Stable isotopes and trophic positions of littoral fishes from a Mediterranean marine protected area

Stable isotope analyses were employed to explore feeding and foraging habitats and trophic levels of littoral fishes in a western Mediterranean Marine Protected Area (Egadi Islands, Sicily, Italy). Carbon and nitrogen stable isotope ratios were measured in primary producers, invertebrates and fishes collected in December 2001 and January 2002. Fishes of the littoral region of the Egadi Islands had isotopic signatures that fell into a wider range for δ ¹³C (about 6‰) than for δ ¹⁵N (about 3‰). Carbon isotope ratios were consistent with a food web based on mixed sources and two trophic pathways leading to different fish species. Differences in the isotopic composition between islands were higher for benthivorous than for planktivorous fishes. The overall picture gained from this study is of a isotopic distinction between planktivorous and benthivorous fishes, resource partitioning facilitating the coexistence of similar species within the same ecosystem, and spatial variability in the isotopic signatures and trophic level of fishes. Asymmetrical analysis of variance showed that estimated trophic levels were lower in the area with the highest level of protection (Zone A) for only two out of the nine fishes analysed. As a consequence, overall spatial differences do not seem to be a consequence of protection, since in most cases trophic levels did not change significantly between zone A and zones C where professional fishing (trawling apart) is permitted, but of natural sources of variation (e.g. variability in food availability and site-specific food preferences of fishes). However, the results of this study suggest a different response at the species compared to the community level.     

Parasite infection alters host stable‐isotope composition under controlled feeding

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Zooplankton interactions in an enclosure experiment: insights from stable isotope analyses

1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on the plankton of a filter feeder (Daphnia hyalina × galeata) with that of more selective feeders (calanoid and cyclopoid copepods). The experiment was conducted in situ over 25 days during spring in a mesotrophic lake, Schöhsee, Germany. 2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (<1000 μm³) species, whereas copepods did not show any impact on algae. 3. At the end of the experiment, Daphnia and remaining cyclopoid copepods were harvested and sorted manually, prior to analyses for stable isotopes of carbon and nitrogen. Daphniids from mesocosms stocked purely with differing densities of Daphnia showed little variability in stable isotope values, whereas those that thrived in enclosure bags together with copepods exhibited lower δ¹³C values. 4. The change in Daphniaδ¹³C indicates a change of food sources, modified by the presence of the copepods: the higher the mean abundance of copepods in the enclosures, the more ¹³C‐depleted the daphniids. Increasing abundance of high nucleic acid (HNA) bacteria in the copepod bags may account for the trend in Daphniaδ¹³C via increased grazing on the bacteria themselves, or via grazing on phytoplankton utilising isotopically light CO₂ from respiratory release. 5. Cyclopoid copepod stable isotope signatures were related to Daphnia and copepod abundances in copepod bags, suggesting that cyclopoids preyed on the available zooplankton.     

Differential Impacts of Copepods and Cladocerans on Lake Seston,and Resulting Effects on Zooplankton Growth

In an enclosure study in Schöhsee, a small mesotrophic lake in Northern Germany, the impact of copepods and daphniids on the seston community was studied. In general, these two guilds differ in their feeding behaviour. Copepods actively select their food, with a preference for larger particles, whereas most cladocerans are unselective filter-feeders. In this study we investigate how the impact of the two different grazers affects zooplankton growth. We combine results obtained in the laboratory with results measured in situ in the enclosures. Copepods and cladocerans were cultured on seston from enclosures that were inhabited by density gradients of copepods or daphniids. We observed that Daphnia grew faster on seston that was pre-handled by copepods than on seston that was pre-handled by daphniids, and that somatic growth decreased with increasing densities of daphniids in the enclosures. In contrast, we observed no differences in development rates for copepods grown on the different media. The population growth rates of Daphnia in the Daphnia treatments were determined in the enclosures. Growth differences in both somatic- and population growth of Daphnia were correlated to food quality aspects of the seston. In the laboratory we found that Daphnia growth was correlated with several fatty acids. The strongest regression was with the concentration of 20:43 (r ²= 0.37). This particular fatty acid also showed the highest correlation with growth after normalisation of the fatty acids to the carbon content of the enclosures (r ²= 0.33). On the other hand, in the enclosure the population growth correlated most to the particulate nitrogen content (r ²= 0.78) and only to the N:C ratio, when normalised to carbon (r ²= 0.51).     

Fractionation of stable isotopes in the Arctic marine copepod Calanus glacialis: Effects on the isotopic composition of marine particulate organic matter

Fractionation of δ¹³C and δ¹⁵N between food, consumer, and faecal pellets was studied in the Arctic marine copepod Calanus glacialis Jaschnov, fed with isotopically distinct algal monocultures. Temporal variations in δ¹³C and δ¹⁵N of copepods that were fed ice algae and phytoplankton followed those of a control group consisting of starved animals. There were no significant trends in the δ¹³C and δ¹⁵N values of copepods that were starved for 42 days, suggesting that the isotopic composition of non-lipid body tissues is unaffected by the metabolic processes during prolonged periods of starvation. The stable isotopic composition of starved copepods therefore seems to reflect food consumed during the previous period of feeding and growth. Faecal pellets produced by feeding copepods were depleted in ¹³C and ¹⁵N by 6.3-11.2‰ and 0.7-9.1‰, respectively, relative to the food ingested. These results indicate that faecal pellet production is an important pathway for the trophic fractionation of δ¹³C, whereas other fractionation pathways, such as excretion of ammonia, may be relatively more important for δ¹⁵N. The strong depletion of ¹³C in faecal pellets compared to the food suggests that grazing by herbivorous copepods on primary production adds to the variability of δ¹³C in marine particulate organic matter.     

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.     

δ13C and δ15N indicators of fish and shrimp community diet and trophic structure in a mangrove ecosystem in Thailand

Stable carbon and nitrogen isotope ratios were used to elucidate primary carbon sources and trophic relationships of the fish and shrimp community in the Klong Ngao mangrove ecosystem, southern Thailand. There were no significant differences in isotopic compositions of biota between mangrove and offshore sites (Welch–Aspin test). The δ¹⁵N values of eight fish species and two shrimp species at both sites were also not significantly different by the test, meaning that at both sites they feed on the same diets due to the discharge of large quantities of mangrove sediments. The δ¹⁵N isotopic enrichment of consumers suggested that there are four trophic levels in the Klong Ngao food web, with at least two fish species capable of switching feeding strategies and thus altering their apparent trophic positions. Phytoplankton culture experiments indicated that mangrove-derived sediments could play an important role in stimulating phytoplankton growth for low turbidity offshore areas, thus providing an alternate food source. The isotopic associations among sources and consumers indicated that mangroves were the major carbon source supporting aquatic food webs in the Klong Ngao ecosystem.     

Intrapopulation variation in carbon and nitrogen stable isotope ratios in the earthworm Aporrectodea longa

The natural abundance variations in carbon and nitrogen stable isotope ratios in a population of the earthworm Aporrectodea longa, a species known to feed on both soil and plant litter, is reported in this paper. Worms were collected from a small land area of an old white clover field and body tissue and mucus were analyzed separately. The range of isotopic values was small, but patterns of variation were not random. Tissue carbon and nitrogen isotope ratios were significantly higher in adult than in juvenile A. longa and tissue nitrogen isotope ratios tended to increase with increasing biomass of individuals. Further, carbon and nitrogen isotope ratios were positively correlated in both tissue and mucus. Possible causes of the observed patterns, including physiological effects, body composition and assimilation of C and N from different plant, soil and microbial sources are discussed. It is concluded that the causes of natural variability in isotopic composition must be understood and validated experimentally before natural abundance stable isotope methods can be used for the analysis of trophic relations among detritivorous soil invertebrates.     

Temporal variation in trophic relationships among three congeneric penguin species breeding in sympatry

Penguins are a monophyletic group in which many species are found breeding sympatrically, raising questions regarding how these species coexist successfully. Here, the isotopic niche of three sympatric pygoscelid penguin species was investigated at Powell Island, South Orkney Islands, during two breeding seasons (austral summers 2013–2014 and 2015–2016). Measurements of carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios were obtained from blood (adults) or feather (chicks) samples collected from Adélie Pygoscelis adeliae, chinstrap P. antarctica, and gentoo P. papua penguins. Isotopic niche regions (a proxy for the realized trophic niches) were computed to provide estimates of the trophic niche width of the studied species during the breeding season. The isotopic niche regions of adults of all three species were similar, but gentoo chicks had noticeably wider isotopic niches than the chicks of the other two species. Moderate to strong overlap in isotopic niche among species was found during each breeding season and for both age groups, suggesting that the potential for competition for shared food sources was similar during the two study years, although the actual level of competition could not be determined owing to the lack of data on resource abundance. Clear interannual shifts in isotopic niche were seen in all three species, though of lower amplitude for adult chinstrap penguins. These shifts were due to variation in carbon, but not nitrogen, isotopic ratios, which could indicate either a change in isotopic signature of their prey or a switch to an alternative food web. The main conclusions of this study are that (1) there is a partial overlap in the isotopic niches of these three congeneric species and that (2) they responded similarly to changes that likely occurred at the base of their food chain between the 2 years of the study.     

Isotopic variation in five species of stream fishes under the influence of different land uses

The aim of this study was to test if changes in land use alter the isotopic signature of fish species, promoting changes in the trophic position and food resource partitioning between these consumers. Three different systems were investigated: pasture streams (n = 3), streams in sugar cane plantations (n = 3) and reference streams (n = 3). Fish species Aspidoras fuscoguttatus, Astyanax altiparanae, Characidium zebra, Hisonotus piracanjuba and Knodus moenkhausii were selected, and their nitrogen and carbon isotopic compositions were estimated to assess changes in the trophic level and partitioning of food items consumed. The composition of δ¹³C (‰) only differed among the land use categories for A. altiparanae, H. piracanjuba and K. moenkhausii. Resource partitioning was different for all species, with changes in the sources or proportions they consumed in each land use category, but only A. altiparanae introduced new food sources in large quantity in altered land uses. It is important to note, however, that the results from the resource partitioning analysis are limited due to large overlapping of isotopic signatures between the analysed food resources. All fish species exhibited variation in δ¹⁵N (‰), with the highest values found in streams under sugar cane or pasture influence. Despite the variation in nitrogen isotopic values, only C. zebra and H. piracanjuba displayed changes in trophic level. Therefore, it is believed that the increase in the δ¹⁵N (‰) value of the individuals collected in streams under the influence of sugar cane or pasture was due to the greater influence of livestock dung and chemical and organic fertilizers. The results also highlight the importance of studying consumer species along with all forms of resources available at each location separately, because the signatures of these resources also vary within different land uses.     

Field evidence for stoichiometric relationships between zooplankton and N and P availability in a shallow calcareous lake

1. According to stoichiometric theory, zooplankters have a species‐specific elemental composition. Daphniids have a relatively high phosphorus concentration in their tissues and copepods high nitrogen. Daphniids should, therefore, be more sensitive to phosphorus limitation and copepods more sensitive to nitrogen. A 2‐year study of a shallow marl lake in the west of Ireland investigated whether population fluctuations of the two dominant taxa, Daphnia spp. and the calanoid Eudiaptomus gracilis, were associated with the availability of phosphorus and nitrogen. 2. In accordance with stoichiometric predictions, Daphnia and Eudiaptomus reproduction had contrasting relationships with dietary phosphorus and nitrogen availability. Egg production by Daphnia was negatively associated with the ratio of dissolved inorganic nitrogen (DIN) : total phosphorus (TP) and the ratio of light to TP which was used as an indirect index for seston carbon (C) : phosphorus (P). Conversely calanoid egg production had a positive relationship with the DIN : TP ratio and was unrelated to the estimated C : P (light : TP) ratio. 3. Daphnia biomass was not, however, correlated with phosphorus availability, and neither was calanoid biomass correlated with nitrogen. The high ratio of DIN : TP when Daphnia dominated the zooplankton biomass and the low ratio when calanoids dominated, is consistent with Daphnia acting as a sink for phosphorus and calanoids as a sink for nitrogen and suggests consumer‐driven nutrient recycling.     

Resilience to changes in lake trophic state: Nutrient allocation into Daphnia resting eggs

During past decades, many lakes underwent drastic human‐caused changes in trophic state with strong implications for population dynamics and food web processes. We investigated the influence of trophic state on nutrient allocation into Daphnia resting eggs. The production of resting eggs is an important survival strategy, allowing Daphnia to cope with unfavorable environmental conditions. Allocation of essential nutrients into resting eggs may crucially influence embryonic development and offspring survival and thus is of great ecological and evolutionary interest. The capacity of Daphnia to adjust the allocation of nutrients into resting eggs may depend on the dietary nutrient supply, which may vary with trophic state‐related changes in the phytoplankton community composition. Resting eggs were isolated from sediment cores taken from Lake Constance, a large prealpine lake with a distinct eutrophication and reoligotrophication history, and analyzed for elemental (carbon, nitrogen, and phosphorus) and biochemical (sterols and fatty acids) nutrients. Carbon allocation into Daphnia resting eggs continuously decreased over time, irrespective of changes in trophic state. The allocation of nitrogen into Daphnia resting eggs followed the changes in trophic state, that is, nitrogen concentrations in resting eggs increased with eutrophication and decreased again with reoligotrophication. The allocation of phosphorus, sterols and long‐chain polyunsaturated fatty acids, such as eicosapentaenoic acid, into Daphnia resting eggs did not change significantly over time. Changes in trophic state strikingly influenced all trophic levels in Lake Constance. However, nutrient allocation into Daphnia resting eggs was mostly resilient to changes in lake trophic state.     

Chytrids enhance Daphnia fitness by selectively retained chytrid-synthesised stearidonic acid and conversion of short-chain to long-chain polyunsaturated fatty acids

1. Chytrid fungal parasites convert dietary energy and essential dietary molecules, such as long-chain (LC) polyunsaturated fatty acids (PUFA), from inedible algal/cyanobacteria hosts into edible zoospores. How the improved biochemical PUFA composition of chytrid-infected diet may extend to zooplankton, linking diet quality to consumer fitness, remains unexplored.
2. Here, we assessed the trophic role of chytrids in supporting dietary energy and PUFA requirements of the crustacean zooplankton Daphnia, when feeding on the filamentous cyanobacterium Planktothrix.
3. Only Daphnia feeding on chytrid-infected Planktothrix reproduced successfully and had significantly higher survival and growth rates compared with Daphnia feeding on the sole Planktothrix diet. While the presence of chytrids resulted in a two-fold increase of carbon ingested by Daphnia, carbon assimilation increased by a factor of four, clearly indicating enhanced carbon transfer efficiency with chytrid presence.
4. Bulk carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes did not indicate any treatment-specific dietary effects on Daphnia, nor differences in trophic position among diet sources and the consumer. Compound-specific carbon isotopes of fatty acids (δ¹³C_FA), however, revealed that chytrids bioconverted short-chain to LC-PUFA, making it available for Daphnia. Chytrids synthesised the ω-3 PUFA stearidonic acid de novo, which was selectively retained by Daphnia. Values of δ¹³C_FA demonstrated that Daphnia also bioconverted short-chain to LC-PUFA.
5. We provide isotopic evidence that chytrids improved the dietary provision of LC-PUFA for Daphnia and enhanced their fitness. We argue for the existence of a positive feedback loop between enhanced Daphnia growth and herbivory in response to chytrid-mediated improved diet quality. Chytrids upgrade carbon from the primary producer and facilitate energy and PUFA transfer to primary consumers, potentially also benefitting upper trophic levels of pelagic food webs.

     

Gut content and stable isotope analyses provide complementary understanding of ontogenetic dietary shifts and trophic relationships among fishes in a tropical river

1. Despite widespread recognition of the role of body size in fish trophic ecology, little attention has been focused on this issue in isotopic studies, particularly in tropical systems. 2. We used analyses of stomach contents and stable isotopes to examine size‐related shifts in diet in a terapontid fish assemblage in the Australian wet–dry tropics. Stomach content analysis identified substantial ontogenetic dietary shifts in all species, corresponding to changes in body size–isotope trajectories for two species. Shifts away from relatively specialised diets of heavily ¹³C‐depleted insect larvae to consumption of a range of items across multiple basal carbon sources appeared to be the proximate cause of observed isotopic changes. 3. Allochthonous organic matter in the form of C3 riparian vegetation was particularly important to smaller terapontids before larger fish shifted to a broad range of dietary items and similarly broad range of basal carbon sources. 4. While there was general agreement between δ¹³C and stomach content analysis, there was minimal concurrence between the latter and δ¹⁵N isotopic derivation of estimates of trophic position. Due to factors such as omnivory, isotopically overlapping basal sources and uncertainties about rates of isotopic fractionation in both predator and prey species, stomach content analysis provides an essential complement to isotopic methodologies in tropical systems. 5. Given that basal sources supporting any individual species can change markedly with ontogeny, consideration of intraspecific, size‐related variation is necessary in isotopic studies of food web structure.     

Food‐web structure and mercury dynamics in a large subarctic lake following multiple species introductions

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Energy sources for aquatic animals in the Orinoco River floodplain: evidence from stable isotopes

Summary Stable carbon and nitrogen isotope ratios in autotrophs, aquatic invertebrates and fishes from the Orinoco River floodplain of Venezuela reveal that microalgae, including both phytoplankton and epiphytic (attached) forms, are predominant energy sources for many aquatic animals, even though aquatic vascular plants are much more abundant. Floating mats of the grass Paspalum repens and the water hyacinth Eichhornia spp. harbor particularly high densities of aquatic animals, but isotopic evidence indicates that few species are dependent on organic carbon originating from these plants. The stable isotopic evidence for the trophic importance of algae contradicts traditional interpretations of food webs in freshwater wetlands, which are generally thought to be based largely on detritus originating from vascular plants.     

Trophic level and basal resource use of soil animals are hardly affected by local plant associations in abandoned arable land

Plants provide resources and shape the habitat of soil organisms thereby affecting the composition and functioning of soil communities. Effects of plants on soil communities are largely taxon‐dependent, but how different functional groups of herbaceous plants affect trophic niches of individual animal species in soil needs further investigation. Here, we studied the use of basal resources and trophic levels of dominating soil meso‐ and macrofauna using stable isotope ratios of carbon and nitrogen in arable fallow systems 3 and 14–16 years after abandonment. Animals were sampled from the rhizosphere of three plant species of different functional groups: a legume (Medicaco sativa), a nonlegume herb (Taraxacum officinale), and a grass (Bromus sterilis). We found virtually no consistent effects of plant identity on stable isotope composition of soil animals and on thirteen isotopic metrics that reflect general food‐web structure. However, in old fallows, the carbon isotope composition of some predatory macrofauna taxa had shifted closer to that of co‐occurring plants, which was particularly evident for Lasius, an aphid‐associated ant genus. Trophic levels and trophic‐chain lengths in food webs were similar across plant species and fallow ages. Overall, the results suggest that variations in local plant diversity of grassland communities may little affect the basal resources and the trophic level of prey consumed by individual species of meso‐ and macrofauna belowground. By contrast, successional changes in grassland communities are associated with shifts in the trophic niches of certain species, reflecting establishment of trophic interactions with time, which shapes the functioning and stability of soil food webs.     

Intra‐lake stable isotope ratio variation in selected fish species and their possible carbon sources in Lake Kyoga (Uganda): implications for aquatic food web studies

The stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) provide powerful tools for quantifying trophic relationships and carbon flow to consumers in food webs; however, the isotopic signatures of organisms vary within a lake. Assessment of carbon and nitrogen isotopic signatures in a suite of plants, invertebrates, and fishes in Lake Kyoga, indicated significant variation between two sites for δ¹³C (paired t = 6.305; df = 14, P < 0.001 and δ¹⁵N paired t = 1.292; df = 14; P < 0.05). The fish fauna in Bukungu was generally more ¹³C enriched (mean δ¹³C = –16.37 ± 1.64‰) than in Iyingo (mean δ¹³C = –20.80 ± 2.41‰) but more δ¹⁵N depleted (mean δ¹⁵N = 5.57 ± 0.71‰) than in Iyingo (mean δ¹⁵N = 6.92 ± 0.83‰). The simultaneous shifts in phytoplankton and consumer signatures confirmed phytoplankton as the major source of carbon for the food chain leading to fish. Limited sampling coverage within lakes may affect lake wide stable isotope signatures, and the same error is transferred into trophic position estimation. Consideration of potential intra‐lake spatial variability in isotope ratios and size is essential in evaluating the spatial and trophic structure of fish assemblages.     

Rapid Carbon Turnover During Growth of Atlantic Salmon (Salmo salar) Smolts in Sea Water, and Evidence for Reduced Food Consumption by Growth-Stunts

Wild Atlantic salmon smolts were captured during spring out-migration in the Northwest Miramichi River, New Brunswick, Canada, and placed on an isotopically distinct hatchery diet to determine the relative contributions of growth and metabolic turnover to isotopic change. As expected for an ectothermic species, growth explained a large amount of isotopic variation in changing stable carbon ratios of muscle tissue (average r ²= 0.46 ) for the 3 years of study. Turnover rates of muscle carbon in all 3 years in growing fish (0.24–0.66 month^–1) were higher than previously reported values for other ectothermic species, but there was little evidence for isotopic change in non-growers (average r ²= 0.041, p > 0.1). It is unlikely that non-growers had consumed any of the hatchery diet over a 2-month period, thus preventing them from acquiring the new carbon isotopic signature. This period of food deprivation resulted in nitrogen-15 enrichment in liver relative to muscle (p= 0.003). It is advised that future isotope studies of metabolic turnover rates in ectotherms be conducted on slow-growing animals over a long time period. This would serve to avoid the obscuring effects of growth on isotopic change, and provide stronger comparisons to endothermic tissue turnover rates.     

Trophic interference by Salmo trutta on Aplochiton zebra and Aplochiton taeniatus in southern Patagonian lakes

The length and mass ratio, diet and isotopic composition of Aplochiton zebra and Aplochiton taeniatus inhabiting a Salmo trutta‐invaded and a S. trutta‐free lake in southern Patagonia were compared. Results indicate that S. trutta exercises important trophic interference over A. zebra and A. taeniatus, causing changes in their dietary composition by reducing the consumption of winged Diptera through changes in feeding behaviours that involve jumping out of the water. This effect is significantly higher in A. zebra than in A. taeniatus a species that has a highly specialized diet. The dietary changes of A. zebra and A. taeniatus in sympatry with S. trutta lead to an impoverishment of their isotopic nitrogen signals (δ¹⁵N), suggesting a reduction of their trophic position. In the case of A. zebra, this translates into a significant decrease in its body condition factor. Such interference could lead to a population decline of this species and would explain the current distribution range decline and allopatry with S. trutta in fluvial systems.