Oral Presentation Freshwater Sciences 2023

The dark side of rocks: an underestimated high-quality food resource in river ecosystems (#134)

Fen Guo 1 , Stuart Bunn 2 , Michael Brett 3 , Hannes Hager 4 , Kainz Martin 4
  1. Guangdong University of Technology, Guangzhou, China, Guangzhou, GUANGZHOU, China
  2. Australian Rivers Institute, Griffith University, Nathan, Qld, Australia, Brisbane
  3. Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington, USA, Seattle
  4. WasserCluster Lunz – Inter-University Centre for Aquatic Ecosystem Research, Lunz am See, Austria, Lunz
  1. Algae are considered to be 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 transfer of essential compounds to consumers. However, there may be other overlooked high-quality resource pools that provide essential compounds to consumers in river ecosystems.
  2. We conducted a field survey along a subalpine river continuum in Austria to identify the nutritional role of a ‘hidden’ food resource for aquatic consumers; the biofilms attached to the bottom of rocks (dark biofilms). Dark and light (i.e., attached to the upper surface of rocks) biofilms, and invertebrates were collected, and their fatty acid (FA) composition was analyzed.
  3. The overall FA profiles of dark biofilms were quite similar to those of light biofilms and invertebrate grazers. Although eicosapentaenoic acid (EPA, 20:5ω3) in dark biofilms was low, it was still significantly retained by invertebrate grazers, similar to that in light biofilms. Bacterial FA (BAFA) in dark biofilms were also strongly correlated with grazer BAFA. Despite the general similarity, our results also show that, compared with light biofilms, dark biofilms contained greater proportions of BAFA, long-chain saturated FA (biomarkers of terrestrial plants), and 18:1w9 (a biomarker of fungi), but less algae-synthesized polyunsaturated FA, especially EPA.
  4. Synthesis. Our study demonstrates that dark biofilms provide a high-quality resource pool for invertebrates in river ecosystems that is often overlooked in field studies. Invertebrates can obtain physiologically important FA on the underside of rocks, even when avoiding predation and/or being dislodged by flow.