Linking aquatic and terrestrial food webs – Odonata in boreal systems

1. It is increasingly realised that aquatic and terrestrial systems are closely linked. We investigated stable isotope variations in Odonata species, putative prey and basal resources of aquatic and terrestrial systems of northern Mongolia during summer. 2. In permanent ponds, δ¹³C values of Odonata larvae were distinctly lower than those of putative prey, suggesting that body tissue comprised largely of carbon originating from isotopically light carbon sources. Presumably, prey consumed during autumn and winter when carbon is internally recycled and/or methanotrophic bacteria form an important basal resource of the food web. In contrast, in a temporary pond, δ¹³C values of Odonata larvae were similar to those of putative prey, indicating that their body carbon originated mainly from prey species present. 3. Changes in δ¹⁵N and δ¹³C values between larvae and adults were species specific and reflected differential replacement of the larval isotopic signature by the terrestrial diet of adult Odonata. The replacement was more pronounced in Odonata species of permanent ponds than in those of the temporary pond, where larvae hatched later in the year. Replacement of larval carbon varied between tissues, with wings representing the larval isotopic signature whereas thoracic muscles and eggs reflected the δ¹⁵N and δ¹³C values of the terrestrial diet of adults. 4. The results suggest that because of their long larval development, Odonata species of permanent ponds carry the larval signature, which is partly replaced during their terrestrial life. Terrestrial prey forms the basis for egg production and thus the next generation of aquatic larvae. In temporary ponds, in contrast, Odonata species rely on prey from a single season, engage in a prolonged aquatic phase and hatch later, leaving less time to acquire terrestrial prey resources for offspring production. Stable isotope analysis provided important insights into the food webs of the waterbodies and their relationship to the terrestrial system.     

Allochthonous and autochthonous particulate organic matter in floodplains of the River Danube: the importance of hydrological connectivity

SUMMARY 1. The elemental composition, the proportion of living organic carbon and the carbon stable isotope signatures of particulate organic matter (POM) were determined in a large river floodplain system in order to elucidate the major carbon sources in relation to the hydrological conditions over a 13‐month period. 2. Two floodplain segments and the main channel of the River Danube downstream of Vienna (Austria), were compared on the basis of discharge and water age estimations. The more dynamic floodplain was connected to the main channel for 46% of the study period and drained up to 12% of total discharge at high water. 3. The mean C : N ratio and δ¹³C signature of the POM increased from the floodplain site that was more isolated from the river (6.6; ?33‰) to the main channel (8.4; ?25‰). At the dynamic floodplain site, the C : N ratio and the δ¹³C signature of the POM increased with hydrological connectivity (expressed as water age). 4. Only during flood events (4% frequency of occurrence), a considerable input of riverine POM was observed. This input was indicated by a C : N ratio of the POM pool of more than 10, the amount of detrital carbon (>80% of the total POM pool) and a δ¹³C signature of POM of more than ?25‰ in the dynamic floodplain. 5. Plankton derived carbon, indicated by C : N ratios less than eight and δ¹³C values lower than ?25‰, dominated the particulate organic carbon (POC) pool at both floodplain sites, emphasising the importance of local (autochthonous) production. Phytoplankton was the major plankton compartment at the dynamic site, with highest biomasses at medium water ages. 6. At the dynamic floodplain site, the Danube Restoration Project has enhanced the duration of upstream surface connection with the main channel from 4 to 46% frequency of occurrence. Therefore, the export of living POC to the main channel is now established during phases of maximum phytoplankton production and doubled the estimated total export of non‐refractory POM compared with prerestoration conditions.     

Detecting trophic‐level variation in consumer assemblages

1. Stable isotopes of nitrogen are useful for quantifying the trophic structure of food webs, but only if the variation in trophic enrichment (Δ_N), which is the difference in δ¹⁵N between a consumer and its food, is small relative to the value of Δ_N itself. 2. We examined the sources of variation in zooplankton Δ_N by measuring the trophic enrichment (Δ_N) of seven species of freshwater cladocerans, and by testing for an effect of age and temperature on the Δ_N of Daphnia pulicaria. 3. We found that Δ_N was similar among Cladocera and was not correlated with body size. Overall, the Δ_N for D. pulicaria was 1.4‰ (SE = 0.69, n = 57), as was expected for the detritus diet that we used in our experiments. We found no effect of temperature (15–25 °C) on Δ_N, but found that Δ_N of D. pulicaria increased with increasing age (10–30 days). 4. We developed a new method to test for trophic‐level variation in a group of consumers that explicitly accounts for the uncertainty in Δ_N. Using this approach, we confirmed that natural assemblages of zooplankton feed at several trophic levels in lake food webs.     

Reach-scale geomorphology affects organic matter and consumer δ13C in a forested Piedmont stream

1. We investigated the spatial (longitudinal position and reach geomorphology) and seasonal (spring and autumn) influences on the variation of δ¹³C among organic matter sources and consumers in a forested Piedmont river, South Carolina, U.S.A. 2. Six sites were sampled along a continuum and varied in basin area from approximately 30 to 300 km². Sites fell into two geomorphic categories (i) high‐gradient, rock bed (‘rock’) or (ii) low‐gradient, sand bed (‘sand’) sites. 3. Variation in δ¹³C was more strongly related to reach geomorphology than longitudinal position. δ¹³C of biofilm and consumers was consistently enriched at rock sites. Leaf litter (i.e. coarse particulate organic matter, CPOM) δ¹³C did not vary with bed type. There was significant δ¹³C enrichment at rock sites for biofilm, seston, fine benthic organic matter (FBOM), and eight of nine consumer trophic guilds (e.g. grazing invertebrates, insectivorous fishes). δ¹³C of biofilm and four trophic guilds was also positively correlated with drainage area, but the magnitude of enrichment was less than between bed types. 4. δ¹³C was generally enriched in spring, but this varied among organic matter types, consumers, and by bed type. CPOM and seston were enriched in spring, FBOM was enriched in autumn, and biofilm showed no trend. Five consumer guilds were enriched in spring, and only one fish guild, generalised carnivores, showed enrichment of muscle tissue in autumn. 5. Consumer δ¹³C enrichment at rock sites suggests greater reliance on algal carbon than for consumers at sand sites, but we also found δ¹³C enrichment of biofilm at rock sites. Thus, differences in consumer δ¹³C between bed types could be related to (i) increased consumption of biofilm at rock compared with sand sites, or (ii) consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites or (iii) both mechanisms. 6. δ¹³C signatures in local food webs appear to respond to processes operating at multiple spatial scales. Overall downstream enrichment of biofilm and consumers was disrupted by strong local effects related to bed morphology. These results suggest that human alteration of channel habitat will have corresponding effects on stream food webs, as assessed by changes in δ¹³C.     

Temporal and spatial variation in ecosystem metabolism and food web carbon transfer in a wet‐dry tropical river

1. High light availability and stable base flow during the dry season promote primary production in perennial rivers of the wet–dry tropics, in contrast to production during the wet season which is often limited by turbidity and scouring. The Mitchell River of northern Queensland (Australia) was studied to understand controls on aquatic production and respiration in the dry season in relation to spatial and temporal gradients of light and temperature. 2. At three sites along the river, whole‐ecosystem gross primary production (GPP) and respiration (ER) were measured from diel changes of dissolved oxygen using the open‐channel single station method. Using stable carbon and nitrogen isotope analysis, aquatic consumers and their potential basal food resources were also assessed to determine food web relationships at the beginning and end of the dry season. 3. Nutrient limitation of aquatic net primary production was implied from the oligotrophic conditions and high algal C:N ratios. Rates of GPP were comparable with other tropical and temperate rivers and were regulated by light availability. 4. Respiration rates were high and similar to other tropical and subtropical rivers. Up to 52% of temporal variation of ER was explained by temperature, while P/R was lowest at the downstream site. 5. Benthic algae were the major carbon source for primary and secondary benthic consumers (insects) in the dry season but not for higher consumers (fish and crustaceans). Despite high rates of ER, which were probably supported by decaying terrestrial C3 plant material, this carbon source was not identified as contributing to animal consumer biomass. 6. While benthic algal production in the dry season sustained benthic invertebrates, the importance of external subsidies of carbon along the river, probably from the floodplain, was emphasised for fish and large invertebrates, which evidently were feeding on carbon sources not present in channel waterholes during the dry season.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Longitudinal patterns of organic matter transport and turnover along a New Zealand grassland river

1. We determined the longitudinal pattern of organic matter concentration, quality, size composition and spiralling length along a 310-km grassland river system (Taieri River, New Zealand). 2. Organic seston concentration (0.24–4.05 mg ash-free dry mass (AFDM) l^–1) and dissolved organic carbon (DOC) concentration (2.3–5.7 mg C l^–1) showed no obvious longitudinal patterns. In contrast, there was a significant downstream increase in inorganic seston concentration (0.13–13.73 mg ash l^–1), presumably because of a downstream increase in the proportion of the catchment developed for agriculture. 3. Although there was a trend toward increasing particle size in the first 25 km of the river continuum, organic seston was primarily composed of ultrafine particles (0.6–30 μm) at all study sites. The ratio of coarse (> 250 μm) to ultrafine organic seston decreased logarithmically down the continuum. Organic content generally decreased with particle size. Ultrafine particles, however, had significantly higher organic fractions than fine (60–100 μm) and very fine (30–60 μm) particles. 4. Daily organic carbon turnover length ranged from 10 to 98 km and increased downstream. This is consistent with other studies along river continua and suggests that organic carbon turnover length is largely controlled by the relationship between channel dimensions and discharge, rather than the presence of specific retention devices. 5. Concentrations and nutritional quality of organic seston and concentrations of DOC were highest in an unconstrained floodplain reach in the upper river. These data suggest that new material enters the river channel in this reach, potentially providing an important energy source for the river community downstream. The effect of this reach on the longitudinal pattern of organic matter concentration and quality emphasizes how changes in channel form can alter river ecosystem structure and function.     

Isotope variability of particulate organic matter at the PN section in the East China Sea

The source of particulate organic matter at the PN section in the East China Sea has been evaluated using stable carbon and nitrogen isotopes. The results showed that the ¹³C and ¹⁵N compositions varied from –19 to –31 and 0.7–9.5 respectively, and the isotope compositions were statistically distinct, enabling, by use of a simple components mixing equations, assessment of the ability of each tracer to estimate the terrestrial, Kuroshio Water, marine and remineralized sources' contributions. The dominance of terrestrial inputs of the Changjiang could be observed 250 km far from the river mouth in the East China Sea. In the shelf water column, the remineralization of biogenic organic matter becomes an important source except for the terrigenous and marine sources. The estimation of sources recorded by ¹³C data was partly confirmed by equivalent ¹⁵N and C/N compositions that reflected greater control by organic matter diagenesis and biological processing. However, the lighter contribution of ¹³C data of the Kuroshio samples also indicates the alteration of the isotope values by microbial or other processes. The net export flux of POC in the PN section is estimated to be 4.1 kmol C/s and the annual export is 129 Gmol C/yr, which is account for 20% of the East China Sea.     

Carbon source accounting for fish using combined DNA and stable isotope analyses in a regulated lowland river weir pool

Determining the source and flow of carbon, energy and nutrients through food webs is essential for understanding ecological connectivity and thus determining the impact of management practices on biodiversity. We combined DNA sequencing, microarrays and stable isotope analyses to test whether this approach would allow us to resolve the carbon flows through food webs in a weir pool on the lower Murray River, a highly impacted, complex and regulated ecosystem in southern Australia. We demonstrate that small fish in the Murray River consume a wide range of food items, but that a significant component of carbon and nitrogen entering the food web during dry periods in summer, but not spring, is derived from nonconventional sources other than in-channel primary producers. This study also showed that isotopic analyses alone cannot distinguish food sources and that a combined approach is better able to elucidate food-consumer dynamics. Our results highlight that a major river ecosystem, stressed by reduced environmental flows, can rapidly undergo significant and previously undetected changes that impact on the ecology of the system as a whole.     

Sources of organic carbon supporting the food web of an arid zone floodplain river

SUMMARY 1. Many Australian inland rivers are characterised by vast floodplains with a network of anastomosing channels that interconnect only during unpredictable flooding. For much of the time, however, rivers are reduced to a string of disconnected and highly turbid waterholes. Given these features, we predicted that aquatic primary production would be light-limited and the riverine food web would be dependent on terrestrial carbon from floodplain exchanges and direct riparian inputs.
2. To test these predictions, we measured rates of benthic primary production and respiration and sampled primary sources of organic carbon and consumers for stable isotope analysis in several river waterholes at four locations in the Cooper Creek system in central Australia.
3. A conspicuous band of filamentous algae was observed along the shallow littoral zone of the larger waterholes. Despite the high turbidity, benthic gross primary production in this narrow zone was very high (1.7–3.6 g C m⁻² day⁻¹); about two orders of magnitude greater than that measured in the main channel.
4. Stable carbon isotope analysis confirmed that the band of algae was the major source of energy for aquatic consumers, ultimately supporting large populations of crustaceans and fish. Variation in the stable carbon and nitrogen isotope signatures of consumers suggested that zooplankton was the other likely major source.
5. Existing ecosystem models of large rivers often emphasise the importance of longitudinal or lateral inputs of terrestrial organic matter as a source of organic carbon for aquatic consumers. Our data suggest that, despite the presence of large amounts of terrestrial carbon, there was no evidence of it being a significant contributor to the aquatic food web in this floodplain river system.     

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow ( Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of ¹⁵N and ¹³C isotopes and a whole stream ¹⁵N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The ¹⁵N and ¹³C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for ¹⁵N and ¹³C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a ¹⁵N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.     

Seasonal variation in seston and organic matter accumulation in a sheltered fenland pond

Seasonal changes in certain aspects of the seston and organic matter accumulating in sediment collectors are described for a small fenland pond which is surrounded by reed-bed. Much of the variation in organic accumulation rate is not attributable to chlorophyllous matter or fluctuation in the seston concentration, and evidence is provided that some of this accumulating material is derived from the reed-bed.     

Habitat and resource use by larval and juvenile fishes in an arid-land river (Rio Grande, New Mexico)

1. Studies of mesic temperate and tropical rivers suggest an important role for floodplain habitats as nursery areas for larval and juvenile fishes. In arid‐land rivers the extent and duration of flooding is diminished and habitats and resources used by larval fishes are poorly known. Our study documented habitat and resource use of larval fishes in the Rio Grande, New Mexico, an arid‐land river. 2. Spatial and temporal distribution of larval and juvenile fishes and their inferred microhabitat preferences were studied during spring, summer and autumn, 2003. Stable carbon (¹³C : ¹²C) and nitrogen (¹⁵N : ¹⁴N) isotope ratios were measured to identify nutrient sources and characterise trophic positions of young‐of‐year fishes in this system. 3. Some fishes recruited during high flows (in spring), whereas others recruited during low‐flow periods in late summer. Regardless of the timing of reproduction, microhabitats with lower current velocity and higher temperature appeared to serve as vital nursery grounds for Rio Grande fishes. Ephemeral backwaters and disconnected side channels held the highest abundance and diversity of larvae and juveniles. 4. Stable isotope analyses revealed that fish larvae obtained carbon predominately from algal production in early summer, but used organic carbon derived from emergent macrophytes as river discharge decreased in mid‐summer. This shift may have been facilitated by microinvertebrate prey that grazed down edible algae and then switched to macrophytes in mid‐summer. Nitrogen isotope ratios did not differ among species or early life stages, suggesting that larval and juvenile fishes use similar food resources, especially when restricted to isolated pools in summer.     

Sensitivity of soil organic matter to climate and fire in a desert grassland

Biogeochemistry - Drylands contain a third of the organic carbon stored in global soils; however, the long-term dynamics of soil organic carbon in drylands remain poorly understood relative to...     

Flow and retention of particulate organic matter in riparian fluvial habitats under different climates

The flow and retention of particulate organic matter were compared in two fluvial systems under different climates: the French upper Rhone (temperate climate) and the lower Moulouya in Morocco (Mediterranean climate). In both systems, a breakdown gradient was observed. This gradient was correlated with water motion, oxygen content, richness in hyphomycete species and occurrence of their mycelia and spores on submerged leaves, invertebrate density and consumption by shredders.U.A. CNRS 1451 Ecologie des Eaux Douces et des Grands FleuvesU.A. CNRS 1451 Ecologie des Eaux Douces et des Grands Fleuves     

应用稳定同位素测定贵州凤冈麻湾洞洞穴陆生动物的食物来源及营养级

以贵州凤冈麻湾洞洞穴生态系统为研究对象,运用δ¹³C、δ¹⁵N测定了洞穴动物及其有机碳源的同位素比值,分析了洞穴生态系统的营养级关系及洞穴动物食源。结果表明:洞内植物δ¹³C范围为-41. 78‰～-38. 80‰,较洞外植物低;δ¹⁵N范围为-1. 31‰～1.23‰,在洞外陆源有机质δ¹⁵N范围内;洞穴土壤有机质的δ¹³C范围为-31. 09‰～-24.95‰,δ¹⁵N范围为-1.08‰～7.72‰;洞穴动物δ¹³C范围为-30.41‰～-12.02‰,δ¹⁵N范围为2.07‰～8.94‰;洞穴土壤有机质对动物的食源贡献率超过72%,远高于植物对动物的食源贡献率,即洞穴土壤有机质是洞穴动物的主要食物来源。麻湾洞生态系统主要由4个营养层次组成:植物为第一营养层次;闪夜蛾、螺类、马陆类处于第二营养层次;裸灶螽、长头地蜈蚣处于第三营养层次;蜘蛛类处于第三或第四营养层次。即大部分同种(或同类群)...