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.

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.

Headings

• ID - number given to each sample
• Zone - salt marsh zone - separated by vegetation structure and shore height
  • USM - Upper salt marsh - dominated by Elymus athericus (Elytrigia atherica)
  • LSM - Lower salt marsh - dominated by Atriplex portulacoides
  • PZ - Pioneer zone - dominated by Spartina anglica and Salicornia sp.
• Date - season when sampling took place
  • Apr - April (spring)
  • July - summer
  • Oct - October (autumn)
• plotID - combination of zone and transect created to account for multiple sampling along transects in mixed-effects model
• Transect - In total 5 transects across the salt marsh zones, numbered T(1-5)
• Type - What type of resource measured "terrestrial"- vascular plants, "marine" - allochthonous marine algae
• Sample - Taxa measured/resource measured
• 15N - d¹⁵N signature of sample
• 13C - d^13C signature of sample
• Nsp - ¹⁵N range (most ¹⁵N enriched - least ¹⁵N enriched)
  
• Csp - ¹³C range (most ¹³C enriched - least ¹³C enriched