Longitudinal variation in the nutritional quality of basal food sources and its effect on invertebrates and fish in subalpine rivers.

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.

Feeding strategies for the acquisition of high-quality food sources in stream macroinvertebrates: Collecting, integrating, and mixed feeding.

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.

Docosahexaenoic acid in Arctic charr (Salvelinus alpinus): the importance of dietary supply and physiological response during the entire growth period.

The aim of this 14-month feeding study was to investigate the effects of dietary docosahexaenoic acid (DHA) on tissue fatty acid composition, DHA retention, and DHA content per biomass accrual in muscle tissues of Arctic charr (Salvelinus alpinus). A control feed, formulated with a relatively high DHA inclusion level (F1), was compared with feeds containing gradually reduced amounts of DHA (Feeds F2, F3, and F4). Arctic charr were randomly distributed among 12 tanks and fed one of the feeds in triplicate. The DHA content within muscle tissues of fish fed diets F1 and F2 was generally higher compared to fish fed diets F3 and F4. However, there was an interaction between dietary DHA treatment and season, which resulted in fish muscle tissues having similar DHA contents irrespective of dietary supply during specific sampling periods. Although diets F3 and F4 contained ~4-fold less DHA compared to diets F1 and F2, the retention of DHA in dorsal and ventral muscle tissue was up to 5-fold higher relative to the diet content in fish fed diets F3 and F4. However, the difference among treatments was dependent on the month sampled. In addition, younger fish retained DHA more efficiently compared to older fish. DHA (µg DHA/g/day) accrual in muscle tissue was independent of somatic growth, and there was no difference among treatments. The results suggested that dietary DHA may be essential throughout the life cycle of Arctic charr and that the DHA content of muscle tissues was influenced by diet and metabolic/physiological factors, such as specific DHA retention during the entire growth cycle . Finally, this long-term feeding study in Arctic charr indicated a non-linear function in DHA retention in dorsal and ventral muscle tissues throughout the life cycle, which varied in its relationship to dietary DHA.