Basal resource quality and energy sources in three habitats of a lowland river ecosystem

Loss of riparian canopy cover reduces the transfer of polyunsaturated fatty acid (PUFA) and simplifies the trophic links in stream food webs.

Fatty acid (FA) analysis is used increasingly to investigate the trophic structure of soil animal food webs as the technique allows separation of the role of detrital resources such as bacteria, fungi and plant material for consumer nutrition. The applicability of FAs as biomarkers for different diets has been verified for Collembola and Nematoda. However, for the analysis of whole food webs it is crucial to know whether marker FA are valid for different taxa and whether they are transferred along the food chain to higher trophic levels, i.e. predators. Top-predators are integrators of lower level energy fluxes in food webs; therefore analysis of their FAs may allow to identify trophic pathways and to separate bacterial vs. fungal based energy channels. Chilopoda and Arachnida are among the main predators in soil food webs. Our aim was to test the applicability of marker FAs for these two predator taxa and to verify the trophic transfer of marker FAs of different basal resources via first order consumers into predators, i.e. over three trophic levels. Therefore, we investigated the transfer of FAs from different basal resources [fungi (Chaetomium globosum), plant leaf litter (Tilia europaea), Gram-positive (Bacillus amyloliquefaciens) and Gram-negative bacteria (Stenotrophomonas maltophilia)] via Collembola (Heteromurus nitidus) as first order consumers into predators [Lithobius forficatus (Chilopoda) and Pardosa lugubris (Arachnida)]. Fatty acid profiles of predators of food chains with different basal resources differed significantly. Marker FAs of basal resources were clearly detectable in predators, suggesting that FA analysis allows to separate trophic channels of soil food webs. By reflecting basal resources, FAs of predators allow tracking energy/resource fluxes through the food web and thereby clarifying the relative importance of bacterial vs. fungal vs. plant resources for soil animal food webs.

Resource quality and quantity are critical drivers in shaping trophic interactions and food web structures in aquatic ecosystems. However, the lack of clarity on how these drivers distinctly influence energy flow and trophic pyramids represents a significant gap in understanding the mechanisms governing ecosystem stability and productivity. This study explored how resource quality and quantity influenced trophic interactions and food web pyramids across a spatial gradient of aqueous nutrient levels. Resource quality was assessed by omega‐3 (ω3) long‐chain polyunsaturated fatty acids (LC‐PUFA), while resource quantity was evaluated based on biomass. Food web components were collected, including algae (phytoplankton and periphyton) and their consumers (zooplankton, macroinvertebrates and fish). Our results showed that higher ambient nutrient concentrations significantly boosted phytoplankton biomass, leading to a bottom‐heavy biomass pyramid in the low‐quality food group. However, this increase in quantity was accompanied by a notable reduction in ω3 LC‐PUFA in primary producers, resulting in a distinct FA stock pyramid with a narrowed base and middle. This pattern suggests that despite high phytoplankton biomass, poor food quality created a resource quality bottleneck that constrained the transfer of essential fatty acids. This reduction in resource quality simplified the transfer pathways of ω3 LC‐PUFA to piscivorous fish, limiting their dietary options and weakening trophic connections. Notably, the unusual role of planktivorous fish in accumulating FA at the second trophic level in the study area, in contrast to typical trends observed in other regions, highlights how variations in species composition and resource quality can reshape trophic structure and influence energy flow. Our findings emphasise that declines in resource quality exerted a greater influence on food web dynamics than declines in resource quantity. Our study underscores the importance of considering resource quality, alongside quantity, to maintain ecosystem stability and resilience in nutrient‐enriched aquatic systems.

Essential biomolecules, such as physiologically essential fatty acids, can critically influence consumers' performance and the ecosystem's functioning. Eicosapentaenoic (EPA; 20:5ω3) and docosahexaenoic (DHA; 22:6ω3) fatty acids are physiologically crucial for consumers, and they must be either obtained from the diet or bioconverted from precursors. We monitored the synthesis of EPA and DHA by primary producers in the largest man‐made ecosystem (Lake Kariba) and in situ fatty acid production, trophic transfer, and endogenous production of EPA and DHA in the tropical lake food web using 13C‐labeling, compound‐specific isotopes, and gene expression of fads2 and elovl5 genes in most abundant fish species. Seston pigment analysis and 23S rRNA sequencing revealed that cyanobacteria dominated primary producers throughout three seasons, and the biosynthesis rate of EPA and DHA was under the detection limit. Moreover, due to the low zooplankton densities and EPA and DHA content in zooplankton, the transfer of EPA and DHA from phytoplankton–zooplankton to upper trophic levels is low. The low production of EPA and DHA by primary producers is mitigated by bioconversion of α‐linolenic acid to EPA and DHA in two tilapia species, especially by Nile tilapia (Oreochromis niloticus) known to feed on cyanobacteria. Compound‐specific isotope analysis revealed that tigerfish (Hydrocynus vittatus), the main predatory fish on the lake, was more closely related to Nile tilapia than to lake planktivorous fish (Limnothrissa miodon). Therefore, trophic interaction between cyanobacteria and algivorous fish has replaced traditional phytoplankton and zooplankton trophic interaction in the synthesis and transfer of EPA and DHA to upper trophic levels.

Fatty acid (FA) content and composition of zooplankton in Puget Sound, Washington (USA) was studied to investigate the nutritional quality of diverse zooplankton prey for juvenile salmon (Oncorhynchus spp.) in terms of their essential fatty acid (EFA) content. The study focus was on eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (ARA) as these are key FA needed to maintain growth and development of juvenile fish. The different zooplankton taxa varied in their FA composition. Much of the variation in FA composition was driven by 18:1ω9 (a biomarker of carnivory), ARA, DHA, and FA characteristic of diatoms, which are linked to zooplankton diet sources. Gammarid and hyperiid amphipods contained the highest amount of EFA, particularly the gammarid amphipod Cyphocaris challengeri, while shrimp and copepods had much lower EFA content. Crab larvae, which are important prey for juvenile salmon in Puget Sound, had intermediate EPA + DHA content and the lowest DHA/EPA ratio, and were rich in diatom biomarkers. Temporal and spatial trends in zooplankton lipids were less apparent than the taxonomic differences, although the EFA content increased from spring to summer in Cancridae zoeae and the amphipod C. challengeri. These results on taxon-specific EFA content provide baseline information on the nutritional quality of zooplankton that can be applied in food web models. Combining zooplankton fatty acid data (quality) with taxon-specific zooplankton biomass data (quantity) enables development of new, sensitive indicators of juvenile fish production to help assess recent declines in salmon production in the Pacific Northwest and predict future adult returns.

Lake eutrophication and brownification downgrade availability and transfer of essential fatty acids for human consumption

BackgroundLittle is known about the transfer of essential fatty acids (FA) across the human blood‐brain barrier (BBB) in adulthood.ObjectiveWill oral supplementation for 6 mo with omega‐3 fatty acids (n‐3 FA), high in DHA, change the FA profile in cerebrospinal fluid (CSF) in patients with Alzheimer's disease (AD)?MethodsFA in CSF were analyzed by gas chromatography and mass spectrometry in 33 patients; 18 received n‐3 FA and 15 placebo. Participants were part of the OmegAD Study, where 204 patients with mild AD received 2.3 g n‐3 FA daily or placebo for 6 mo in a randomized fashion. Correlation analyses were made with plasma FA, CSF markers of AD and of inflammation.ResultsAt 6 mo a significant increase in CSF (and plasma) EPA, DHA and total n‐3 levels were noted in the n‐3 FA group, whereas no changes were observed in the placebo group. Changes of CSF DHA levels correlated significantly to changes of total and phosphorylated Tau and soluble IL‐1RII in CSF, i.e. the more the FA increased, the more the AD biomarkers decreased.SummaryN‐3 FA supplementation conferred discrete changes of n‐3 FA in CSF, suggesting transfer over the adult BBB. Significant correlations between CSF n‐3 FA and CSF AD markers suggest possible relationships.

Little is known about the transfer of essential fatty acids (FAs) across the human blood-brain barrier (BBB) in adulthood. In this study, we investigated whether oral supplementation with omega-3 (n-3) FAs would change the FA profile of the cerebrospinal fluid (CSF). A total of 33 patients (18 receiving the n-3 FA supplement and 15 receiving placebo) were included in the study. These patients were participants in the double-blind, placebo-controlled randomized OmegAD study in which 204 patients with mild Alzheimer's disease (AD) received 2.3 g n-3 FA [high in docosahexaenoic acid (DHA)] or placebo daily for 6 months. CSF FA levels were related to changes in plasma FA and to CSF biomarkers of AD and inflammation. At 6 months, the n-3 FA supplement group displayed significant increases in CSF (and plasma) eicosapentaenoic acid (EPA), DHA and total n-3 FA levels (P < 0.01), whereas no changes were observed in the placebo group. Changes in CSF and plasma levels of EPA and n-3 docosapentaenoic acid were strongly correlated, in contrast to those of DHA. Changes in DHA levels in CSF were inversely correlated with CSF levels of total and phosphorylated tau, and directly correlated with soluble interleukin-1 receptor type II. Thus, the more DHA increased in CSF, the greater the change in CSF AD/inflammatory biomarkers. Oral supplementation with n-3 FAs conferred changes in the n-3 FA profile in CSF, suggesting transfer of these FAs across the BBB in adults.

Freshwater ecosystems reflect the condition of their surrounding landscape, and thus are particularly vulnerable to anthropogenic stressors associated with human land-use. One of the most prevalent stressors on stream ecosystems in agricultural regions, such as the Canterbury Plains of New Zealand, is eutrophication, or increased primary productivity. e aim of this thesis was to investigate effects of eutrophication on stream communities, specifically food web structure and ecosystem function. Froma foodwebperspective, eutrophication is a shi in the formand amount of available energy from externally-produced (allochthonous) to internal (autochthonous) basal resources. Such shis are frequently associated with land-use intensification, due to riparian vegetation removal and increased nutrient inputs, both of which enhance autochthonous production. A field survey across a gradient of eutrophication showed that eutrophic stream food webs are largely autochthonously-based and oen contain large numbers of defended primary consumers, which form trophic bottlenecks and prevent energy from reaching higher trophic levels. Consequently, while there is more total energy available, less of that energy is in a usable form for stream food webs. Moreover, I found that eutrophic streams are largely composed of generalist consumers, which shi their diets to refocus on autochthonous resources with increasing productivity. Given that eutrophication causes food web resources to become more homogenous and was a primary driver of food web change, I tested whether reintroducing allochthonous subsidies would alter or reverse the negative effects of eutrophication. To do this I conducted a shortterm community assembly experiment and a year-long population biomass accrual study. I found that the simplified, generalist-dominated communities in eutrophic streams did not respond to changes in resource diversity as predicted by food web theories, which are based on more diverse food webs. Aer restoration of allochthonous subsidies, defended generalist taxa continued to dominate the invertebrate communities. However, while restoring allochthonous subsidies did not mitigate the numerical dominance of defended consumers, the biomass accrual of other, previously excluded desirable taxa, such as mayflies and predatory invertebrates, increased following resource additions. is indicates that more energy reached the top of the food web, suggesting that resource additions alleviated trophic bottlenecks. Overall, my findings have advanced current knowledge about key mechanisms driving food web responses to both anthropogenic stress and to restoration efforts, which can be applied to improvemanagement and restoration of stream ecosystems.

Docosahexaenoic acid [22:6(n-3); 22:6(4,-7,10,13,16,19) (DHA)] is required in quantity by the developing nervous system of the fetus. This need could be met through synthesis of DHA from linolenic acid in the fetus or through placental transfer of DHA directly. To study the placental transfer of n-3 fatty acids, we obtained umbilical and maternal blood samples from 26 healthy women and infants at parturition and measured the fatty acid composition and content of both plasma and erythrocytes. A striking finding was a considerable venous-arterial difference for DHA in the umbilical erythrocytes as a proportion of total fatty acids and in absolute concentration. This difference of 2.2 micrograms per billion erythrocytes was 6 times larger than the difference in fetal plasma, when the plasma and erythrocyte concentrations were normalized to whole blood. Most other erythrocyte fatty acids showed a similar trend. In umbilical plasma, significant venous-arterial differences were found for 16:0, 16:1, 18:2, and total saturated fatty acids. There was a similar trend for most other plasma fatty acids. Compared with maternal blood, fetal plasma and erythrocytes had higher levels of 20:4 and DHA and lower levels of 18:2 and 18:3(n - 3) fatty acids as a proportion of total fatty acids. These results suggest that erythrocytes play a major role in the necessary transport of the essential fatty acid DHA into the fetus.

Polyunsaturated fatty acids (PUFA) are essential components of cell membranes and reproductive and sensory organs in vertebrates and are largely acquired through their diets. Accordingly, identification of the dietary sources of PUFA is an important consideration in food web studies. We collected fish, macroinvertebrates (aquatic and terrestrial), and plants (aquatic and terrestrial) from floodplain and river channel habitats in a tropical river catchment in northern Australia, to identify food sources and habitats that provided lipid and PUFA rich food that sustain fish populations. The composition of most FA in fish was similar to that reported from other freshwater tropical environments, with the exception of their higher arachidonic acid content. Fish were found to derive their fatty acids primarily from aquatic sources of food and had similar FA composition to aquatic macroinvertebrates. Aquatic macroinvertebrates fed on aquatic plants (algae) and had higher total lipids and FA contents than plants and fish, providing a more concentrated source of PUFA for fish. Fish obtained most of their FA from their diet except for docosahexaenoic acid (DHA), which they must synthesize due to low DHA in algae and macroinvertebrates. There was no overall difference in basal dietary FA composition between floodplain and river channel habitats. However, macroinvertebrate taxa varied in their lipid and PUFA content. Food quality for fish may therefore vary between habitats as a consequence of differences in macroinvertebrate community composition. Given the high algal production on floodplains compared to the river channels, these habitats are likely to represent the major source of high quality food for fish and other aquatic consumers, and conserving habitats that are rich in lipids and PUFA is important for maintaining healthy fish communities.

Earthworm invasion shifts trophic niches of ground-dwelling invertebrates in a North American forest

Alteration of riverine flows can modify the structure and function of ecosystems, changing energy pathways and patterns of micronutrient transfer between trophic levels. Fatty acids (FAs) commonly are used to evaluate food quality, since some FAs required for somatic growth and physiological functions in animals must be obtained from their diet. FAs also are used in food‐web studies as biotracers as a consequence of their constrained metabolic biosynthesis by animals. However, their utility may be confounded by selective retention or modification of dietary FAs by consumers. We conducted a 70‐day feeding trial to compare growth and survival of an abundant and widespread mesoconsumer (Cherax destructor, the common yabby or crayfish) fed three contrasting diets: a poor‐quality detritus‐based diet; a high protein invertebrate diet; and a high‐quality commercial aquaculture pellet. Fatty acid profiles were obtained for each dietary treatment and contrasted with crayfish FA profiles at the end of the experiment to examine patterns of FA retention and integration. We also collected wild crayfish from floodplain wetland and river habitats, and obtained FA profiles from their stomach contents and body tissue to compare with experimental crayfish. Experimental crayfish fed high‐quality commercial pellets doubled in mass during the 70‐day assay, invertebrate fed crayfish growth was intermediate, and growth of crayfish fed detritus was negligible. Fatty acid profiles of crayfish fed our three contrasting diets differed significantly at the end of the experiment. Proportions of the polyunsaturated omega‐6 FA linoleic acid (LIN, 18:2ω6) in crayfish followed the same inequality observed in growth and diets: pellets &gt; invertebrates &gt; detritus. Pellet‐fed crayfish preferentially assimilated greater proportions of FAs 20:4ω6 (ARA), 20:5ω3 (EPA) 18:1ω9 (OA) and 16:1ω7 (POA) into their tissue. Fatty acid profiles of floodplain crayfish differed to profiles of riverine crayfish, and floodplain crayfish had higher proportions of essential FAs ARA and LIN in their tissues. Fatty acid biosynthesis by crayfish was best described by a hypothesis of FA allostasis rather than homeostasis; in this, FA profiles of crayfish were shaped by their diet, and selective integration and modification of high‐quality FAs from basal resources rich in these micronutrients led to higher proportions in crayfish tissues. Here we present evidence for the conversion of shorter‐chain essential FAs by freshwater crayfish to compensate for a lack of long‐chain FAs in their diet. We provide a necessary step for improving our understanding of micronutrient dynamics and the transfer of essential molecules between trophic levels in lowland river food webs. Floodplain habitats are known to provide higher‐quality basal food resources for mesoconsumers than riverine habitats, and here we identify one mechanism by which that may be extended to subsequent trophic levels.

The highly directional maternal-to-fetal transfer of essential fatty acids (EFAs) across the placenta plays a critical role in guiding proper fetal development. Exposure to xenobiotics that may alter the fetal supply of EFAs/lipids could lead to fetal toxicity. Since the placenta is the first fetal arising organ that regulates fetal fatty acid homeostasis, the fatty acid/lipid composition in the placenta may serve as an indicator of fetal composition. In this study, we investigated the effects of the peroxisome proliferator chemical di-(2-ethylhexyl)-phthalate (DEHP), a widely used plasticizer and ubiquitous environmental contaminant, and its selective metabolites, mono-(2-ethylhexyl)-phthalate (MEHP) and 2-ethylhexanoic acid (EHA) on the lipid metabolome in a rat HRP-1 trophoblast model. The concentrations of ten lipid classes (cholesterol esters, diacylglycerol, triacylglycerides, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, lysophosphatidylcholine, cardiolipin, and sphingomyelin) were determined, as well as the individual fatty acid compositions, especially the omega-3 and omega-6 family of EFAs. The level of each lipid class was significantly increased upon exposure to the agents, with MEHP and EHA generally showing higher increases than DEHP. The same trends were observed in comparing the fatty acid compositions. For example, the omega-3/omega-6 fatty acids ratio did not change, although the levels of omega-3 and omega-6 fatty acids were significantly elevated upon exposure. These results suggest that DEHP and its metabolites can alter lipid metabolome in a rat placental cell line, implying that these compounds may contribute to aberrant placental EFA/lipid homeostasis caused by peroxisome proliferation, and potentially result in abnormal fetal development.

Fatty acid requirements for the preterm infant