Red and brown macroalgae in temperate stream food webs: Are we missing an important trophic link?

In temperate low-order streams and rivers, the macroinvertebrate community has a key role in the flow of matter and energy. Despite its relative scarcity, in-stream primary producers have a high functional importance for the macroinvertebrates as the main source of Long-Chained PolyUnsaturated Fatty Acids (LC-PUFAs), essential biochemical compounds for macroinvertebrates fitness. Among these primary producers, the trophic role of microalgae in periphytic biofilms has been largely studied over the last decades, while macroalgae (Rhodophyta, Ochrophyta) have been neglected or even ignored. We call for future studies quantifying macroalgal species and production in streams, their biochemical composition, as well as their nutritional value for stream invertebrates to assess their potential trophic role in stream food webs. Our literature review indicates that macroalgae can be seasonally and locally abundant in freshwater streams and that they are frequently found in the guts of numerous macroinvertebrate taxa. From a biochemical perspective, the macroalgal species studied are rich in LC-PUFAs, particularly in Arachidonic Acid (ARA), an omega-6 fatty acid, contrasting with periphytic biofilms, which are rich in the omega-3 series, especially Eicosapentaenoic Acid (EPA). Consuming Rhodophyta and Ochrophyta macroalgae as a complementary resource could enable stream invertebrates to optimize their dietary EPA : ARA ratio.