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Organisms at the base of stream food webs are typically poor in long‐chain polyunsaturated fatty acids (LC‐PUFA), especially in docosahexaenoic acid (DHA), whereas consumers at higher trophic levels are often rich in LC‐PUFA. For example, fish tissues, especially the brain, are DHA‐rich. This obvious mismatch between consumer LC‐PUFA and their basal dietary supply may result from selective retention and/or endogenous conversion of dietary precursors to LC‐PUFA.To determine which is more likely, we investigated compound‐specific carbon stable isotopes in PUFA (δ13CPUFA) of potential basal resources (stream epilithon, leaf litter) and consumers (invertebrates, European bullhead, and two salmonid species and their brain, eye, liver, and muscle tissues). We predicted that consumer‐PUFA, depleted in 13C values relative to their dietary sources, would indicate internal de novo PUFA synthesis. Alternatively, higher consumer‐δ13CPUFA values would imply selective retention of aquatic (epilithon and conditioned leaves) rather than terrestrial resources or internal production, irrespective of trophic levels.Invertebrate grazers and predators resembled δ13C values of essential fatty acids (δ13CEFA) of benthic algae, while shredder‐δ13CEFA values reflected those of conditioned rather than fresh leaves. Lower eye‐δ13CEFA values of salmonids than in livers indicated high retention of dietary PUFA sources (invertebrates and epilithon). Stable isotope values of eicosapentaenoic acid suggest that all consumers retained algal EPA, while insectivorous fish produced DHA in their liver. There is no further evidence from carbon stable isotopes for local PUFA conversion within neural tissues.Our study demonstrates that δ13CPUFA can be used to track sources of these highly functional molecules in aquatic consumers and highlights the importance of algal derived‐PUFA for these consumers in oligotrophic headwater streams. [ABSTRACT FROM AUTHOR]

Many freshwater systems receive substantial inputs of terrestrial organic matter. Terrestrially derived dissolved organic carbon (t-DOC) inputs can modify light availability, the spatial distribution of primary production, heat, and oxygen in aquatic systems, as well as inorganic nutrient bioavailability. It is also well-established that some terrestrial inputs (such as invertebrates and fruits) provide high-quality food resources for consumers in some systems., In small to moderate-sized streams, leaf litter inputs average approximately three times greater than the autochthonous production. Conversely, in oligo/mesotrophic lakes algal production is typically five times greater than the available flux of allochthonous basal resources., Terrestrial particulate organic carbon (t-POC) inputs to lakes and rivers are comprised of 80%-90% biochemically recalcitrant lignocellulose, which is highly resistant to enzymatic breakdown by animal consumers. Further, t-POC and heterotrophic bacteria lack essential biochemical compounds that are critical for rapid growth and reproduction in aquatic invertebrates and fishes. Several studies have directly shown that these resources have very low food quality for herbivorous zooplankton and benthic invertebrates., Much of the nitrogen assimilated by stream consumers is probably of algal origin, even in systems where there appears to be a significant terrestrial carbon contribution. Amino acid stable isotope analyses for large river food webs indicate that most upper trophic level essential amino acids are derived from algae. Similarly, profiles of essential fatty acids in consumers show a strong dependence on the algal food resources., Primary production to respiration ratios are not a meaningful index to assess consumer allochthony because respiration represents an oxidised carbon flux that cannot be utilised by animal consumers. Rather, the relative importance of allochthonous subsidies for upper trophic level production should be addressed by considering the rates at which terrestrial and autochthonous resources are consumed and the growth efficiency supported by this food., Ultimately, the biochemical composition of a particular basal resource, and not just its quantity or origin, determines how readily this material is incorporated into upper trophic level consumers. Because of its highly favourable biochemical composition and greater availability, we conclude that microalgal production supports most animal production in freshwater ecosystems. [ABSTRACT FROM AUTHOR]

1. Analyses of zooplankton fatty acid (FA) composition in laboratory experiments and samples collected from lakes in New Zealand spanning a wide gradient of productivity were used to assess the extent to which FAs might infer their diet. We used the cladocerans, Daphnia and Ceriodaphnia, and the calanoid copepod, Boeckella, as test organisms, and monocultures of cryptophytes, chlorophytes and cyanobacteria as food. Based on reproductive success, cryptophytes were the highest food quality, chlorophytes were intermediate and cyanobacteria the poorest. 2. Several FA groups were highly correlated between zooplankton and their diets. They were monounsaturated fatty acids (MUFAs), and ω3 and ω6 polyunsaturated fatty acids (PUFAs) for cladocerans, and saturated fatty acids (SAFAs) and ω3 PUFAs for copepods. Several FAs varied significantly less in the zooplankton than in their monoculture diets, e.g. MUFAs in Daphnia, and ω3 and ω6 PUFAs in Ceriodaphnia, despite clear dietary dependency for these FAs. 3. Zooplankton collected from lakes in New Zealand had more eicosapentaenoic acid (EPA) ( Daphnia), more highly unsaturated ω3 and ω6 FAs (C, C; Daphnia, Ceriodaphnia, Boeckella) and less ω3 C PUFAs ( Daphnia, Ceriodaphnia, Boeckella) and ω6 C PUFAs ( Daphnia, Ceriodaphnia) than measured in the same species reared on phytoplankton in the laboratory. 4. Analyses of FA composition of seston and freshwater zooplankton globally showed that, in general, zooplankton had a significantly higher proportion of arachidonic acid and EPA than seston, and copepods also had a higher percentage of docosahexaenoic acid than seston. 5. These results suggest that zooplankton selectively incorporate the most physiologically important FAs. This could be a consequence of preferential assimilation, selective feeding on more nutritious cells or locating and feeding within higher food quality food patches. [ABSTRACT FROM AUTHOR]

1. We conducted a statistical reassessment of data previously reported in the lake total phosphorus (TP) input/output literature ( n = 305) to determine which lake characteristics are most strongly associated with lake phosphorus concentration and retention. We tested five different hypotheses for predicting lake TP concentrations and phosphorus retention. 2. The Vollenweider phosphorus mass loading model can be expressed as: TPout = TPin/(1 + σ τw), where TPin is the flow-weighted input TP concentration, τw is the lake hydraulic retention time and σ is a first-order rate constant for phosphorus loss. 3. The inflow-weighted TP input concentration is a moderately strong predictor ( r2 = 0.71) of lake phosphorus concentrations when using log–log transformed data. Lake TP retention is negatively correlated with lake hydraulic retention time ( r2 = 0.35). 4. Of the approaches tested, the best fit to observed data was obtained by estimating σ as an inverse function of the lake's hydraulic retention time. Although this mass balance approach explained 84% of the variability in log–log transformed data, the prediction error for individual lakes was quite high. 5. Estimating σ as the ratio of a putative particle settling velocity to the mean lake depth yielded poorer predictions of lake TP ( r2 = 0.77) than the approach described above, and in fact did not improve model performance compared with simply assuming that σ is a constant for all lakes. 6. Our results also demonstrate that changing the flow-weighted input concentration should always have a directly proportionate impact on lake phosphorus concentrations, provided the type of phosphorus loaded (e.g. dissolved or particulate) does not vary. [ABSTRACT FROM AUTHOR]

1. A series of experiments examined growth of Daphnia magna on three algal diets ( Rhodomonas minuta , Scenedesmus acutus and Synechococcus sp.) at varying physiological states [nitrogen and phosphorus (P) limitations] to test whether variation in algal fatty acid and/or elemental composition can predict Daphnia growth. 2. These algae differed widely in their essential fatty acid (EFA) composition while phosphorus (P) or nitrogen limitation had only a small influence on their ω3-polyunsaturated fatty acid (PUFA) content. 3. Individually, algal ω3-PUFA content explained 69% of the variation in the normalised growth of D. magna, while algal phosphorus content explained 11% of the variation. Quantitative models for D. magna growth used both algal ω3-PUFA content and algal C : P ratio as food quality indices. Together, algal ω3-PUFA content and C : P ratio explained 70% of the variation in the normalised growth rate of D. magna . 4. Our results indicate that EFA influenced algal food quality much more strongly than P. The EFA and mineral P impacts appear to be independent. [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]

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]

The importance of allochthonous organic matter contributions to aquatic food webs has long been debated. We studied the aging process of terrestrial particulate organic matter (tPOM) in aquatic ecosystems by incubating senescent plant material in the presence of natural microbial communities in either lake water or artificial growth medium. We hypothesised that the aging would change the biomarker composition of tPOM, and would improve its nutritional quality for Daphnia due to microbial conditioning. We also hypothesised that when algal availability is low, Daphnia can partially compensate for the low quality of tPOM when more of this resource is available to them.The fatty acid biomarkers of tPOM changed very little during short‐ and long‐term incubations. In particular, the proportion of long‐chain saturated fatty acids was quite stable even during long‐term incubations. A decrease in carbohydrate and ω‐3 polyunsaturated fatty acid content, and an increase in the bacterial and fungal lipid biomarkers indicated that microbial decomposition of tPOM took place during the incubation. The δ15N value of aged tPOM increased, which indicated microbial colonisation, while the δ13C value did not change during aging.Daphnia magna survival and reproductive output were low when fed diets containing fresh or aged tPOM. The reproductive output of Daphnia was slightly enhanced in the presence of aged tPOM as compared to fresh tPOM, but only when tPOM comprised ≥ 95% of the diet.When Daphnia were offered low (0.5 mg C/L) concentrations of algae, even high additions of tPOM (up to 15 mg C/L) did not increase Daphnia growth rates. Daphnia reproduction increased with tPOM additions, presumably due to the sterols and ω‐3 polyunsaturated fatty acid in the tPOM that Daphnia retained from their diet.Our results indicate that tPOM is a poor resource for pelagic cladocerans, even when aged in the presence of microbes or offered in large quantities. [ABSTRACT FROM AUTHOR]

Streams and rivers provide crucial sources of long-chain polyunsaturated fatty acids ( LC- PUFA, a subset of PUFA with ≥20 C in their acyl chains) for freshwater and terrestrial animals. LC- PUFA are primarily produced by particular algal taxa and are subsequently transferred to fish and higher consumers, including humans. Although research on the trophic transfer of LC- PUFA in streams is very limited, studies on the PUFA composition of organisms at various trophic levels within stream food webs are well documented. Here, we systematically analyze the literature to synthesise LC- PUFA distribution and retention in stream ecosystems., In general, stream food webs are highly enriched in omega-3 LC- PUFA, in particular eicosapentaenoic acid ( EPA, 20:5n-3) and docosahexaenoic acid ( DHA, 22:6n-3). Algae from temperate rivers had more EPA compared with algae from subtropical rivers. Stream invertebrates from both temperate and subtropical rivers preferentially retain algal EPA and their PUFA tend to vary with algal PUFA. Invertebrates may be capable of regulating their fatty acid ( FA) composition, but this ability is limited, to the extent that they must obtain physiologically important LC- PUFA primarily from algae., DHA is selectively and highly retained in all fish taxa irrespective of their feeding sites (i.e. temperate or subtropical rivers). However, DHA content is variable among stream fish taxa and is likely linked to resource utilisation and their endogenous metabolism. Freshwater salmonids have a very high DHA content, which suggests they preferentially retain this FA or biosynthesise it from EPA. The PUFA profiles of other fish taxa more closely reflect their dietary PUFA supply. However, so far very few studies have considered the differences in the PUFA biosynthesis of fish taxa in rivers or compared the dietary PUFA effect on different fish taxa., The supply of LC- PUFA from streams and rivers is known to be affected by altered light availability, greater nutrient inputs and other anthropogenic activities, and any decrease in algal LC- PUFA production will negatively affect the consumers that are nutritionally dependent on these molecules. Further investigations are needed to understand how environmental changes affect LC- PUFA production and retention in stream ecosystems, and how to protect streams from threats to this essential ecosystem service. [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]

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]