DOI: 10.5061/dryad.m37pvmd1v
title: The dark side of rocks: an underestimated high-quality food resource in river ecosystems

description: This dataset contains data of fatty acid profiles described in the paper:
 “Guo et al. (2021) The dark side of rocks: an underestimated high-quality
 food resource in river ecosystems. Journal of Ecology,
 https://doi.org/10.1111/1365-2745.13647”. This study was a field
 investigation conducted along a subalpine river continuum in Austria and
 aimed 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
 analyzed. Main results were: (1) 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). (2) The ω3 PUFA composition in dark biofilms
 was strongly correlated with that in light biofilms. (3) the overall FA
 profiles of dark biofilms were significantly associated with invertebrate
 FA profiles. (4) Strong correlations were also observed between
 invertebrates and dark biofilms for bacterial FA and the ω3 PUFA
 eicosapentaenoic acid (EPA, 20:5ω3).
description:   <b>Sample
 collection</b> Biofilms and macroinvertebrates
 were sampled at 8 riffles from upstream to downstream in the subalpine
 River Ybbs catchment, Austria,  across three seasons, i.e., summer (July),
 fall (October) and winter (November) in 2017. Three paired samples of
 light and dark biofilms were collected for FA analyses along a 20-m reach
 from each riffle. Each paired sample was collected using a quadrat (1.5 m
 * 1.5 m), and five cobbles (diameters ranging from 8 to 16 cm) within the
 quadrat were randomly picked. Light and dark biofilms were scraped from
 the substrate with a toothbrush and kept separate.
 Macroinvertebrates clinging to the above cobbles within each
 quadrat were washed into white trays. <i>Ecdyonurus </i>sp.
 (Heptageniidae, Ephemeroptera), which is the most abundant algal grazer in
 the study streams (Kühmayer et al., 2020), was selected for analysis.
 Although there are four <i>Ecdyonurus</i> species from our
 study streams, including <i>E. helveticus</i> (Eaton, 1885),
 <i>E. venosus</i> (Fabricius, 1775), <i>E.
 dispar</i> (Curtis, 1834) and <i>E. picteti</i>
 (Meyer-Dur, 1864), more than 95% of the larvae were late instars of
 <i>E. helveticus </i>(Moog, 2002). All four species have
 identical feeding habits and are assigned to the same functional feeding
 groups: 50% grazer and 50% detritivore/gatherer/collector (Moog, 2002).
 One replicate with at least 10 individuals of <i>Ecdyonurus
 </i>sp was collected from each riffle. All samples were immediately
 placed in zip-lock plastic bags, stored on ice and kept in the dark in a
 portable freezer. Samples were brought to the laboratory within 4 hours
 and placed in a -80°C freezer until further processing.
 Due to the heavy snow in early November, there was no access to
 one of the upstream sites (WO), where biofilms and invertebrates were not
 collected. There was limited access to the sites FB, TB and WO during
 winter and invertebrates were not collected.
 <b>Sample processing</b> All
 biofilm and macroinvertebrate samples were freeze-dried (Virtis Genesis
 Freeze Dryer). Dry mass from each biofilm sample (~10 mg) and from each
 invertebrate sample (5-7 mg) was used for the lipid extractions. Lipids
 were extracted and methylated according to the methods reported in Guo et
 al (2016c), with nonadecanoic acid (19:0) used as an internal standard.
 Fatty acid methyl esters (FAME) were analysed using a gas chromatograph
 (THERMO Trace; FID 260°C, Carrier gas: He: 1 ml/min, Detector gases:
 H<sub>2</sub>: 40 ml/min, N<sub>2</sub>: 45
 ml/min, air: 450 ml/min, temperature ramp: 140°C (5 min)– 4°C/min–240°C
 (20 min) = 50 min) equipped with a temperature-programmable injector and
 an autosampler. FAME were separated by a Supelco<sup>TM</sup>
 SP-2560 column (100 m, 25 mm i.d., 0.2 mm film thickness), and FA peaks
 were identified by comparison of their retention times with known
 standards, i.e., 37-component FAME mix (Supelco 47885-U) and Bacterial
 Acid Methyl Ester Mix (Supelco 47080-U), and quantified with reference to
 seven-point calibration curves based on known standard concentrations. FA
 compositions were expressed as percentages relative to the total FA
 (FA%).