Data from: Trophic position and niche overlap of an Asian weatherfish (Misgurnus bipartitus), western tubenose goby (Proterorhinus semilunaris), and native benthic fish species

The dataset belonging to the paper "Trophic position and niche overlap of an Asian weatherfish (Misgurnus bipartitus), western tubenose goby (Proterorhinus semilunaris), and native benthic fish species" published in Aquatic Invasions (paper in press; doi link will be added later), is provided here. The dataset consists of δ13C and δ15N (‰) stable isotope ratios of taxa of fish, macroinvertebrates, plants, alga, and soil. If applicable, the length of the taxon was included. Below, methodological information is provided on the study site, the sampling process, the sampling preparation, and the stable isotope analysis. For references, see the published paper in Aquatic Invasions.

Study site
The study site concerned a section of the lowland brook Tungelroysebeek (51°14.38'N, 005°52.086'E – 51°14.26'N, 005°47.77'E) near the village Tungelroy in the Province of Limburg, the Netherlands. This brook of 35 km length has several tributaries before discharging into the River Meuse. Over most of its course, the brook was meandering and had a well-developed riparian and hydrophyte vegetation. During sampling the mean water temperature was 16.0 °C, conductivity 664 µS/cm, pH 7.3, water velocity 0.2 m/s, depth 70 cm, and Secchi depth 60 cm. The brook width ranged between 5-8 m and its bed substrate predominantly consisted of sand.

Sampling
Samples of fish, macroinvertebrates, macrophytes, dead organic material, and bottom soil substrate were collected in October 2019. The samples were collected in the stretch of the brook that is denoted in Fig. 1 of the paper. Fish were caught using handheld electrofishing equipment (Bretschneider EFGI 650). After catching, the fish were euthanized using a neutralized benzocaine solution of 100 mg l-1. Macroinvertebrates were collected using 70x55 cm dip nets with a mesh size of 1 mm. Helophytes, floating-leaved, and submerged aquatic macrophytes were collected by hand. Soil samples were collected by means of a hollow soil sampling tube with a diameter of 5 cm. A Nikon SMZ800 stereo microscope with a 10-63 magnification was used for the identification of small macroinvertebrates.

Sample preparation 
All samples were transported to the laboratory and stored separately at -18° C until preparation. To obtain muscle samples a piece of 0.5-1 cm of the dorsal tissue of each fish was dissected. Muscle tissue samples were dissected from fish, crayfish, and unionid mussels. Other macroinvertebrates were stored alive for two days at 5 °C to empty their intestinal contents. Subsequently, these invertebrates were rinsed with tap water and then with demineralized water before processing. From unionid mussels, muscle tissue of a similar size was dissected. Of small mollusks, all soft body tissues were used. For small mollusks and other macroinvertebrates, individuals of the same species were pooled to obtain enough material (0.22-0.26 mg) for analyses. For crayfish, muscle tissue of the abdomen was used while the intestine was removed. The stems, leaves, and roots of individual plants were pooled. 
After preparation, all samples were stored at -80 °C until freeze drying. Freeze drying was carried out at -90 °C for 24-48 hours for fish and macroinvertebrate samples. Plant, dead organic material, and soil samples were freeze dried at least 48 hours. After freeze drying, the samples were grounded with aluminum balls, for 2 min at 30 rpm, using a Retsch MM 400. Subsequently, the grounded samples were weighted in tin cups (Elemental Microanalysis 8 x 5 mm) and prepared for isotope analyses. For the fish and invertebrate samples, 0.22-0.26 mg was weighted. For plants and soil, separate samples were weighted for carbon (10 mg) and nitrogen (40 mg) analyzes. 

Stable isotope analyses
Carbon and nitrogen stable isotopes were measured using a Thermo Scientific FLASH 2000 HT Elemental Analyzer with a Thermo Scientific DELTA V Advantage Next Generation Isotope Ratio mass spectrometer. Reference gasses were calibrated with the IAEA standards (IAEA-N-2 and IAEA-CH-6), with a maximum deviation of 0.15‰. As an internal standard control, caffeine was used and the 13C/12C and 15N/14N of every sample were determined (in ‰). The isotope ratios (R) δ13C and δ15N are relative to Vienna PDB and atmospheric N2 and were calculated by:

δ13C or δ15N = (Rsample/Rstandard − 1) * 1000

Abstract 
Co-occurring and morphologically similar species have adapted to differential niches for minimizing competition. An invasive alien species can occupy an 'empty niche' in introduced ranges. Alternatively, the invader may occupy an overlapping niche and compete with native species to a certain degree. In a Western European lowland brook with high nutrient loads, we studied a benthic community of five fish species, including two alien species: an Asian weatherfish (Misgurnus bipartitus) and the western tubenose goby (Proterorhinus semilunaris). The native species concerned stone loach (Barbatula barbatula), spined loach (Cobitis taenia), and gudgeon (Gobio gobio). Because of the unknown effects of the invaders on native benthic fish species, the trophic position, isotopic niche overlap, and potential food competition among these species were identified using nitrogen and carbon stable isotopes. The trophic levels of the five fish species indicated that they are secondary consumers. Body size of native fish species correlated significantly negative with their δ15N (‰) signature, in contrast with the invaders indicating that the latter are generalists. Significant isotopic niche overlap was observed among all benthic species. The degree of niche overlap of M. bipartitus was the highest (91.8%) with the G. gobio. Proterorhinus semilunaris had the highest degree of niche overlap (91.2%) with the (B. barbatula. It was notable that the observed niche overlap between the native B. barbatula and C. taenia was high (99.2%). Overlap between M. bipartitus and P. semilunaris was low (8.9% overlap), indicating little resource competition between these alien species. Native species showed wider isotopic niches than the invaders. Bayesian mixing models revealed that native and alien species slightly differ in their main diet. The results suggest that the invaders are plastic in their resource use, leading to niche differentiation and promoting co-existence of benthic fish species.