DOI: 10.5061/dryad.k98sf7mgh
title: Data from: Intraspecific body size determines isotopic trophic structure of a large river fish community

description: This dataset includes stable isotopes δ15N and δ13C to explore the effects
 of body size on fish community trophic structure and niches in one of
 Australia’s largest river basins — the Murray-Darling. First, we test
 whether Trophic Position (TP) and δ13C scale with body mass within and
 among species and functional guilds (predator; micro-carnivore; omnivore;
 algivore-detritivore). Secondly, we test whether isotopic niche breadth
 scales with body size within and among species or community mass-classes
 ranging from &lt; 1 g to &gt; 8192 g. The importance of within
 species, size-based, trophic structure in our study contrasts with
 previous evidence suggesting that river food webs are not size-structured.
  Food web models and tests of theory which have assumed a single,
 species-level, TP or δ13C are unlikely to fully capture the complex
 intraspecific size-based trophic dynamics of river fish
 communities. 
description: Between March and May 2018, fish tissues and basal carbon sources
 from primary producers were collected from five large rivers of the MDB.
 Rivers sampled were the Edward-Wakool, Goulburn, Lachlan, Lower Murray,
 and Murrumbidgee (Supplementary Fig. 1).  Primary producers were sampled
 from the Gwydir River, but fish tissue samples were not available from
 this system. Fin-clips from eight additional Murray cod ranging from 600
 mm to 1100 mm TL sampled from the Mid-Murray River, and 36 juvenile Golden
 perch (<em>Macquaria ambigua</em>) from the Tala Creek in the
 Murrumbidgee River system were also included to fill gaps in small and
 large individuals. The fish species sampled make-up the bulk of fish
 community abundance and biomass in lowland river-floodplain ecosystems of
 the MDB (Davies et al., 2010), but did not include several rare
 small-bodied species, predominantly inhabitants of upland or coastal river
 reaches. Fish were sampled using standardised boat-electrofishing and fyke
 netting. In each river, fish were sampled at 10 sites, spanning lengths of
 20-100 km. Each site was sampled once using 10 fyke nets and 2880 s of
 electrofishing ‘on time’. Fish were identified and the length of the first
 50 individuals of each species per site was measured (mm).  Body mass of
 individuals was estimated from length using previously published
 length-weight relationships (Llewellyn, 2011). Whole
 fish or a caudal fin-clip of up to 30 individuals from each fish species
 per river, spanning the sampled body size range, were collected for stable
 isotope analysis.  A caudal fin-clip (30-50 mm2) from the upper lobe was
 collected from fish larger than 150 mm, while fish smaller than 150 mm
 were euthanised to ensure enough tissue for stable isotope analyses and to
 minimise post-release animal welfare concerns.  Fin-clips were stored in
 1.5 mL non-stick centrifuge tubes and whole fish in plastic zip-lock bags
 on ice and then frozen before preparation for stable isotope
 analyses. <em>Basal carbon
 sources</em> To provide baseline δ15N for trophic
 position estimates, and to estimate δ13C of basal resources, four basal
 resources were sampled during the same period as fish community sampling
 activities were being undertaken. These included two replicates from each
 of three reaches within each river located between 10-120 km apart (Figure
 S1). At each site, the basal resources sampled were: 1) periphyton
 (benthic algae, filamentous algae and associated biofilm); 2) seston
 (phytoplankton and suspended fine particulate matter); 3) macrophytes; and
 4) terrestrial riparian plants. Periphyton was scraped from woody debris,
 rocks and macrophytes using five, 2 cm diameter, circular dish scouring
 pads and placed in zip-lock bags. Seston was collected from epilimnetic
 water samples in 1 L bottles prefiltered through 250‐μm mesh and then
 filtered onto precombusted Whatman GF/F filters (pore size 0.7 um).
 Macrophytes, grasses and plant leaves, seeds and stems were collected from
 a mixed variety of species visually assessed within a 500 m reach at each
 site, cut into pieces, placed into plastic zip-lock bags and
 frozen. <em>Stable isotope and laboratory
 analyses</em> Frozen basal resources, fin-clips
 and whole small-bodied fish were thawed and rinsed (excluding seston
 filter papers) with reverse osmosis purified water and dried in glass
 vials or petri dishes in an oven at 60°C for 48 h (Arrington and
 Winemiller, 2002). Where possible, fin-clips from a total of at least 20
 individuals of each species, representing the full body size range sampled
 and each river, were processed for bulk stable isotope ratios of nitrogen
 (15N/14N) and carbon (13C/12C) at the University of Western Australia. 
 Samples were analysed for δ15N and δ13C, using a continuous flow system
 consisting of a Delta V Plus mass spectrometer connected with a Thermo
 Flush 1112 via Conflo IV (Thermo-Finnigan/Germany). The isotope values
 δ15N and δ13C are reported in relation to [‰, Air] and [‰, VPDB]
 respectively according to international standards (Skrzypek,
 2013). Trophic guilds and trophic position
 Fishes were classified into four trophic guilds based on a diet
 analysis of adult Australian freshwater fishes by Stoffels (2013). The
 trophic guilds included: 1) Predator/Piscivore (P): fish that primarily
 consume fish and in some cases decapod crustaceans and other
 macroinvertebrates; 2) Omnivore (O): trophic generalists that consume a
 range of phytoplankton, benthic algae and aquatic or terrestrial
 invertebrates; 3) Algivore-Detritivore (AD): fish that consume detritus or
 algae; 4) Micro-Carnivore (MC): fish with diets dominated by insects,
 aquatic microcrustaceans or other invertebrates. We combined the guilds
 ‘microcrustacivores’ and ‘aquatic insectivores’  (Stoffels, 2013) into MC
 since some species could not be clearly distinguished between the two
 guilds.  The trophic position (TP) of fishes was
 calculated according to the equation (Post, 2002, Vander Zanden and
 Rasmussen, 1999): TP = 1+ (δ15Nconsumer - δ15Nbasal source) / TDF, where
 δ15Nconsumer is the signature of individual fish, and δ15Nbasal source was
 the mean δ15N of the basal resources within respective rivers where fish
 were sampled including (δ15N, mean ± SE): EW (2.85 ± 1.28); GB (2.90 ±
 1.30 SE); LC (4.44 ± 1.98); LM (2.54 ± 1.14); and MB (3.59 ± 1.61).  The
 trophic discrimination factor (TDF) represents the shift in nitrogen
 between its ingestion by a consumer and its assimilation into the
 consumers’ tissue. Firstly, we used a constant TDF of 3.4 which is a
 common baseline used in freshwater food web studies (Post, 2002). To
 evaluate the sensitivity of our results to uncertainties associated with
 the TDF, we compared TPs of models based on the constant TDF
 (TP<em>constant</em>) with scaled
 (TP<em>scaled</em>) based on a global meta-analysis (Hussey et
 al., 2014) and TDFs estimated for fish predators (5.7), microcarnivores
 (3.4), omnivores (4.3) and herbivores (3.9) in Australian rivers
 (TP<em>Aus.riv.</em>) (Bunn et al., 2013).
 TP<em>scaled</em> (Hussey et al., 2014) was calculated
 according to the equation: TP<em>scaled</em> =1+
 (δ15N<em>consumer</em> - δ15N<em>basal
 source</em>) / 2.94 + 0.22 × (δ15N<em>basal source</em>
 - 3.4).  The TP values shown in the dataset represent TPscaled
 values.
description: # Intraspecific body size determines isotopic trophic structure of a large
 river fish community
 [https://doi.org/10.5061/dryad.k98sf7mgh](https://doi.org/10.5061/dryad.k98sf7mgh) **Column headings:** Family=taxonomic family of fish species. river=Edward-Wakool (EW); Murrumbidgee (MB); Lachlan (LC); Lower Murray (LM): Goulburn (GB); Middle reach of the Murray River (MidMurray); Tala Creek (Tala). group = fish species:  | ***Bidyanus bidyanus*** | | :-------------------------------- | | *Carassius auratus* | | *Cyprinus   carpio* | | *Craterocephalus stercusmuscarum* | | *Gambusia holbrooki* | | *Hypseleotris  spp.* | | *Macquaria ambigua* | | *Melanotaenia fluviatilis* | | *Maccullochella peelii* | | *Nematalosa erebi* | | *Philypnodon grandiceps* | | *Philypnodon macrostomus* | | *Retropinna semoni* | | *Tandanus tandanus* | log10length=log10transformed body length (mm); guild= trophic guild, Algivore-Detritivore (AD), Predator (P), Omnivore (O), Invertivore-Microcrustacivore (IM); length = body length (mm), mass = body mass (g), isoo1=trophic position scaled estimate; n= δ15N bulk stable isotope; iso2=δ13C; log2mass=logbase2 body mass (g) transformed; log2massclass = logbase2 body mass (g) class grouping; stage= small juvenile (SJ), large juvenile (LJ), small adult (SA), large adult (LA); guild*_*stage=Algivore-Detritivore (AD), Predator (P), Omnivore (O), Invertivore-Microcrustacivore (IM) *_* small juvenile (SJ), large juvenile (LJ), small adult (SA), large adult (LA).