Polyunsaturated fatty acids in stream food webs – high dissimilarity among producers and consumers

Abstract

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

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

“…Predator FA profiles have been found to be distinct at low taxonomic resolution, among classes or higher levels (Lau et al ), which indicates the low effect of species identity and their potential ability to maintain a quasi‐homoeostatic FA profile regardless of different food quality. Furthermore, although the periphyton FA effect on predators was low, EPA% is increasingly retained along the trophic gradient from periphyton to predators via primary consumers (grazers and shredders), confirming the importance of EPA in stream ecosystems for efficient trophic transfer to upper trophic levels (Gladyshev et al ; Lau et al ; Guo et al ).…”

“…Our study suggests that stream macroinvertebrates exhibit a mixed feeding strategy, rather than simply collecting or integrating high‐quality LC‐PUFA, especially EPA, relative to their basal food sources. Although the periphyton FA effect on macroinvertebrate FA profiles decreased with increasing trophic levels, the EPA% in macroinvertebrate body tissues increased (Gladyshev et al ; Lau et al ; Brett et al ; Guo et al ), indicating EPA accumulation is a general ecological response in stream food webs. We propose three hypotheses regarding the mechanism that macroinvertebrates use to accumulate EPA: (1) macroinvertebrates may selectively feed on high‐quality periphyton instead of terrestrial leaves, (2) although macroinvertebrates feed on a mixed periphyton assemblage, they may assimilate EPA more efficiently than other FA, or preferentially assimilate EPA over other FA, and/or, (3) macroinvertebrates may actively control their LC‐PUFA distribution through metabolic processing to support their somatic growth, reproduction, and survival.…”

“…High LC‐PUFA retention in metazoan consumers means high availability for the next trophic level (Brett and Müller‐Navarra ; Taipale et al ). However, even though LC‐PUFA are highly retained in macroinvertebrates (Guo et al ), little is known how macroinvertebrates utilize dietary LC‐PUFA. Do macroinvertebrates simply collect LC‐PUFA from their food sources (“LC‐PUFA collector”), or are they able to regulate their PUFA content relative to food sources (“LC‐PUFA integrator”)?…”

Urbanisation alters fatty acids in stream food webs

“…In our study of a large catchment, macroinvertebrates had greater proportions of Σω3 FAs and lower proportions of Σother FAs than those in periphyton, whereas their proportions of Σω6 were similar. These patterns probably resulted from macroinvertebrates having the ability to selectively consume and retain periphyton‐derived FAs even if food sources have low amounts of them, especially ALA, EPA, and ARA (Brett et al., ; Crenier et al., ; Guo, Bunn, Brett, & Kainz, ). These compounds also indicate higher food quality than shorter‐chained or saturated FAs (Guo et al., ; Torres‐Ruiz et al., ).…”

“…Although proportions of Σω3, Σω6, and Σother FA groups did not significantly differ among shredders, collector/gatherers, and predators, their proportions of the five essential FAs did. These differences probably resulted from dissimilar diets among feeding groups, given that FA content and proportions differ among basal resources in streams, and most macroinvertebrates are unlikely, or are at least greatly limited in their ability, to alter their content of these FAs via desaturation and elongation (Guo et al., , ; Torres‐Ruiz et al., , ).…”