The dark side of rocks: An underestimated high‐quality food resource in river ecosystems

Abstract

1. Algae are generally a high-quality diet source because they provide essential compounds to aquatic consumers. In forested stream ecosystems, the availability of high-quality algae is low compared to terrestrial organic matter, which may constrain the dietary transfer of essential compounds to consumers. However, there could be other overlooked high-quality resource pools that provide essential compounds to consumers in river ecosystems.2. We conducted a field study along a subalpine river continuum in Austria to identify the nutritional role of a 'hidden' food resource for aquatic consumers; the biofilms growing on the underside of rocks (dark biofilms). Dark and light (i.e. upper surface of rocks) biofilms and invertebrates were collected, and their fatty acid (FA) composition was analysed.3. Compared with light biofilms, dark biofilms contained greater proportions of bacterial FA, long-chain saturated FA (biomarkers of terrestrial plants) and oleic acid (18:1ω9; a fungal biomarker), but a lower proportion of algal FA, especially omega-3 polyunsaturated FA (ω3 PUFA). The ω3 PUFA composition in dark biofilms was strongly correlated with that in light biofilms. Furthermore, the overall FA profiles of dark biofilms were significantly associated with invertebrate FA profiles. Strong correlations were also observed between invertebrates and dark biofilms for bacterial FA and the ω3 PUFA eicosapentaenoic acid (EPA, 20:5ω3). 4.Synthesis. This field study demonstrates that dark biofilms are a high-quality resource pool for invertebrates in river ecosystems that is often overlooked. Similar to light biofilms, dark biofilms provide physiologically important FA and bacterial FA for stream invertebrates. However, these high-quality resources are threatened by increasing human disturbances to river ecosystems. Future research is required to better understand how the quality of both dark and light biofilms changes in response to human disturbance, and how this affects dietary energy transfer to upper trophic consumers, including fish and humans. K E Y W O R D Sbacteria, diatoms, fatty acids, food quality, invertebrates, subalpine riversThis is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Dark matters: Contrasting responses of stream biofilm to browning and loss of riparian shading

“…The traditional view of terrestrial leaf litter and associated microbial decomposers, fungi and bacteria (which lack EPA and DHA) as the foundation of lotic food webs in forested landscapes has been challenged by studies showing that the nutritional quality of leaf litter per se is relatively low but greatly enhanced by the presence of algal biofilm on leaf surfaces (Guo, Kainz, Valdez, et al, 2016). Algae, even if available in very low amounts, may reduce the EPA limitation of lotic consumers and therefore have a primary role in supporting secondary production (Crenier et al, 2017; Guo et al, 2021).…”

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

“…Grazers and shredders are frequently used in studies when examining the importance of autochthonous and allochthonous diet sources for stream food webs, since these consumers have contrasting feeding behaviours on algae and terrestrial leaves respectively. It is predictable that grazer EPA varied with periphyton EPA from upstream to downstream, since grazers primarily feed on periphyton and retain large amounts of periphyton FAs (Guo et al., 2018; Guo, Bunn, et al., 2021). Surprisingly, shredders, which feed on allochthonous sources, also tracked the variations in periphyton EPA, although the pattern was not significant from upstream to downstream.…”

“…Since most freshwater macroinvertebrates have a very low ability to synthesize LC‐PUFAs, they must obtain LC‐PUFAs directly from the diet. Given that periphyton are the exclusive source of LC‐PUFAs (Guo, Kainz, Sheldon, et al., 2016; Torres‐Ruiz et al., 2007), invertebrate primary consumers should have a FA composition that largely reflects the FA composition of periphyton (Guo, Bunn, et al., 2021; Torres‐Ruiz et al., 2010) and this periphyton‐derived LC‐PUFAs can be consequently transferred from primary consumers to predators (Guo et al., 2018). It is thus likely that the FA profiles of macroinvertebrates should reflect and/or track the spatial patterns of periphyton diet quality in river ecosystems.…”

Dietary composition and fatty acid content of giant salmonflies (Pteronarcys californica) in two Rocky Mountain rivers

“…We detected small differences in diets of salmonflies collected on rocky habitat compared to woody debris, with more diatoms consumed by salmonflies sampled from rocks. Perhaps salmonflies using rocks are consuming more diatoms, which is a high-quality food source for invertebrates (Guo et al, 2021;Marcarelli et al, 2011). The underside of rocks could also provide a refuge from potential predation.…”