Abstract1. 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 highquality food resources for consumers in some systems.2. 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.3. 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. 4.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.5. 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.6. 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. | WHY DOES ALLOCHTHONY MATTER?A better understanding of where and how allochthony modifies aquatic food web processes will improve our ability to predict how land-use and climate change affect organic carbon export from watersheds to lakes and rivers and how this matter influences upper trophic level production in aquatic systems. If invertebrate and fish consumers in rivers and lakes are strongly subsidised by allochthonous carbon inputs, then watersheds and especially riparian zone management will potentially have as mu...

SUMMARY1. While many streams and rivers are dominated by terrestrial inputs of organic carbon, algae are an important trophic base for stream food webs. However, the nutritional importance of algae for stream invertebrates only recently has been highlighted. Algae are acknowledged as higher quality food than terrestrial organic matter for the growth and reproduction of invertebrates. In part, this is because of higher algal polyunsaturated fatty-acid (PUFA) content. Here, we review the important influence of algal food quality, as assessed by PUFA, in stream food webs. 2. Current field investigations have mainly focused on the fatty-acid dynamics of macroinvertebrates, and indicate that algal eicosapentaenoic acid (EPA), a-linolenic acid (ALA) and linoleic acids (LIN) are present in all macroinvertebrates. However, fungal and bacterial tracers have also been observed in a range of macroinvertebrates. The omega-3 (x3)/omega-6 (x6) ratio >1 in most macroinvertebrates strongly indicates that dietary energy of algae is highly retained in stream food webs. Interspecific differences in PUFA composition seem to be affected by dietary PUFA and consumer physiology. 3. Some studies have suggested that besides dietary EPA, the shorter chain C18 PUFA LIN and ALA also can improve growth and reproduction of stream invertebrate consumers. Some macroinvertebrates may preferentially retain or synthesise long-chain PUFA from C18 PUFA when experiencing low-quality food. However, this process is controversial since other species have shown very limited ability to synthesise long-chain PUFA. 4. Algal PUFA composition is strongly influenced by abiotic factors, particularly light, nutrients, and temperature. Human disturbance (i.e. riparian vegetation removal and nutrient inputs) on algal PUFA content and their consequent effects on macroinvertebrates and fish clearly warrant further scientific attention. Controlled feeding trials and manipulative studies are required to measure PUFA conversion capacities and reproductive investment of stream macroinvertebrates under different food quality conditions, which will provide insights into how freshwater species can cope with different nutritional food conditions due to human disturbance and climate change.

Multivariate statistical methods, such as cluster analysis (CA), discriminant analysis (DA) and principal component analysis (PCA), were used to analyze the water quality dataset including 13 parameters at 18 sites of the Daliao River Basin from 2003-2005 (8424 observations) to obtain temporal and spatial variations and to identify potential pollution sources. Using Hierarchical CA it is classified 12 months into three periods (first, second and third period) and the 18 sampling sites into three groups (groups A, B and C). Six significant parameters (temperature, pH, DO, BOD(5), volatile phenol and E. coli) were identified by DA for distinguishing temporal or spatial groups, with close to 84.5% correct assignment for temporal variation analysis, while five parameters (DO, NH(4)(+)-N, Hg, volatile phenol and E. coli) were discovered to correctly assign about 73.61% for the spatial variation analysis. PCA is useful in identifying five latent pollution sources for group B and C (oxygen consuming organic pollution, toxic organic pollution, heavy metal pollution, fecal pollution and oil pollution). During the first period, sites received more oxygen consuming organic pollution, toxic organic pollution and heavy metal pollution than those in the other two periods. For group B, sites were mainly affected by oxygen consuming organic pollution and toxic organic pollution during the first period. The level of pollution in the second period was between the other two periods. For group C, sites were mainly affected by oil pollution during the first period and oxygen consuming organic pollution during the third period. Furthermore, source identification of each period for group B and group C provided useful information about seasonal pollution. Sites were mainly affected by fecal pollution in the third period for group B, indicating the character of non-point source pollution. In addition, all the sites were also affected by physical-chemistry pollution. In the second and third period for group B and second period for group C sites were also affected by natural pollution.

Aquatic macroinvertebrates play an important functional role in energy transfer in food webs, linking basal food sources to upper trophic levels that include fish, birds, and humans. However, the trophic coupling of nutritional quality between macroinvertebrates and their food sources is still poorly understood. We conducted a field study in subalpine streams in Austria to investigate how the nutritional quality (measured by long‐chain polyunsaturated fatty acids, LC‐PUFAs) in macroinvertebrates changes relative to their basal food sources. Samples of macroinvertebrates, periphyton, and leaves were collected from 17 streams in July and October 2016 and their fatty acid (FA) composition was analyzed. Periphyton FA varied strongly with time and space, and their trophic effect on macroinvertebrate FA differed among functional feeding groups. The match between periphyton FA and macroinvertebrate FA decreased with increasing trophic levels, but LC‐PUFA content increased with each trophic step from periphyton to grazers and finally predators. Macroinvertebrates fed selectively on, assimilated, and/or actively controlled their LC‐PUFA, especially eicosapentaenoic acid (EPA, 20 : 5ω3) relative to their basal food sources in the face of spatial and temporal changes. Grazer FA profiles reflected periphyton FA with relatively good fidelity, and especially their EPA feeding strategy was primarily linked to periphyton FA variation across seasons. In contrast, shredders appeared to preferentially assimilate more EPA over other FA, which was determined by the availability of high‐quality food over seasons. Predators may more actively control their LC‐PUFA distribution with respect to different quality foods and showed less fidelity to the basal FA profiles in plants and prey. Overall, grazers and shredders showed relatively good fidelity to food FA profiles and performed as both “collectors” and “integrators” for LC‐PUFA requirements across seasons, while predators at higher trophic levels were more “integrators” with added metabolic complexity leading to somewhat more divergent FA profiles. These results are potentially applicable for other aquatic consumers in freshwater and marine ecosystems.

Algae are higher-quality food resources than allochthonous plant litter for stream invertebrates, in part, because of their higher content of polyunsaturated fatty acids (PUFAs). We tested the hypothesis that algal biofilms on the surfaces of leaf litter improve the nutritional quality of terrestrial inputs for invertebrate shredders. We used a laboratory feeding experiment with 2 light levels (open and shaded) and 2 nutrient regimes (ambient and enriched) to manipulate the algal biofilms that form on leaf surfaces (Lophostemon confertus). We assessed how these treatments affected the fatty acid (FA) composition of these biofilms and the somatic growth of a stream invertebrate shredder (Anisocentropus bicoloratus, Trichoptera) that feeds on the conditioned leaf litter. Shredders reached a significantly larger size when nutrients were added, and leaf mass loss was significantly greater in these treatments than in treatments without nutrients. Shredder growth was affected significantly by leaf PUFA content, and variations in shredder PUFA content were consistent with those in leaf PUFAs. Our results suggest that high-quality algae attached to leaf litter regulated the PUFA composition and improved the somatic growth of these shredders. Our data provide evidence that the availability of high-quality algae enhances dietary use of low-quality riparian leaf litter in stream food webs.

Algal polyunsaturated fatty acids (PUFA), essential for somatic growth and reproduction of aquatic animals, are influenced by ambient environmental conditions, including light and nutrients. Few studies have addressed the extent to which changes in algal PUFA can influence stream herbivore PUFA profiles and the implications for stream food webs. We manipulated subtropical stream periphyton by applying two light levels (open and shaded canopy) and two nutrient regimes (ambient and enriched) to investigate the response of PUFA and somatic growth in stream herbivores. After 6 weeks, the relative content of periphyton PUFA (%) changed distinctly and differed among treatments. Periphyton in the control treatment with open canopy showed a decline in eicosapentaenoic acid (EPA) relative to initial conditions, whereas shading increased EPA and total highly unsaturated FA (HUFA), but decreased α-linolenic acid (ALA), linoleic acid and total C18 PUFA. The interaction of open canopy and added nutrients increased periphyton ALA compared with initial conditions, while the combined effects of shading and added nutrients led to greater total HUFA. FA similarity between stream grazers (the mayfly Austrophlebioides and caddisfly Helicopsyche) and periphyton increased with periphyton HUFA content. In addition, the growth of large instars of both grazers also increased in response to increased periphyton HUFA %. Our findings show that environmental changes, associated with riparian canopy and nutrients, can lead to changes in periphyton PUFA composition that in turn affect growth and PUFA composition in stream grazers.

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.3. 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.4. 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. K E Y W O R D Salgae, biomarker, freshwater fish, invertebrates, trophic transfer

1. Benthic algae play an important role in many stream food webs, not only serving as a major carbon source but also influencing energy transfer as the primary source of polyunsaturated fatty acids (PUFA). Dietary deficiency in PUFA can limit animal growth and reproduction. Algal PUFA content can vary considerably in response to abiotic factors. 2. We conducted a field investigation in streams in South-East Queensland, Australia, to identify environmental factors affecting the fatty acid composition of periphyton. 3. Riparian canopy cover and NO x -N concentrations explained most of the observed variation in periphyton fatty acid profiles. NO x -N concentrations showed negative relationships with most saturated fatty acids (SAFA), while mono-unsaturated fatty acids (MUFA) and the fatty acids 16:0 and 16:1x7 were negatively correlated with canopy cover. In contrast, the percentage of highly unsaturated fatty acids (HUFA) in periphyton was greater with increasing canopy cover regardless of NO x -N concentrations. 4. Variation in riparian canopy cover and nutrients gave rise to opposite outcomes in terms of periphyton food quality and quantity. The highest periphyton food quality, measured by HUFA content, occurred in streams with high canopy cover and low nutrients, while the highest periphyton food quantity occurred in streams with low canopy cover. 5. Our study suggests that changes in riparian vegetation and/or nutrient inputs can significantly alter the fatty acid content of periphyton, with important implications for aquatic consumers. We recommend that future studies on impacts of riparian clearing or nutrient enrichment in stream ecosystems place more attention on the effects of food quality rather than quantity.