DOI: 10.5061/dryad.tdz08kq1t
title: Trophic structure and origin of resources of soil macrofauna in the salt marsh of the Wadden Sea: a stable isotope (15N,13C) study

description: Salt marshes exist along the gradient of the marine mudflat to the
 terrestrial dunes, with a gradient of shore height and associated plant
 zonation. The lower salt marsh (LSM) extends from the mean high tidal
 level to 35 cm above that level and is followed by the upper salt marsh
 (USM). Despite changes in the amount of allochthonous marine input and in
 abiotic conditions, little is known about changes in the trophic structure
 and used of basal resources by the soil macrofauna along marine –
 terrestrial boundaries. Natural variations in carbon stable isotope ratios
 (δ¹³C signatures) allow insight into basal resources of consumers such as
 marine algae, terrestrial C3 and C4 photosynthesising plants. Furthermore,
 variations in nitrogen stable isotope ratios (δ¹5N signatures) allow
 insight into the trophic position of consumers. We investigated spatial
 and temporal changes in stable isotope signatures in salt marsh soil
 macrofauna of the island of Spiekeroog, German Wadden Sea. The range of
 δ¹⁵N signatures indicated no changes in food chain length across salt
 marsh zones with consumers in both zones comprising primary decomposer,
 secondary decomposer and first order predators. However, the trophic
 position of individual species changed between zones, but in particular
 with season. Contrasting δ¹⁵N signatures, the range in δ¹³C signatures in
 the LSM was twice that in the USM indicating a wider range of resources
 consumed. Bayesian mixing models indicated predominant autochthonous
 resource use in both the LSM and USM, with the use of marine allochthonous
 resources never exceeding 29.6%. However, the models also indicate an
 increase in the use of marine resources in certain species in the LSM with
 no use in the USM. Overall, the results indicate that the resource use of
 salt marsh macrofauna varies more in space than in time, with the food web
 being generally based on autochthonous rather than allochthonous
 resources. However, there also is trophic plasticity in certain species
 across both temporal and spatial scales including variations in the use of
 allochthonous resources. Generally, however, marine input contributes
 little to the nutrition of salt marsh soil macroinvertebrates.
description: Sampling was performed on the 16th of April (spring), 16th of
 July (summer) and 22nd of October 2019 (autumn) on the island of
 Spiekeroog during low tide along five transects (53°45’2” - 53° 47’1”N, 7°
 40’0” - 7° 49’1”E). Per transect and zone one soil core (ø 20 cm, depth 10
 cm) was taken. Animals were extracted by heat and stored in 70 % ethanol
 at -20°C. Animals were determined to group or species level. Additionally,
 litter, soil, vascular plant species and macroalgae were collected and
 stored at -20°C. Samples were dried at 60°C for 24 h
 and weighed into tin capsules using a fine scale (Cubis MSE 3.6P,
 Sartorius). Variations in stable isotope ratios (¹³C/¹²C and ¹⁵N/¹⁴N) were
 measured by an elemental analyser (Euro EA 3000, EuroVector S.p.A; Milano
 Italy) modified for small samples coupled with an isotope mass
 spectrometer (Delta V Plus, Thermo Electron, Bremen Germany) [47]. Ratios
 of stable isotopes were expressed as δX (‰) =
 [(Rsample—Rstandard)/Rstandard] x 1000, with ‘X’ representing the target
 isotope and ‘R’ the heavy-to-light isotope ratios (¹³C/¹²C and ¹⁵N/¹⁴N) of
 the sample and standard, respectively. Vienna PD Belemnite (PDB) was used
 as standard for δ¹³C and atmospheric nitrogen for δ¹⁵N. Acetanilide was
 used for internal calibration. Table 1 shows the data
 (15N and 13C) of taxa for the full factorial linear mixed-effects
 model. Table 2 includes the full raw data file (15N and
 13C) of taxa used for all further analysis Table 3
 includes the 15N and 13C ranges calculated. Cores where two or more
 species occurred, the ranges in ¹³C and ¹⁵N signatures of the species were
 calculated (most ¹³C or ¹⁵N enriched – least ¹³C or ¹⁵N
 enriched) Table 4 includes the δ¹³C and δ¹⁵N of
 resources, including litter material, soil, vascular plants and marine
 algae.
description: Headings 
 <strong>ID</strong> - number given to each sample
 <strong>Zone </strong>- salt marsh zone - separated
 by vegetation structure and shore height USM - Upper
 salt marsh - dominated by <em>Elymus athericus</em>
 (<em>Elytrigia atherica</em>) LSM - Lower
 salt marsh - dominated by <em>Atriplex portulacoides</em>
 PZ - Pioneer zone - dominated by <em>Spartina
 anglica</em> and <em>Salicornia</em> sp.
 <strong>Date</strong> -
 season when sampling took place Apr - April
 (spring) July - summer Oct -
 October (autumn) 
 <strong>plotID</strong> - combination of zone and transect
 created to account for multiple sampling along transects in mixed-effects
 model <strong>Transect</strong> - In
 total 5 transects across the salt marsh zones, numbered T(1-5)
 <strong>Type </strong>- What type of resource
 measured "terrestrial"- vascular plants, "marine" -
 allochthonous marine algae 
 <strong>Sample</strong> - Taxa measured/resource
 measured <strong>15N</strong> -
 d<sup>15</sup>N signature of sample 
 <strong>13C </strong>- d<sup>13C </sup>signature
 of sample <strong>Nsp </strong>-
 <sup>15</sup>N range (most <sup>15</sup>N enriched
 - least <sup>15</sup>N enriched)<br> <strong>Csp </strong>- <sup>13</sup>C range (most <sup>13</sup>C enriched - least <sup>13</sup>C enriched