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The ecological success of shallow water reef-building corals has been linked to the symbiosis between the coral host and its dinoflagellate symbionts (herein ‘symbionts’). As mixotrophs, symbiotic corals depend on nutrients 1) transferred from their photosynthetic symbionts (autotrophy) and 2) acquired by host feeding on particulate organic resources (heterotrophy). However, coral species differ in the extent to which they depend on heterotrophy for nutrition and these differences are typically poorly defined. Here, a multi-tracer fatty acid approach was used to evaluate the trophic strategies of three species of common reef-building coral (Galaxea fascicularis, Pachyseris speciosa, and Pocillopora verrucosa) whose trophic strategies had previously been identified using carbon stable isotopes. The composition and various indices of fatty acids were compared to examine the relative contribution of symbiont autotrophy and host heterotrophy in coral energy acquisition. A linear discriminant analysis (LDA) was used to estimate the contribution of polyunsaturated fatty acids (PUFA) derived from various potential sources to the coral hosts. The total fatty acid composition and fatty acid indices revealed differences between the more heterotrophic (P. verrucosa) and more autotrophic (P. speciosa) coral hosts, with the coral host G. fascicularis showing overlap with the other two species and greater variability overall. For the more heterotrophic P. verrucosa, the fatty acid indices and LDA results both indicated a greater proportion of copepod-derived fatty acids compared to the other coral species. Overall, the LDA estimated that PUFA derived from particulate resources (e.g., copepods and diatoms) comprised a greater proportion of coral host PUFA in contrast to the lower proportion of symbiont-derived PUFA. These estimates provide insight into the importance of heterotrophy in coral nutrition, especially in productive reef systems. The study supports carbon stable isotope results and demonstrates the utility of fatty acid analyses for exploring the trophic strategies of reef-building corals. [ABSTRACT FROM AUTHOR]

We modified the stable isotope mixing model MixSIR to infer primary producer contributions to consumer diets based on their fatty acid composition. To parameterize the algorithm, we generated a ‘consumer-resource library’ of FA signatures of Daphnia fed different algal diets, using 34 feeding trials representing diverse phytoplankton lineages. This library corresponds to the resource or producer file in classic Bayesian mixing models such as MixSIR or SIAR. Because this library is based on the FA profiles of zooplankton consuming known diets, and not the FA profiles of algae directly, trophic modification of consumer lipids is directly accounted for. To test the model, we simulated hypothetical Daphnia comprised of 80% diatoms, 10% green algae, and 10% cryptophytes and compared the FA signatures of these known pseudo-mixtures to outputs generated by the mixing model. The algorithm inferred these simulated consumers were comprised of 82% (63-92%) [median (2.5th to 97.5th percentile credible interval)] diatoms, 11% (4-22%) green algae, and 6% (0-25%) cryptophytes. We used the same model with published phytoplankton stable isotope (SI) data for δ13C and δ15N to examine how a SI based approach resolved a similar scenario. With SI, the algorithm inferred that the simulated consumer assimilated 52% (4-91%) diatoms, 23% (1-78%) green algae, and 18% (1-73%) cyanobacteria. The accuracy and precision of SI based estimates was extremely sensitive to both resource and consumer uncertainty, as well as the trophic fractionation assumption. These results indicate that when using only two tracers with substantial uncertainty for the putative resources, as is often the case in this class of analyses, the underdetermined constraint in consumer-resource SI analyses may be intractable. The FA based approach alleviated the underdetermined constraint because many more FA biomarkers were utilized (n < 20), different primary producers (e.g., diatoms, green algae, and cryptophytes) have very characteristic FA compositions, and the FA profiles of many aquatic primary consumers are strongly influenced by their diets. [ABSTRACT FROM AUTHOR]

There is considerable interest in the pathways by which carbon and growth-limiting elemental and biochemical nutrients are supplied to upper trophic levels. Fatty acids and sterols are among the most important molecules transferred across the plant-animal interface of food webs. In lake ecosystems, in addition to phytoplankton, bacteria and terrestrial organic matter are potential trophic resources for Zooplankton, especially in those receiving high terrestrial organic matter inputs. We therefore tested carbon, nitrogen, and fatty acid assimilation by the crustacean Daphnia magna when consuming these resources. We fed Daphnia with monospecific diets of high-quality (Cryptomonas marssonii) and intermediate-quality (Chlcimydomonas sp. and Scenedesmus gracilis) phytoplankton species, two heterotrophic bacterial strains, and particles from the globally dispersed riparian grass, Pliragmites australis, representing terrestrial particulate organic carbon (t-POC). We also fed Daplmia with various mixed diets, and compared Daphnia fatty acid, carbon, and nitrogen assimilation across treatments. Our results suggest that bacteria were nutritionally inadequate diets because they lacked sterols and polyunsaturated omega-3 and omega-6 (CÜ-3 and co-6) fatty acids (PUFAs). However, Daplmia were able to effectively use carbon and nitrogen from Actinobacteria, if their basal needs for essential fatty acids and sterols were met by phytoplankton. In contrast to bacteria, t-POC contained sterols and co-6 and co-3 fatty acids, but only at 22%, 1.4%, and 0.2% of phytoplankton levels, respectively, which indicated that t-POC food quality was especially restricted with regard to co-3 PUFAs. Our results also showed higher assimilation of carbon than fatty acids from t-POC and bacteria into Daphnia, based on stable-isotope and fatty acids analysis, respectively. A relatively high (>20%) assimilation of carbon and fatty acids from t-POC was observed only when the proportion of t-POC was >60%, but due to low PUFA to carbon ratio, these conditions yielded poor Daphnia growth. Because of lower assimilation for carbon, nitrogen, and fatty acids from t-POC relative to diets of bacteria mixed with phytoplankton, we conclude that the microbial food web, supported by phytoplankton, and not direct t-POC consumption, may support Zooplankton production. Our results suggest that terrestrial particulate organic carbon poorly supports upper trophic levels of the lakes. [ABSTRACT FROM AUTHOR]

We explored how dietary bacteria affect the life history traits and biochemical composition of Daphnia magna, using three bacteria taxa with very different lipid composition. Our objectives were to (1) examine whether and how bacteria-dominated diets affect Daphnia survival, growth, and fecundity, (2) see whether bacteria-specific fatty acid ( FA) biomarkers accrued in Daphnia lipids, and (3) explore the quantitative relationship between bacteria availability in Daphnia diets and the amounts of bacterial FA in their lipids. Daphnia were fed monospecific and mixed diets of heterotrophic ( Micrococcus luteus) or methanotrophic bacteria ( Methylomonas methanica and Methylosinus trichosporium) and two phytoplankton species ( Cryptomonas ozolinii and Scenedesmus obliquus). Daphnia neonates fed pure bacteria diets died after 6-12 days and produced no viable offspring, whereas those fed pure phytoplankton diets had high survival, growth, and reproduction success. Daphnia fed a mixed diet with 80% M. luteus and 20% of either phytoplankton had high somatic growth, but low reproduction. Conversely, Daphnia fed mixed diets including 80% of either methane-oxidizing bacteria and 20% Cryptomonas had high reproduction rates, but low somatic growth. All Daphnia fed mixed bacteria and phytoplankton diets had strong evidence of both bacteria- and phytoplankton-specific FA biomarkers in their lipids. FA mixing model calculations indicated that Daphnia that received 80% of their carbon from bacteria assimilated 46 ± 25% of their FA from this source. A bacteria-phytoplankton gradient experiment showed a strong positive correlation between the proportions of the bacterial FA in the Daphnia and their diet, indicating that bacterial utilization can be traced in this keystone consumer using FA biomarkers. [ABSTRACT FROM AUTHOR]

We collected suspended particulate matter (seston) and zooplankton samples from Lake Washington in Seattle, Washington, USA, over a 10-month period to investigate the effects of food availability on zooplankton fatty acid (FA) composition. The percentage of nutritionally critical ω3 polyunsaturated fatty acids (PUFA) in the seston varied from 8% of the FA pool in midsummer to 30% during the spring diatom bloom. Zooplankton accumulated much higher percentages ω3 PUFA than was available in the seston. In particular, cladocerans preferentially accumulated eicosapentaenoic acid (EPA, 20:5ω3), copepods accumulated docosahexaenoic acid (DHA, 22:6ω3), and both copepods and cladocerans accumulated 18 carbon chain ω3 PUFAs (C18 ω3). By comparison, the FA of zooplanktivorous juvenile sockeye salmon (Oncorhynchus nerka) were strongly dominated by EPA (12.5% ± 2.1%) and DHA (28.2% ± 8.7%). The saturated fatty acid and the arachidonic acid (ARA, 20:4ω6) composition of Diaptomus ashlandi was strongly (r2 = 0.76) and moderately (r2 0.54) correlated with the prevalence of these FAs in the seston. Furthermore, the DHA content of Diaptomus was moderately correlated with the seston's DHA content (r2 = 0.45) and very strongly correlated with seston EPA (r2 = 0.89). Since EPA was the most prevalent PUFA in the seston and DHA was the most prevalent PUFA in Diaptomus, these results suggest that Diaptomus may synthesize DHA from the EPA in their food. In general, zooplankton species in Lake Washington were strongly enriched with those FA molecules that are most physiologically important for fish nutrition (i.e., ARA, EPA, and DHA), indicating a clear mechanism by which changes in seston composition influence fisheries ecology. [ABSTRACT FROM AUTHOR]

Terrestrial organic matter inputs have long been thought to play an important role in aquatic food web dynamics. Results from recent whole lake 13C addition experiments suggest terrestrial particulate organic carbon (t-POC) inputs account for a disproportionate portion of zooplankton production. For example, several studies concluded that although t-POC only represented ≈20% of the flux of particulate carbon available to herbivorous zooplankton, this food source accounted for ≈50% of the C incorporated by zooplankton. We tested the direct dietary impact of t-POC (from the leaves of riparian vegetation) and various phytoplankton on Daphnia magna somatic growth, reproduction, growth efficiency, and lipid composition. By itself, t-POC was a very poor quality resource compared to cryptophytes, diatoms, and chlorophytes, but t-POC had similar food quality compared to cyanobacteria. Small additions of high quality Cryptomonas ozolinii to t-POC-dominated diets greatly increased Daphnia growth and reproduction. When offered alone, t-POC resulted in a Daphnia growth efficiency of 5 ± 1%, whereas 100% Cryptomonas and Scenedesmus obliquus diets resulted in growth efficiencies of 46 ± 8% (± SD) and 36 ± 3%. respectively. When offered in a 50:50 mixed diet with Cryptomonas or Scenedesmus, the t-POC fraction resulted in a partial growth efficiency of 22 ± 9% and 15 ± 6%, respectively. Daphnia that obtained 80% of their available food from t-POC assimilated 84% of their fatty acids from the phytoplankton component of their diet. Overall, our results suggest Daphnia selectively allocate phytoplankton-derived POC and lipids to enhance somatic growth and reproduction, while t-POC makes a minor contribution to zooplankton production. [ABSTRACT FROM AUTHOR]

1. Polyunsaturated fatty acids (PUFA) are almost exclusively synthesized by plants. Animals can convert from one form of PUFA to another through elongation and desaturation, but very few can synthesize PUFA de novo. PUFA play an important role in regulating cell membrane properties, serve as precursors for important animal hormones and are essential for animals. 2. In aquaculture studies, highly unsaturated fatty acids (HUFA), a subset of PUFA, have been found to be critical for maintaining high growth, survival and reproductive rates and high food conversion efficiencies for a wide variety of marine and freshwater organisms. 3. The plankton literature suggests high food-quality algae species are rich in HUFA and low food-quality algae are poor in HUFA. Adding semi-pure emulsions of HUFA to algae monocultures can markedly increase the growth rates of zooplankton feeding on these mixtures. 4. A study measuring zooplankton biomass accrual when feeding on natural phytoplankton found a strong correlation between phytoplankton HUFA (specifically eicosapentaenoic acid) content and herbivorous zooplankton production. 5. The aquatic ecology literature suggests that planktonic foodwebs with high HUFA content phytoplankton have high zooplankton to phytoplankton biomass ratios, while systems with low HUFA phytoplankton have low zooplankton biomass. Also, the seasonal succession of plankton in many temperate lakes follows patterns tied to phytoplankton HUFA content, with intense zooplankton grazing and 'clear-waterphases' characteristic of periods when the phytoplankton is dominated by HUFA-rich species. 6. Herbivorous zooplankton production is constrained by the zooplankton's ability to ingest and digest phytoplankton. It is becoming increasingly clear, however, that much of the phytoplankton which is assimilated may be nutritionally inadequate. HUFA may be key nutritional constituents of zooplankton diets, and may determine energetic efficiency across the plant-animal interface, secondary production and the strength of trophic coupling in aquatic pelagic foodwebs. [ABSTRACT FROM AUTHOR]

Algae are generally a high‐quality diet source because they provide essential compounds to aquatic consumers. In forested stream ecosystems, the availability of high‐quality algae is low compared to terrestrial organic matter, which may constrain the dietary transfer of essential compounds to consumers. However, there could be other overlooked high‐quality resource pools that provide essential compounds to consumers in river ecosystems.We conducted a field study along a subalpine river continuum in Austria to identify the nutritional role of a 'hidden' food resource for aquatic consumers; the biofilms growing on the underside of rocks (dark biofilms). Dark and light (i.e. upper surface of rocks) biofilms and invertebrates were collected, and their fatty acid (FA) composition was analysed.Compared with light biofilms, dark biofilms contained greater proportions of bacterial FA, long‐chain saturated FA (biomarkers of terrestrial plants) and oleic acid (18:1ω9; a fungal biomarker), but a lower proportion of algal FA, especially omega‐3 polyunsaturated FA (ω3 PUFA). The ω3 PUFA composition in dark biofilms was strongly correlated with that in light biofilms. Furthermore, the overall FA profiles of dark biofilms were significantly associated with invertebrate FA profiles. Strong correlations were also observed between invertebrates and dark biofilms for bacterial FA and the ω3 PUFA eicosapentaenoic acid (EPA, 20:5ω3).Synthesis. This field study demonstrates that dark biofilms are a high‐quality resource pool for invertebrates in river ecosystems that is often overlooked. Similar to light biofilms, dark biofilms provide physiologically important FA and bacterial FA for stream invertebrates. However, these high‐quality resources are threatened by increasing human disturbances to river ecosystems. Future research is required to better understand how the quality of both dark and light biofilms changes in response to human disturbance, and how this affects dietary energy transfer to upper trophic consumers, including fish and humans. [ABSTRACT FROM AUTHOR]

It is well established that terrestrial and aquatic ecosystems are interlinked in a myriad of ways, yet understanding of the role terrestrial particulate organic matter (OM) plays in supporting aquatic macroinvertebrate production is still incomplete and debated. We investigated the role of terrestrial particulate OM (leaf litter) and autochthonous OM (macrophytes) with and without biofilm as food sources for the model benthic macroinvertebrate Hyalella (Amphipoda). We hypothesised that biofilm (algae, fungi, bacteria) associated with terrestrial and aquatic plants is the main food source for benthic primary consumers. First, we microbially conditioned the leaf litter in situ for 10-weeks. Then, we performed a laboratory feeding experiment with nine treatments: no food, conditioned leaf litter from two native terrestrial plants, non-conditioned leaf litter from these plants, two freshly collected macrophyte species with periphyton, and these macrophytes without periphyton. We measured the growth and analysed the fatty acid (FA) composition of field-collected and experimental amphipods reared on these diets. We found that amphipods fed food sources without biofilm grew poorly and had a FA content and composition resembling that of starved animals. The same was observed for amphipods fed one leaf litter type with biofilm. However, animals fed the other leaf litter type with biofilm grew during the feeding experiment and showed a characteristic FA composition linked to the higher fungal biomass in this food source. We showed that amphipods fed macrophytes with biofilm resembled field-collected animals regarding FA amounts and composition; monounsaturated FA representing >40% of total FA, with 16:1n-7 (palmitoleic acid) and 18:1n-9 (oleic acid) being the most abundant, and polyunsaturated FA representing 30% of total FA. The FA data indicated algae, particularly diatoms, as evidenced by large quantities of its biomarker 16:1n-7, were the main food for the amphipods. We conclude diatoms were the peanut butter on crackers for the amphipods, and diatom consumption supported its somatic growth and polyunsaturated FA composition. In contrast, bacteria and fungi on conditioned leaf litter were lower quality food sources because they do not provide essential FA for macroinvertebrates growth and physiology. These results have important ecological implications from a food web perspective. [ABSTRACT FROM AUTHOR]

We performed 12 experiments where the herbivorous zooplankter Daphnia pulex was fed three different phytoplankton food types (cyanophytes, chlorophytes, or cryptophytes) at two phosphorus (P) deficiency levels (C : P ∼ 400 or 600) and with two different fatty acid (FA) supplements. Phosphatidylcholine liposome amendments were used to manipulate P availability and/or deliver FA in order to simultaneously test the relative importance of phytoplankton P and FA limitation as constraints on Daphnia somatic growth rate and reproduction. We used multiple regression analyses to test the data set for six main effects and three interaction terms. The six main effects tested were phytoplankton taxa, FA supplementation, FA type (EPA or FA-mix), direct (P-D) and indirect (P-ID) phosphorus limitation, and P deficiency level. Food quality was most strongly affected by phytoplankton taxa followed by P-ID, FA supplementation, the interaction between phytoplankton taxa and P-ID, and P-D. Daphnia fed cryptophytes grew 0.18 day-1 faster and had an additional 2.6 eggs individual-1 than Daphnia fed cyanophytes. Indirect P limitation reduced Daphnia somatic growth rates by 0.16 day-1 and egg production by 2.6 eggs individual-1. Direct P limitation reduced Daphnia growth rates by 0.04 day-1 and egg production by 1.2 eggs individual-1. FA supplementation improved growth by 0.09 day-1 and egg production by 2.2 eggs individual-1. These results suggest that FA supplementation exerted stronger effects on Daphnia somatic growth rate and reproduction than did direct P limitation. Furthermore, these results suggest that phytoplankton taxa and indirect P limitation had the greatest effects on Daphnia growth and reproduction. [ABSTRACT FROM AUTHOR]

We conducted a series of experiments feeding Daphnia pulex nine different phytoplankton monocultures with widely varying fatty acid composition and nutritional values to test the extent to which Daphnia fatty acid composition was affected by diet. In general, Daphnia fatty acid composition matched that of their diet much more closely than it did the fatty acid composition of Daphnia consuming other diets. However, Daphnia had consistently less saturated fatty acids and more arachidonic acid than did their diet, and Daphnia consuming cyanobacteria had substantially less saturated fatty acids and more monounsaturated fatty acids than their diets. Daphnia that consumed cryptophytes, which are rich in ϖ3 polyunsaturated fatty acids (PUFAs), had on average 47% ± 8% (61 SD) ϖ3 PUFAs within their fatty acid pool, whereas Daphnia that consumed ϖ3 PUFA-poor cyanophytes only had 6% ± 3% ϖ3 PUFAs. The ratio of ϖ3 to ϖ6 fatty acids in Daphnia was also strongly dependent on diet, and averaged ∼ 10 : 1, 2 : 1, and 1 : 1 for Daphnia that consumed cryptophytes, chlorophytes, and cyanophytes, respectively. Furthermore, the sum of C20 and C22 ϖ3 and ϖ6 fatty acids in Daphnia was highly correlated with that of their diet (r² = 0.94). These results suggest analyses of Daphnia fatty acid composition may be a powerful means of inferring diet in the field. These results also suggest the nutritional benefits of consuming ϖ3-rich phytoplankton will transfer up the food web, making zooplankton both more efficient at converting phytoplankton biomass to their own biomass as well as much more nutritious for the zooplanktivorous fish that consume them. [ABSTRACT FROM AUTHOR]

Physical attributes of rivers control the quantity and quality of energy sources available to consumers, but it remains untested whether geomorphic conditions of whole watersheds affect the assimilation of different resources by stream organisms. We compared the fatty acid (FA) compositions of two invertebrate taxa (caddisflies, mayflies) collected from 16 streams in southwest Alaska, USA, to assess how assimilation of terrestrial organic matter (OM) and algae varied across a landscape gradient in watershed features. We found relatively higher assimilation of algae in high-gradient streams compared with low-gradient streams, and the opposite pattern for assimilation of terrestrial OM and microbes. The strength of these patterns was more pronounced for caddisflies than mayflies. Invertebrates from low-gradient watersheds had FA markers unique to methane-oxidizing bacteria and sulfate-reducing microbes, indicating a contribution of anaerobic pathways to primary consumers. Diversity of FA composition was highest in watersheds of intermediate slopes that contain both significant terrestrial inputs as well as high algal biomass. By controlling the accumulation rate and processing of terrestrial OM, watershed features influence the energetic base of food webs in boreal streams. [ABSTRACT FROM AUTHOR]

There is growing recognition of the importance of food quality over quantity for aquatic consumers. In streams and rivers, most previous studies considered this primarily in terms of the quality of terrestrial leaf litter and importance of microbial conditioning. However, many recent studies suggest that algae are a more nutritional food source for riverine consumers than leaf litter. To date, few studies have quantified longitudinal shifts in the nutritional quality of basal food resources in river ecosystems and how these may affect consumers.We conducted a field investigation in a subalpine river ecosystem in Austria to investigate longitudinal variations in diet quality of basal food sources (submerged leaves and periphyton) and diet source dependence of stream consumers (invertebrate grazers, shredders, filterers and predators, and fish). Fatty acid (FA) profiles of basal food sources and their consumers were measured.Our results indicate systematic differences between the FA profiles of terrestrial leaves and aquatic biota, that is periphyton, invertebrates and fish. Submerged leaves contained very low proportions of long‐chain polyunsaturated fatty acids (LC‐PUFAs), which were conversely rich in aquatic biota. While the FA composition of submerged leaves remained similar among sites, the LC‐PUFAs of periphyton increased longitudinally, which was associated with increasing nutrients from upstream to downstream.Longitudinal variations in periphyton LC‐PUFAs were reflected in the LC‐PUFAs of invertebrate grazers and shredders, and further tracked by invertebrate predators and fish. However, brown trout Salmo trutta contained a large proportion of docosahexaenoic acid (DHA, 22:6ω3), a LC‐PUFA almost entirely missing in basal sources and invertebrates. The fish accumulated eicosapentaenoic acid (EPA, 20:5ω3) from invertebrate prey and may use this FA to synthesize DHA.Our results provide a nutritional perspective for river food web studies, emphasizing the importance of algal resources to consumer somatic growth and the need to account for the longitudinal shifts in the quality of these basal resources. [ABSTRACT FROM AUTHOR]

Zooplankton transfer ecologically important fatty acids (FA) from their diets to upper trophic levels. We used diet-switching experiments with 13C-labeled food sources to determine the time scale at which dietary uptake is manifested in the FA profiles of Daphnia magna. Daphnia dramatically shifted their FA composition in response to diet change within only four days, however Daphnia switched from a high quality (i.e. Cryptomonas) to a moderate quality ( Scenedesmus) diet retained the most physiologically important FA from their original diet source even after 14 days. In particular, Daphnia exhibited long-term retention of eicosapentaenoic (EPA; 20:5ω3) and arachidonic acid (ARA; 20:4ω6) when switched from Cryptomonas to Scenedesmus. Similarly, when switched from Scenedesmus to Cryptomonas, Daphnia took up a high proportion of EPA and ARA after only two days. The phospholipid fatty acid (PLFA) fraction in Daphnia was preferentially enriched with stearic (18:0), oleic (18:1ω9), and linoleic acid (LIN; 18:2ω6). In contrast with studies of marine copepods, dietary FA also strongly affected the PLFA composition (structural lipids) of Daphnia. Results of δ13C signatures of individual FA provided evidence of elongation and desaturation of α-linolenic (ALA; 18:3ω3) or stearidonic acid (SDA; 18:4ω3) to EPA 10 days after a diet switch to EPA-deficient Scenedesmus. Differences in the ARA content of Daphnia fed Cryptomonas and Scenedesmus suggest Daphnia consuming Cryptomonas synthesized ARA via retroconversion of ω6-docosapentaenoic acid (ω6-DPA; 22:5ω6). Daphnia preferentially accumulate and retain, as well as bioconvert, those FA that are also most physiologically important for fish production. Our results also indicate Daphnia FA composition responds to their diet on a short temporal scale and analyses of lipid biomarkers in zooplankton provide strong insights into the food sources that support their production. [ABSTRACT FROM AUTHOR]

According to the River Continuum Concept, headwater streams are richer in allochthonous (e.g. terrestrial leaves) than autochthonous (e.g. algae) sources of organic matter for consumers. However, compared to algae, leaf litter is of lower food quality, particularly ω‐3 polyunsaturated fatty acids (n‐3 PUFA), and would constrain the somatic growth, maintenance, and reproduction of stream invertebrates. It may be thus assumed that shredders, such as Gammarus, receive lower quality diets than grazers, e.g. Ecdyonurus, that typically feed on algae.The objective of this study was to assess the provision of dietary PUFA from leaf litter and algae to the shredder Gammarus and the grazer Ecdyonurus. Three different diets (algae, terrestrial leaves, and an algae–leaf litter mix) were supplied to these macroinvertebrates in a flume experiment for 2 weeks. To differentiate how diet sources were retained in these consumers, algae were isotopically labelled with 13C.Both consumers became enriched with 13C in all treatments, demonstrating that both assimilated algae. For Gammarus, n‐3 PUFA increased, whereas n‐6 PUFA stayed constant. By contrast, the n‐3 PUFA content of Ecdyonurus decreased as a consequence of declining algal supply.Results from compound‐specific stable isotope analysis provided evidence that the long‐chain n‐3 PUFA eicosapentaenoic acid (EPA) in both consumers was more enriched in 13C than the short‐chain n‐3 PUFA α‐linolenic acid, suggesting that EPA was taken up directly from algae and not from heterotrophic biofilms on leaf litter. Both consumers depended on algae as their carbon and EPA source and retained their EPA from high‐quality algae. [ABSTRACT FROM AUTHOR]

Stable isotope mixing models in aquatic ecology require δ13C values for food web end members such as phytoplankton and bacteria, however it is rarely possible to measure these directly. Hence there is a critical need for improved methods for estimating the δ13C ratios of phytoplankton, bacteria and terrestrial detritus from within mixed seston. We determined the δ13C values of lipids, phospholipids and biomarker fatty acids and used these to calculate isotopic differences compared to the whole-cell δ13C values for eight phytoplankton classes, five bacterial taxa, and three types of terrestrial organic matter (two trees and one grass). The lipid content was higher amongst the phytoplankton (9.5±4.0%) than bacteria (7.3±0.8%) or terrestrial matter (3.9±1.7%). Our measurements revealed that the δ13C values of lipids followed phylogenetic classification among phytoplankton (78.2% of variance was explained by class), bacteria and terrestrial matter, and there was a strong correlation between the δ13C values of total lipids, phospholipids and individual fatty acids. Amongst the phytoplankton, the isotopic difference between biomarker fatty acids and bulk biomass averaged -10.7±1.1‰ for Chlorophyceae and Cyanophyceae, and -6.1±1.7‰ for Cryptophyceae, Chrysophyceae and Diatomophyceae. For heterotrophic bacteria and for type I and type II methane-oxidizing bacteria our results showed a -1.3±1.3‰, -8.0±4.4‰, and -3.4±1.4‰ δ13C difference, respectively, between biomarker fatty acids and bulk biomass. For terrestrial matter the isotopic difference averaged -6.6±1.2‰. Based on these results, the δ13C values of total lipids and biomarker fatty acids can be used to determine the δ13C values of bulk phytoplankton, bacteria or terrestrial matter with ± 1.4‰ uncertainty (i.e., the pooled SD of the isotopic difference for all samples). We conclude that when compound-specific stable isotope analyses become more widely available, the determination of δ13C values for selected biomarker fatty acids coupled with established isotopic differences, offers a promising way to determine taxa-specific bulk δ13C values for the phytoplankton, bacteria, and terrestrial detritus embedded within mixed seston. [ABSTRACT FROM AUTHOR]

The zooplankton is a key link in the transfer of energy from primary producers up through aquatic food webs. Previous efforts to quantify the importance of basal resources to aquatic consumers have used stable isotopes ( SI) and simple ternary models, including only 'bulk' phytoplankton, bacteria or terrestrial particulate organic matter (t- POM)., We used a novel Bayesian mixing model based on fatty acids ( FA) to quantify the dietary assimilation of seven basal resources, including five phytoplankton groups, pelagic bacteria and t- POM, to Cladocera in large boreal lakes in Finland. To account for trophic enrichment of FA from the diet to consumers, we parameterised the model with a resource library, from many feeding trials, consisting of Daphnia magna fed 22 diverse basal taxa., The results of the feeding trials show that the distinctive FA profiles of algal groups are transferred to consumers. Moreover, the large number of FA variables ( n = 22) used in the model avoids the limitations of underdetermined mixing problems, common to SI modelling, in cases when the number of resources outnumbers the tracer variables., We show that cladocerans were generally supported by phytoplankton (86-94%), with little use of t- POM (1-9%) and bacteria (1-3%). Cladocerans used primarily high-quality phytoplankton (cryptophytes, diatoms and dinoflagellates) in both summer (51 ± 22%) and autumn (79 ± 12%), and the relative importance of medium-quality resources (cyanobacteria, chlorophytes and chrysophytes) declined from 37 ± 23% in the summer to 8 ± 2% in the autumn., High-quality resources, rich in essential biochemical compounds, are critical in fuelling food webs in large lakes, even those with high concentrations of allochthonous organic matter. [ABSTRACT FROM AUTHOR]

Primary productivity by plants and algae is the fundamental source of energy in virtually all food webs. Furthermore, photosynthetic organisms are the sole source for ω-3 and ω-6 essential fatty acids (EFA) to upper trophic levels. Because animals cannot synthesize EFA, these molecules may be useful as trophic markers for tracking sources of primary production through food webs if different primary producer groups have different EFA signatures. We tested the hypothesis that different marine macrophyte groups have distinct fatty acid (FA) signatures by conducting a phylogenetic survey of 40 marine macrophytes (seaweeds and seagrasses) representing 36 families, 21 orders, and four phyla in the San Juan Archipelago, WA, USA. We used multivariate statistics to show that FA composition differed significantly ( P < 0.001) among phyla, orders, and families using 44 FA and a subset of seven EFA ( P < 0.001). A second analysis of published EFA data of 123 additional macrophytes confirmed that this pattern was robust on a global scale ( P < 0.001). This phylogenetic differentiation of macrophyte taxa shows a clear relationship between macrophyte phylogeny and FA content and strongly suggests that FA signature analyses can offer a viable approach to clarifying fundamental questions about the contribution of different basal resources to food webs. Moreover, these results imply that taxa with commercially valuable EFA signatures will likely share such characteristics with other closely related taxa that have not yet been evaluated for FA content. [ABSTRACT FROM AUTHOR]

Lethal control, which has been used to reduce local abundances of animals in conflict with humans or with endangered species, may not achieve management goals if animal movement is not considered. In populations with emigration and immigration, lethal control may induce compensatory immigration, if the source of attraction remains unchanged. Within the Columbia River Basin (Washington, U.S.A.), avian predators forage at dams because dams tend to reduce rates of emigration of juvenile salmonids (Oncorhynchus spp.), artificially concentrating these prey. We used differences in fatty acid profiles between Caspian Terns (Hydroprogne caspia) at coastal and inland breeding colonies and terns culled by a lethal control program at a mid-Columbia River dam to infer dispersal patterns. We modeled the rate of loss of fatty acid biomarkers, which are fatty acids that can be traced to a single prey species or groups of species, to infer whether and when terns foraging at dams had emigrated from the coast. Nonmetric multidimensional scaling showed that coastal terns had high levels of C20 and C22 monounsaturated fatty acids, whereas fatty acids of inland breeders were high in C18:3n3, C20:4n6, and C22:5n3. Models of the rate of loss of fatty acid showed that approximately 60% of the terns collected at Rock Island Dam were unlikely to have bred successfully at local (inland) sites, suggesting that terns foraging at dams come from an extensive area. Fatty acid biomarkers may provide accurate information about patterns of dispersal in animal populations and may be extremely valuable in cases where populations differ demonstrably in prey base. [ABSTRACT FROM AUTHOR]