A high-quality diet is central for consumer somatic growth, maintenance, reproduction and their ecological interactions within food webs (Paine, 1980; Simpson & Raubenheimer, 2012). At the base of aquatic food webs, algae provide essential compounds, such as amino acids, sterols and polyunsaturated fatty acids (PUFA) to consumers, and thus are considered to be high-quality resources for consumers (Brett et al., 2009). However, in river ecosystems, in particular headwaters, the availability of algae is assumed to be low compared to terrestrial organic matter (Vannote et al., 1980), which may constrain the dietary transfer of essential compounds to consumers. Despite this, animals living in streams and rivers are highly enriched in essential compounds, particularly PUFA (Guo et al., 2017). Most aquatic animals have a limited ability to synthesize these essential compounds and thus must obtain them directly from their diet, originally from basal algae. However, in addition to periphyton and planktonic algae, which have been intensively studied in river ecology (Allan & Castillo, 2007), there may be other overlooked high-quality resource pools that provide essential compounds to consumers.
Biofilms growing on submerged substrata are an important food source for consumers in river ecosystems (Allan & Castillo, 2007; Battin et al., 2016). Biofilms are mostly composed of algae and readily develop on the surface of leaves, woody debris and rocks. In particular, biofilms growing on rocks, also called epilithon, have been found to better support stream food webs than do terrestrial leaves due to their high PUFA content (Guo et al., 2016a; Lau et al., 2009; McInerney et al., 2020). However, in most previous studies, the method to sample these biofilms has not been uniform; a few studies collected biofilms on the upper side of the rock (light-exposed biofilms; in short ‘light biofilms’), whereas most studies do not clarify whether the biofilms were only from the upper side or a combination of the upper side and underside of rocks.
Many invertebrates cling to the underside of rocks, which may be a strategy to avoid predators, high ultraviolet radiation and/or being dislodged by flow (Allan & Castillo, 2007). It is therefore also likely that some invertebrates feed on biofilms from the underside of rocks (dark-exposed biofilms; in short ‘dark biofilms’). Sedentary invertebrates (e.g. cased caddis larvae) are found to spend much of their time on rock surfaces (Hynes, 1970) and may not feed on dark biofilms. In contrast, mobile invertebrates (e.g. mayflies and stoneflies) drift mostly at night, but during the day, they hide in sheltered locations (Brittain & Eikeland, 1988; Hynes, 1970), such as the underside of rocks. Dark biofilms may be an overlooked resource pool for these aquatic consumers. However, little is known about the nutritional composition and thus trophic importance of dark biofilms for aquatic consumers.
Long-chain polyunsaturated fatty acids (LC-PUFA) are an informative and increasingly important indicator of diet quality for aquatic consumers (Brett et al., 2017). Among LC-PUFA, eicosapentaenoic acid (EPA, 20:5ω3) and docosahexaenoic acid (DHA, 22:6ω3) are required for invertebrate development, reproduction and hormone regulation (Stanley-Samuelson, 1994). Diatoms with high contents of EPA and DHA are considered as high-quality diet for consumers (Brett et al., 2009), while cyanobacteria are usually low-quality diet because they lack these specific PUFA as well as sterols (Martin-Creuzburg et al., 2008). Diatoms have been shown to support higher growth rates and reproduction of consumers (Guo et al., 2016a) and enhance dietary energy transfer efficiency to upper trophic levels (Lau et al., 2013; Müller-Navarra et al., 2000).
Biofilms are a mixture of terrestrial particles, fungi, bacteria and algae, with light biofilms being more autotrophic and dark biofilms more heterotrophic (Allan & Castillo, 2007; Romani et al., 2004), which results in different nutritional quality for consumers. Terrestrial plants are considered low-quality diets for aquatic consumers because they lack LC-PUFA (Brett et al., 2009). However, many terrestrial plants are rich in the short-chain PUFA linoleic acid (LIN, 18:2ω-6) and α-linolenic acid (ALA, 18:3ω-3), which are precursors for LC-PUFA, as well as saturated fatty acids (SAFA), which are generally used for energy storage and to support catabolism (Brett et al., 2017). Initially, as terrestrial leaves decompose in water, their ALA and LIN proportions decrease, but SAFA does not change (Guo et al., 2018; Hiltunen et al., 2019). Fungi and bacteria are considered less nutritious than algal diets for aquatic consumers because they lack LC-PUFA. Studies on terrestrial and marine fungi reported that 16:0, 18:0 and 18:1ω9 are the most common FA in fungi (Cooney et al., 1993; Stahl & Klug, 1996). Bacteria are characterized by odd-chain FA, for example, 15:0, 17:0 and their branched homologues, as well as vaccenic acid (18:1ω7) (Desvilettes et al., 1997). Therefore, algae within light and dark biofilms are an important determinant of their nutritional quality for consumers.
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