become evident, are key management needs, but appropriately scaled surveys are surprisingly few.

2. We surveyed macroinvertebrates and superficial fine sediments at two spatial resolutions (reach‐ and patch‐scale) in tributaries of the River Usk, a temperate, montane catchment in rural Wales (U.K.). Land use, habitat and geomorphological character were measured on‐site or derived from an existing database (=Fluvial Audit). We aimed to identify: (i) how in‐stream sediments varied with land use and associated geomorphology; (ii) likely consequences for macroinvertebrates and (iii) any scale‐dependence in relationships between macroinvertebrates and sediment character.

3. At both the reach‐ and patch‐scales, bed cover by fine sediment was related directly to the extent of eroding banks 500 m upstream. In turn, sedimentation and bank erosion were negatively correlated with catchment or riparian woodland extent.

4. At the reach scale, macroinvertebrate composition varied with catchment land use and stream chemistry, with richness declining as rough grazing or woodland was replaced by improved grassland. There was no response to deposited sediment except for weak increase in the relative abundance of oligochaetes.

5. By contrast, at the patch scale, fine sediments were accompanied by pronounced changes in invertebrate composition, and we ranked the 27 most common taxa according to their apparent sediment tolerance. General estimating equations showed that total and Ephemeroptera, Plecoptera and Trichoptera richness decreased significantly by 20% and 25% at the most sediment impacted sites (30% cover) by comparison with sediment‐free sites.

6. We conclude that sediment deposition in the upper Usk system mostly reflects local bank erosion, with riparian woodland likely to mediate this process through bank stability. Fine sediment release had marked ecological effects, but these were detectable only at patch‐to‐patch scales. We suggest that investigation of localized sediment release in streams will benefit from scale‐dependent or scale‐specific sampling, and some effects could go undetected unless sample resolution is selected carefully.