Density of red alder (Alnus rubra) in headwaters influences invertebrate and detritus subsidies to downstream fish habitats in Alaska

We investigated the influence of red alder (Alnus rubra) stand density in upland, riparian forests on invertebrate and detritus transport from fishless headwater streams to downstream, salmonid habitats in southeastern Alaska. Red alder commonly regenerates after soil disturbance (such as from natural landsliding or timber harvesting), and is common along streams in varying densities, but its effect on food delivery from headwater channels to downstream salmonid habitats is not clear. Fluvial transport of invertebrates and detritus was measured at 13 sites in spring, summer and fall during two years (2000–2001). The 13 streams encompassed a riparian red alder density gradient (1–82% canopy cover or 0–53% basal area) growing amongst young-growth conifer (45-yr-old stands that regenerated after forest clearcutting). Sites with more riparian red alder exported significantly more invertebrates than did sites with little alder (mean range across 1–82% alder gradient was about 1–4 invertebrates m^?3 water, and 0.1–1 mg invertebrates m^?3 water, respectively). Three-quarters of the invertebrates were of aquatic origin; the remainder was of terrestrial origin. Aquatic taxa were positively related to the alder density gradient, while terrestrially-derived taxa were not. Streams with more riparian alder also exported significantly more detritus than streams with less alder (mean range across 1–82% alder gradient was 0.01–0.06 g detritus m^?3 water). Based on these data, we predict that headwater streams with more riparian alder will provide more invertebrates and support more downstream fish biomass than those basins with little or no riparian alder, provided these downstream food webs fully utilize this resource subsidy.     

Flow intermittency influences the trophic base,but not the overall diversity of alpine stream food webs

Alpine streams can exhibit naturally high levels of flow intermittency. However, how flow intermittency in alpine streams affects ecosystem functions such as food web trophic structure is virtually unknown. Here, we characterized the trophic diversity of aquatic food webs in 28 headwater streams of the Val Roseg, a glacierized alpine catchment. We compared stable isotope (δ¹³C and δ¹⁵N) trophic indices to high temporal resolution data on flow intermittency. Overall trophic diversity, food chain length and diversity of basal resource use did not differ to a large extent across streams. In contrast, gradient and mixing model analysis indicated that primary consumers assimilated proportionally more periphyton and less allochthonous organic matter in more intermittent streams. Higher coarse particulate organic matter (CPOM) C:N ratios were an additional driver of changes in macroinvertebrate diets. These results indicate that the trophic base of stream food webs shifts away from terrestrial organic matter to autochthonous organic matter as flow intermittency increases, most likely due to reduced CPOM conditioning in dry streams. This study highlights the significant, yet gradual shifts in ecosystem function that occur as streamflow becomes more intermittent in alpine streams. As alpine streams become more intermittent, identifying which functional changes occur via gradual as opposed to threshold responses is likely to be vitally important to their management and conservation.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Longitudinal variation in food sources and their use by aquatic fauna along a subtropical river in Taiwan

1. River food webs rely on two major food sources: autochthonous primary production within the river and allochthonous organic matter transferred to the river. We characterised the consumer communities and assessed the food sources of dominant consumers along a subtropical mountainous river (the Lanyang River of north‐eastern Taiwan) at the catchment scale from the headwater to the estuary using natural abundances of stable carbon and nitrogen isotopes. 2. The downstream transport of fine particulate organic matter (FPOM) was two orders of magnitude greater than that of coarse particulate organic matter (CPOM). Transport of both materials increased from the headwater and reached a maximum in the midstream reach. CPOM composition exhibited a gradual shift from leaves and branches in the headwater, an area characterised by high canopy cover, to algae in the midstream reaches and marsh plants in the downstream reaches. 3. Consumer communities can be classified into two regional categories: the upland category in the headwater and upstream and midstream reaches and the lowland category comprised of samples from the downstream reach and estuary. The upland category revealed a clear and gradual seasonal shift in community composition, but a seasonal shift was not apparent for the lowland category. Nutrient concentrations and water temperature were the main factors explaining longitudinal and seasonal variations. 4. The use of sources of organic matter by dominant consumers along the Lanyang River was primarily determined by their availability. Riparian C₃ plants were the major food sources in the headwater, upstream reach and estuary, but the contribution of periphyton increased in the upper midstream reach where the river flows through an agricultural area. In the lower midstream and downstream reaches, the contribution of riparian C₄ plants became dominant. 5. The trophic transfer of organic materials in the Lanyang River may be influenced by the fast current velocity and by sewage nutrient loading in the river, both of which have important implications for predicting how the functioning of subtropical river food webs will respond to human‐related changes in land use.     

Top-down and bottom-up processes in grassland and forested streams

The influence of predatory fish on the structure of stream food webs may be altered by the presence of forest canopy cover, and consequent differences in allochthonous inputs and primary production. Eight sites containing introduced brown trout (Salmo trutta) and eight sites that did not were sampled in the Cass region, South Island, New Zealand. For each predator category, half the sites were located in southern beech (Nothofagus) forest patches (range of canopy cover, 65–90%) and the other half were in tussock grassland. Food resources used by two dominant herbivores-detritivores were assessed using stable isotopes. ¹³C/¹²C ratios were obtained for coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), algal dominated biofilm from rocks, and larvae of Deleatidium (Ephemeroptera) and Olinga (Trichoptera). Total abundance and biomass of macroinvertebrates did not differ between streams with and without trout, but were significantly higher at grassland sites than forested sites. However, taxon richness and species composition differed substantially between trout and no-trout sites, irrespective of whether streams were located in forest or not. Trout streams typically contained more taxa, had low biomass of predatory invertebrates and large shredders, but a high proportion of consumers with cases or shells. The standing stock of CPOM was higher at forested sites, but there was less FPOM and more algae at sites with trout, regardless of the presence or absence of forest cover. The stable carbon isotope range for biofilm on rocks was broad and encompassed the narrow CPOM and FPOM ranges. At trout sites, carbon isotope ratios of Deleatidium, the most abundant invertebrate primary consumer, were closely related to biofilm values, but no relationship was found at no-trout sites where algal biomass was much lower. These results support a role for both bottom-up and top-down processes in controlling the structure of the stream communities studied, but indicate that predatory fish and forest cover had largely independent effects.     

Chemical cues modify species interactions: the ecological consequences of predator avoidance by freshwater snails

Chemical signals released by predators or injured prey often induce shifts in the traits of prey species, which may in turn affect species interactions. Here we investigate the role that chemical cues play in mediating species interactions in the littoral food web of lakes. Previous studies have shown that predators induce shifts in the morphology, life history, and behavior of the freshwater snail Physella, but the ecological consequences of developing these inducible defenses are not well documented. We observed habitat use of the freshwater snail Physella gyrina along a depth gradient in a natural lake, and found they increased their use of covered habitats with increasing depth. We hypothesized that this habitat shift was due to changes in the level and type of predation risk, and that the habitat shift would affect periphyton standing crops. These hypotheses were tested in a mesocosm experiment in which we manipulated the presence of molluscivorous fish and crayfish. Predators were confined to cages and snail density was identical in all treatments, so any effects of predators were mediated through trait shifts induced by chemical cues. In the presence of fish, Physella moved under cover, but in the presence of crayfish, Physella avoided cover and moved to the water surface. These non‐lethal effects of predators on snail habitat use influenced the interaction between snails and their periphyton resources. In the presence of fish, periphyton standing crop in covered habitats was reduced to just 8% of periphyton in the absence of fish. Crayfish had no significant effect on periphyton in covered habitats, but they reduced periphyton in near‐surface habitats to 39% of the standing crop in the absence of crayfish. The combined effects of fish and crayfish were generally intermediate to their individual effects. We conclude that because chemical cues often have strong effects on individual traits and trophic interactions are sensitive to trait values, chemical cues may play an important role in shaping the structure and dynamics of food webs.     

Prey use by web-building spiders: stable isotope analyses of trophic flow at a forest-stream ecotone

A forest-stream trophic link was examined by stable carbon isotope analyses which evaluated the relationship of aquatic insects emerging from a stream to the diets of web-building spiders. Spiders, aquatic and terrestrial prey, and basal resources of forest and stream food webs were collected in a deciduous forest along a Japanese headwater stream during May and July 2001. The ¹³C analyses suggested that riparian tetragnathid spiders relied on aquatic insects and that the monthly variation of such dependence is partly associated with the seasonal dynamics of aquatic insect abundance in the riparian forest. Similarly, linyphiid spiders in the riparian forest exhibited ¹³C values similar to aquatic prey in May. However, their ¹³C values were close to terrestrial prey in both riparian and upland (150m away from the stream) forests during June to July, suggesting the seasonal incorporation of stream-derived carbon into their tissue. In contrast, araneid spiders relied on terrestrial prey in both riparian and upland forests throughout the study period. These isotopic results were consistent with a previous study that reported seasonal variation in the aquatic prey contribution to total web contents for each spider group in this forest, implying that spiders assimilate trapped prey and that aquatic insect flux indeed contributes to the energetics of riparian tetragnathid and linyphiid spiders.     

Lateral input of particulate organic matter from bank slopes surpasses direct litter fall in the uppermost reaches of a headwater stream in Hokkaido, Japan

In forested streams, surrounding riparian forests provide essential supplies of organic matter to aquatic ecosystems. We focused on two pathways of particulate organic matter inputs: direct input from upper riparian forests and indirect lateral input from bank slopes, for which there are limited quantitative data. We investigated the inputs of coarse particulate organic matter (CPOM) and carbon and nitrogen in the CPOM into the uppermost reaches of a headwater stream with steep bank slopes in Hokkaido, Japan. CPOM collected by litter traps was divided into categories (e.g., leaves, twigs) and weighed. Monthly nitrogen and carbon inputs were also estimated. The annual direct input of CPOM (ash-free dry mass) was 472 g m⁻², a common value for temperate riparian forests. The annual lateral CPOM input was 353 g m⁻¹ and 941 g m⁻² when they were converted to area base. This value surpassed the direct input. Organic matter that we could not separate from inorganic sediments contributed to the total lateral input from the bank slopes (124 g m⁻¹); this organic matter contained relatively high amounts of nitrogen and carbon. At uppermost stream reaches, the bank slope would be a key factor to understanding the carbon and nitrogen pathways from the surrounding terrestrial ecosystem to the aquatic ecosystem.     

Tracing carbon sources in small urbanising streams: catchment‐scale stormwater drainage overwhelms the effects of reach‐scale riparian vegetation

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Importance of Terrestrial Arthropods as Subsidies in Lowland Neotropical Rain Forest Stream Ecosystems

The importance of terrestrial arthropods has been documented in temperate stream ecosystems, but little is known about the magnitude of these inputs in tropical streams. Terrestrial arthropods falling from the canopy of tropical forests may be an important subsidy to tropical stream food webs and could also represent an important flux of nitrogen (N) and phosphorus (P) in nutrient‐poor headwater streams. We quantified input rates of terrestrial insects in eight streams draining lowland tropical wet forest in Costa Rica. In two focal headwater streams, we also measured capture efficiency by the fish assemblage and quantified terrestrially derived N‐ and P‐excretion relative to stream nutrient uptake rates. Average input rates of terrestrial insects ranged from 5 to 41 mg dry mass/m²/d, exceeding previous measurements of aquatic invertebrate secondary production in these study streams, and were relatively consistent year‐round, in contrast to values reported in temperate streams. Terrestrial insects accounted for half of the diet of the dominant fish species, Priapicthys annectens. Although terrestrially derived fish excretion was found to be a small flux relative to measured nutrient uptake rates in the focal streams, the efficient capture and processing of terrestrial arthropods by fish made these nutrients available to the local stream ecosystem. This aquatic‐terrestrial linkage is likely being decoupled by deforestation in many tropical regions, with largely unknown but potentially important ecological consequences.     

New Zealand stream crayfish: functional omnivores but trophic predators?

1. The feeding ecology of the crayfish Paranephrops planifrons in streams draining catchments in native forest and pastoral land was investigated using analyses of both stomach contents and stable isotopes of carbon and nitrogen. We aimed to (1) identify the energy sources of crayfish, (2) determine whether these were affected by ontogeny or land use change, and (3) assess the functional and trophic roles of crayfish in New Zealand hill-country streams.
2. In native forest streams, crayfish stomach contents were dominated volumetrically by leaf detritus (>60%), but in pasture streams aquatic invertebrates constituted more than 40% of stomach volumes and leaf detritus <30%. Stable isotope analyses revealed that crayfish from both native forest and pasture streams incorporated energy from aquatic invertebrates into their body tissue but did not appear to utilize detritus for growth. Therefore, deforestation had little impact on crayfish energy sources.
3. In native forest streams, adult crayfish (≥20 mm orbit-carapace length (OCL)) consumed greater amounts of leaf detritus than juvenile crayfish, but a corresponding change in stable isotope signatures was not detected. Ontogenetic shifts in diet were not consistent between land use suggesting that change in local habitat and food resources, as a result of land use change, affect crayfish food choice more than factors related solely to age or size.
4. Crayfish appear to occupy the trophic position of a predator, but by functioning as omnivores, they have dual roles as both predators and processors of organic matter. The use of gut content analysis in conjunction with stable isotope analyses revealed that the functional and trophic roles of P. planifrons differed, with implications for the interpretation of diet studies and understanding of the role of omnivores in food webs.     

Urban catchment hydrology overwhelms reach scale effects of riparian vegetation on organic matter dynamics

1. Urbanisation severely affects stream hydrology, biotic integrity and water quality, but relatively little is known about effects on organic matter dynamics. Coarse particulate organic matter (CPOM) is a source of energy and nutrients in aquatic systems, and its availability has implications for ecosystem productivity and aquatic communities. In undisturbed environments, allochthonous inputs from riparian zones provide critical energy subsidies, but the extent to which this occurs in urbanised streams is poorly understood. 2. We investigated CPOM inputs, standing stocks, retention rates and retention mechanisms in urban and peri‐urban streams in Melbourne, Australia. Six streams were chosen along a gradient of catchment urbanisation, with the presence of reach scale riparian canopy cover as a second factor. CPOM retention was assessed at baseflow via replicate releases of marked Eucalyptus leaves where the retention distance and mechanism were recorded. CPOM and small wood (>1 cm diameter) storage were measured via cores and direct counts, respectively, while lateral and horizontal CPOM inputs were assessed using riparian litter traps. Stream discharge, velocity, depth and width were also measured. 3. CPOM inputs were not correlated with urbanisation, but were significantly higher in ‘closed’ canopy reaches. Urbanisation and riparian cover altered CPOM retention mechanisms, but not retention distances. Urban streams showed greater retention by rocks; while in less urban streams, retention by small wood was considerably higher. CPOM and small wood storage were significantly lower in more urban streams, but we found only a weak effect of riparian cover. 4. These findings suggest that while riparian vegetation increases CPOM inputs and has modest/weak effects on storage, catchment scale urbanisation decreases organic matter availability. Using an organic matter budget approach, it appears likely that the increased frequency and magnitude of high flows associated with catchment urbanisation exerts an overriding influence on organic matter availability. 5. We conclude that to maintain both organic matter inputs and storage, the restoration and protection of streams in urban or rapidly urbanising environments relies on the management of both riparian vegetation and catchment hydrology.     

Herbs and grasses as an allochthonous resource in open-canopy headwater streams

1. The organic matter dynamics of streams dominated by herbs and grass on their banks are poorly understood, despite the fact that such streams are common worldwide. Further, herbs and grasses can provide large quantities of detritus to stream food webs, and particularly small streams can be heavily shaded by overhanging vegetation, perhaps limiting in‐stream primary production. 2. We quantified the standing crop of edge vegetation and associated macroinvertebrate communities along three headwater streams with herbaceous and grass riparian vegetation on agricultural land in the Piedmont of Maryland, U.S.A., measured the decomposition of four common species of herbs and grasses using experimental leaf packs, and removed edge vegetation experimentally to determine the effect of shading on benthic algal production. 3. Large standing crops of plant material (average range: 68–276 g ash‐free dry mass per m⁻²), composed largely of monocotyledons, were found at all three study streams. These values are similar to those for coarse particulate organic matter in deciduous forested streams in the eastern U.S.A. In addition, diverse assemblages of shredding macroinvertebrates were observed at all three study sites. 4. Decomposition of the herbs was faster than that of the grasses, and both decomposed faster than most deciduous tree leaf litter. The decomposition rates of the herbs and grasses were significantly related to leaf quality as measured by leaf nitrogen content. Macroinvertebrate shredders colonized all experimental leaf packs, and the colonization of the herbs was faster than that of the grasses. 5. The accrual of chlorophyll‐a after the removal of shading vegetation was faster than that measured prior to removal as well as that in an unmanipulated control reach. 6. Given that the standing crop of organic matter in streams with herbs and grass along their banks was similar to that in forested streams, that the organic matter was rich in nitrogen and used by detritivores, and riparian shading limited algal growth, we suggest that herbaceous and grass plant material may be an important allochthonous food resource in such systems.     

Biomass and Decay Rates of Roots and Detritus in Sediments of Intermittent Coastal Plain Streams

Biomass and breakdown of tree roots within streambed sediments were compared with leaf and wood detritus in three Coastal Plain headwater intermittent streams. Three separate riparian forest treatments were applied: thinned, clearcut, and reference. Biomass of roots (live and dead) and leaf/wood was significantly higher in stream banks than in the channel and declined with depth strata (0–10 > 10–20 > 20–30 cm). Riparian roots (live and dead combined) contributed on average 24 and 42% of coarse particulate organic matter (CPOM) biomass within the top 30 cm of channel and streambank sediments, respectively. Estimated mean surface area of live riparian roots within sediments was 1084 cm² m⁻³. Streambed temperatures showed greater fluctuation at the clearcut site compared to thinned and reference treatments. However, breakdown rates among buried substrate types or riparian treatments did not differ after 1 y. Slow decay rates were associated initially with anaerobic conditions within sandy sediments and later with dry sediment conditions. Riparian roots represent a direct conduit between streamside vegetation and the hyporheic zone. In addition to contributing to organic matter storage, the abundance of riparian roots within streambed sediments suggests that roots play an important role in biogeochemical cycling within intermittent headwater streams of the Coastal Plain.     

Growth and population size of crayfish in headwater streams: individual- and higher-level consequences of acidification

1. Environmental stress may have indirect positive effects on population size through modification of food‐web interactions, despite having negative effects on individuals. Here we evaluate the individual‐ and population‐level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams. 2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity. 3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites. 4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.     

Beyond cool: adapting upland streams for climate change using riparian woodlands

Managed adaptation could reduce the risks of climate change to the world's ecosystems, but there have been surprisingly few practical evaluations of the options available. For example, riparian woodland is advocated widely as shade to reduce warming in temperate streams, but few studies have considered collateral effects on species composition or ecosystem functions. Here, we use cross‐sectional analyses at two scales (region and within streams) to investigate whether four types of riparian management, including those proposed to reduce potential climate change impacts, might also affect the composition, functional character, dynamics and energetic resourcing of macroinvertebrates in upland Welsh streams (UK). Riparian land use across the region had only small effects on invertebrate taxonomic composition, while stable isotope data showed how energetic resources assimilated by macroinvertebrates in all functional guilds were split roughly 50:50 between terrestrial and aquatic origins irrespective of riparian management. Nevertheless, streams draining the most extensive deciduous woodland had the greatest stocks of coarse particulate matter (CPOM) and greater numbers of ‘shredding’ detritivores. Stream‐scale investigations showed that macroinvertebrate biomass in deciduous woodland streams was around twice that in moorland streams, and lowest of all in streams draining non‐native conifers. The unexpected absence of contrasting terrestrial signals in the isotopic data implies that factors other than local land use affect the relative incorporation of allochthonous subsidies into riverine food webs. Nevertheless, our results reveal how planting deciduous riparian trees along temperate headwaters as an adaptation to climate change can modify macroinvertebrate function, increase biomass and potentially enhance resilience by increasing basal resources where cover is extensive (>60 m riparian width). We advocate greater urgency in efforts to understand the ecosystem consequences of climate change adaptation to guide future actions.     

Isotopic analysis of three food web theories in constricted and floodplain regions of a large river

Analyses of stable isotope (δ¹³C and δ¹⁵N) and C:N ratios of food webs within a floodplain and a constricted-channel region of the Ohio River during October 1993 and July 1994 indicate that the increasingly influential flood pulse concept (FPC) does not, for either location, adequately address food web structure for this very large river. Furthermore, results of this study suggest that the riverine productivity model (RPM) is more appropriate than the widely known river continuum concept (RCC) for the constricted region of this river. These␣conclusions are based on stable isotope analyses of potential sources of organic matter (riparian C₃ trees, riparian C₄ grasses and agricultural crops, submerged macrophytes, benthic filamentous algae, benthic particulate organic matter, and transported organic matter containing detritus and phytoplankton) and various functional feeding groups of invertebrate and fish consumers. The FPC, which stresses the key contribution of organic matter, particularly terrestrial organic matter, originating from the floodplain to riverine food webs, was judged inappropriate for the floodplain region of the Ohio River for hydrodynamic and biotic reasons. The rising limb and peak period of discharge typically occur in November through March when temperatures are low (generally much less than 10°C) and greater than bank-full conditions are relatively unpredictable and short-lived. The major food potentially available to riverine organisms migrating into the floodplain would be decaying vegetation because autotrophic production is temperature and light limited and terrestrial insect production is minimal at that time. It is clear from our data that terrestrial C₄ plants contribute little, if anything, to the consumer food web (based on δ¹³C values), and δ¹⁵N values for C₃ plants, coarse benthic organic matter, and fine benthic organic matter were too depleted (∼7–12‰ lower than most invertebrate consumer values) for this organic matter to be supporting the food web. The RPM, which emphasizes the primary role of autotrophic production in large rivers, is the most viable of the remaining two ecosystem models for the constricted-channel region of the Ohio based on stable isotope linkage between sources and consumers of organic matter in the food web. The most important form of food web organic matter is apparently transported (suspended) fine (FTOM) and ultra-fine particulate organic matter. We propose that phytoplankton and detritus of an autochthonous origin in the seston would represent a more usable energy source for benthic (bivalve molluscs, hydropsychid caddisflies) and planktonic (microcrustaceans) suspension feeders than the more refractory allochthonous materials derived from upstream processing of terrestrial organic matter. Benthic grazers depend heavily on nonfilamentous benthic algae (based on gut analysis from a separate study), but filamentous benthic algae have no apparent connection to invertebrate consumers (based on δ¹³C values). Amphipod and crayfish show a strong relationship to aquatic macrophytes (possibly through detrital organic matter rather than living plant tissue). These observations contrast with the prediction of the RCC that food webs in large rivers are based principally on refractory FTOM and dissolved organic matter from upstream inefficiencies in organic-matter processing and the bacteria growing upon these suspended or dissolved detrital compounds. The conclusions drawn here for the Ohio River cannot yet be extended to other floodplain and constricted-channel rivers in temperate and tropical latitudes until more comparable data are available on relatively pristine and moderately regulated rivers. Received: 3 January 1997 / Accepted: 28 August 1998     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

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Beaver invasion alters terrestrial subsidies to subantarctic stream food webs

North American beavers (Castor canadensis) were introduced to Tierra del Fuego Island in 1946 for their fur, and have since spread across the archipelago and onto the South American mainland. We assessed the impact of invasive beavers on streams of these forested watersheds by quantifying the trophic basis of production (TBP) and consumptive organic matter flows of benthic macroinvertebrate assemblages. TBP was determined in two streams: clear- and black-water. Stable isotopes were used across four streams to further elucidate food web structure and dominant pathways. TBP and stable isotopes showed that terrestrially derived organic matter (amorphous detritus, leaves, and wood) supported a majority of secondary production in the benthic food webs at all sites (forested reaches, beaver ponds, and sections downstream of ponds with foraged riparian zones). The magnitude of these flows was enhanced in beaver-modified sites compared with forested habitats (4.0–5.3× increase g AFDM m⁻² year⁻¹ in pond habitats, 1.1–2.1× increase in downstream habitats). Diatoms were the only autochthonous resource identified in macroinvertebrate guts, but their contribution to secondary production was small. Consumptive flows mirrored trends in TBP (i.e., dominance of terrestrial sources and greater magnitude in beaver ponds). Collector–gatherer consumption of amorphous detrital material dominated food web flows in all habitats, but was higher in beaver ponds relative to other habitats. Food web structure was simplified in beaver ponds; only two of the five possible functional groups contributed >1% of total organic matter flow in ponds (collector–gatherers and predators). Consumptive flows to predators increased in ponds, and stable isotopes of nitrogen and carbon (δ¹⁵N and δ¹³C) corroborated a relatively greater importance of predators (greater trophic distance), as well as less diversity of basal resources (less variation in δ¹³C) in ponds. Our findings indicate that invasive beaver’s engineering activities resulted in greater flows of terrestrial organic matter subsidies to in-stream food webs, which had a relatively greater change in the clear-water than in the black-water stream. Owing to the fact that these streams were naturally dependent on allochthonous resources for a majority of production and material flows, changes wrought by beavers to streams in forested environments are probably less than in watersheds with inherently greater dependence on autochthonous production such as the adjacent steppe biome.     

Allochthonous and autochthonous carbon sources for fish in floodplain lagoons of an Australian dryland river

Dryland rivers associated with arid and semi-arid land areas offer an opportunity to explore food web concepts and models of energy sources in systems that experience unpredictable flooding and long dry spells. This study investigated the sources of energy supporting three species of fish feeding at different trophic levels within floodplain lagoons of the Macintyre River in the headwaters of the Murray-Darling river system, Australia. Stable isotope analyses revealed that fish consumers derived, on average, 46.9% of their biomass from zooplankton, 38.1% from Coarse Particulate Organic Matter (CPOM) and 24.0% from algae. Ambassis agassizii derived on average 57.6% of its biomass carbon from zooplankton and 20.4–27.8% from algae or CPOM. Leiopotherapon unicolor derived most of its carbon from zooplankton and CPOM (38.3–39.5%), with relatively high contributions from algae compared to the other species (33.3%). An average of 48.4% of the biomass of Nematalosa erebi was derived from zooplankton, with CPOM contributing another 38.1%. Zooplankton was the most important source of organic carbon supporting all three fish species in floodplain lagoons. Phytoplankton, and possibly, particulate organic matter in the seston, are the most likely energy sources for the planktonic suspension feeders (zooplankton) and, consequently, the fish that feed on them. These results indicate a stronger dependence of consumers on autochthonous sources and on locally produced organic matter from the riparian zone (i.e., the Riverine Productivity Model), than on other resources.