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

1. 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. 2. 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. 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.